Merge pull request #45 from KGAudioLab/feat/2026-05-17-misc

Feat/2026 05 17 misc

.vscode/tasks.json

@@ -2,15 +2,15 @@
     "version": "2.0.0",
     "tasks": [
         {
-            "type": "npm",
-            "script": "dev",
+            "type": "shell",
             "label": "npm: dev",
             "detail": "vite",
             "isBackground": true,
+            "command": "source ~/.nvm/nvm.sh && nvm use 20 && npm run dev",
             "options": {
                 "shell": {
                     "executable": "/bin/zsh",
-                    "args": ["-c", "source ~/.nvm/nvm.sh && nvm use 20 && npm run dev"]
+                    "args": ["-c"]
                 }
             },
             "problemMatcher": {
@@ -25,11 +25,11 @@
             }
         },
         {
-            "type": "npm",
-            "script": "dev",
+            "type": "shell",
             "label": "npm: dev (Windows)",
             "detail": "vite",
             "isBackground": true,
+            "command": "npm run dev",
             "options": {
                 "shell": {
                     "executable": "cmd.exe",
@@ -48,4 +48,4 @@
             }
         }
     ]
-}
+}

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "K.G.Studio",
-  "version": "0.16.0-build.20260510",
+  "version": "0.17.3-build.20260515",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "K.G.Studio",
-      "version": "0.16.0-build.20260510",
+      "version": "0.17.3-build.20260515",
       "dependencies": {
         "@breezystack/lamejs": "^1.2.7",
         "class-transformer": "^0.5.1",

public/config.json

@@ -27,8 +27,12 @@
             "model": ""
         },
         "local_browser": {
-            "context_length": 32768
+            "context_length": 32768,
+            "model_url": "https://huggingface.co/notabilia/gemma-4-E4B-it-litert-lm/resolve/main/gemma-4-E4B-it-web.task"
         }, 
+        "uvr5_web_runtime": {
+            "mdx_net_model_url": "https://huggingface.co/notabilia/uvr5-models/resolve/main/UVR-MDX-NET-Inst_HQ_3.onnx"
+        },
         "soundfont": {
             "base_url": "https://cdn.jsdelivr.net/npm/soundfont-for-samplers/FluidR3_GM/"
         },
@@ -49,7 +53,9 @@
             "copy": "ctrl+c",
             "cut": "ctrl+x",
             "paste": "ctrl+v",
-            "save": "ctrl+s"
+            "save": "ctrl+s",
+            "split_region": "ctrl+t",
+            "merge_regions": "ctrl+j"
         },
         "piano_roll": {
             "switch": "tab", 
@@ -76,6 +82,7 @@
         "default_open": true
     },
     "audio": {
+        "bounce_starts_from_beat_1": true,
         "enable_audio_capture_for_screen_sharing": false,
         "input_device_id": "default",
         "lookahead_time": 0.05,

src/agent/llm/LocalBrowserLLMProvider.ts

@@ -12,7 +12,7 @@ import {
 import {
   LOCAL_LLM_DEFAULT_CONTEXT_LENGTH,
   LOCAL_LLM_MODEL_FILENAME,
-  LOCAL_LLM_MODEL_URL,
+  LOCAL_LLM_DEFAULT_MODEL_URL,
   normalizeLocalLLMContextLength,
 } from '../../util/localLLMConfig';
 import { LocalLLMModelCache } from '../../util/localLLMModelCache';
@@ -67,12 +67,13 @@ export class LocalBrowserLLMProvider implements LLMProvider {
     }
 
     const maxTokens = this.getConfiguredContextLength();
+    const modelUrl = this.getConfiguredModelUrl();
     console.log(`[localLLM] Initializing with max context length: ${maxTokens} tokens`);
 
     const [{ FilesetResolver, LlmInference }, modelLoad] = await Promise.all([
       this.getMediaPipeModule(),
       LocalLLMModelCache.loadModelReaderWithCache(
-        LOCAL_LLM_MODEL_URL,
+        modelUrl,
         LOCAL_LLM_MODEL_FILENAME,
         progress => {
           LocalLLMModelManager.notifyLoadProgress(progress.receivedBytes, progress.totalBytes, progress.fromCache);
@@ -132,6 +133,16 @@ export class LocalBrowserLLMProvider implements LLMProvider {
     }
   }
 
+  private getConfiguredModelUrl(): string {
+    try {
+      const configManager = ConfigManager.instance();
+      const configured = configManager.get('general.local_browser.model_url');
+      return typeof configured === 'string' && configured.trim() ? configured : LOCAL_LLM_DEFAULT_MODEL_URL;
+    } catch {
+      return LOCAL_LLM_DEFAULT_MODEL_URL;
+    }
+  }
+
   private applyTemplate(message: { role: 'user' | 'model'; text: string }): string {
     const template = PROMPT_TEMPLATE[message.role];
     return `${template.pre}${message.text}${template.post}`;

src/components/KGOnePanel.test.tsx

@@ -5,6 +5,10 @@ import KGOnePanel from './KGOnePanel';
 import { KGAudioRegion } from '../core/region/KGAudioRegion';
 import { KGAudioTrack } from '../core/track/KGAudioTrack';
 
+const { mockLocalSeparatorDownload } = vi.hoisted(() => ({
+  mockLocalSeparatorDownload: vi.fn(async (_url?: string, _filename?: string, _onProgress?: unknown) => undefined),
+}));
+
 let kgoneEnabled = false;
 let selectedRegionIds: string[] = [];
 let localModelCached = false;
@@ -31,6 +35,7 @@ vi.mock('../core/config/ConfigManager', () => ({
       get: (key: string) => {
         if (key === 'general.kgone.enabled') return kgoneEnabled;
         if (key === 'general.kgone.base_url') return 'http://127.0.0.1:8000';
+        if (key === 'general.uvr5_web_runtime.mdx_net_model_url') return 'https://example.com/custom-uvr5.onnx';
         return undefined;
       },
     }),
@@ -79,8 +84,9 @@ vi.mock('../util/audioUtil', () => ({
 vi.mock('../util/localSeparatorModelCache', () => ({
   LocalSeparatorModelCache: {
     exists: vi.fn(async () => localModelCached),
-    download: vi.fn(async () => {
+    download: vi.fn(async (url: string, filename: string, onProgress: (progress: unknown) => void) => {
       localModelCached = true;
+      return mockLocalSeparatorDownload(url, filename, onProgress);
     }),
     delete: vi.fn(async () => {
       localModelCached = false;
@@ -124,6 +130,7 @@ describe('KGOnePanel local separator mode', () => {
       { name: 'Instrumental', blob: new Blob(['instrumental'], { type: 'audio/wav' }) },
       { name: 'Vocals', blob: new Blob(['vocals'], { type: 'audio/wav' }) },
     ];
+    mockLocalSeparatorDownload.mockClear();
     mockRefreshProjectState.mockReset();
     mockExecuteCommand.mockReset();
   });
@@ -155,6 +162,20 @@ describe('KGOnePanel local separator mode', () => {
     expect(screen.getByLabelText('MDX overlap')).toBeInTheDocument();
   });
 
+  it('uses the configured UVR5 model URL when downloading the local model', async () => {
+    render(<KGOnePanel isVisible={true} />);
+
+    fireEvent.click(await screen.findByRole('button', { name: 'Download Model' }));
+
+    await waitFor(() => {
+      expect(mockLocalSeparatorDownload).toHaveBeenCalledWith(
+        'https://example.com/custom-uvr5.onnx',
+        'UVR-MDX-NET-Inst_HQ_3.onnx',
+        expect.any(Function),
+      );
+    });
+  });
+
   it('prompts for an audio region when the model is cached but nothing is selected', async () => {
     localModelCached = true;
 

src/components/KGOnePanel.tsx

@@ -18,7 +18,7 @@ import { showAlert } from '../util/dialogUtil';
 import {
   LOCAL_SEPARATOR_MODEL_CONFIG,
   LOCAL_SEPARATOR_MODEL_FILENAME,
-  LOCAL_SEPARATOR_MODEL_URL,
+  LOCAL_SEPARATOR_DEFAULT_MODEL_URL,
 } from '../util/localSeparatorConfig';
 import { LocalSeparatorModelCache } from '../util/localSeparatorModelCache';
 import { runLocalSeparator } from '../util/localSeparatorRunner';
@@ -959,6 +959,13 @@ const SeparatorTab: React.FC<{ mode: KGOneMode }> = ({ mode }) => {
     return null;
   }, [selectedRegionIds]);
 
+  const getConfiguredLocalSeparatorModelUrl = useCallback(() => {
+    const configured = ConfigManager.instance().get('general.uvr5_web_runtime.mdx_net_model_url');
+    return typeof configured === 'string' && configured.trim()
+      ? configured
+      : LOCAL_SEPARATOR_DEFAULT_MODEL_URL;
+  }, []);
+
   const isGenerating = genStatus !== 'idle' && genStatus !== 'done' && genStatus !== 'error';
 
   const handleDownloadLocalModel = useCallback(async () => {
@@ -969,7 +976,7 @@ const SeparatorTab: React.FC<{ mode: KGOneMode }> = ({ mode }) => {
 
     try {
       await LocalSeparatorModelCache.download(
-        LOCAL_SEPARATOR_MODEL_URL,
+        getConfiguredLocalSeparatorModelUrl(),
         LOCAL_SEPARATOR_MODEL_FILENAME,
         progress => {
           const receivedMb = (progress.receivedBytes / (1024 * 1024)).toFixed(1);
@@ -992,7 +999,7 @@ const SeparatorTab: React.FC<{ mode: KGOneMode }> = ({ mode }) => {
     } finally {
       setIsDownloadingLocalModel(false);
     }
-  }, [refreshLocalModelCacheState]);
+  }, [getConfiguredLocalSeparatorModelUrl, refreshLocalModelCacheState]);
 
   const handleDeleteLocalModel = useCallback(async () => {
     setIsDeletingLocalModel(true);

src/components/MainContent.test.tsx

@@ -3,13 +3,20 @@ import { beforeEach, describe, expect, it, vi } from 'vitest';
 import { fireEvent, render, screen } from '@testing-library/react';
 import MainContent from './MainContent';
 import { KGMidiRegion } from '../core/region/KGMidiRegion';
+import { KGAudioRegion } from '../core/region/KGAudioRegion';
+import { KGAudioTrack } from '../core/track/KGAudioTrack';
 import { createMockMidiTrack } from '../test/utils/mock-data';
 
 const midiRegion = new KGMidiRegion('region-1', '1', 0, 'Region 1', 0, 4);
-const track = createMockMidiTrack({ id: 1, regions: [midiRegion] });
+const anotherMidiRegion = new KGMidiRegion('region-2', '1', 0, 'Region 2', 8, 4);
+const audioRegion = new KGAudioRegion('audio-1', '2', 1, 'Audio 1', 4, 4);
+const midiTrack = createMockMidiTrack({ id: 1, regions: [midiRegion, anotherMidiRegion] });
+const audioTrack = new KGAudioTrack('Audio Track', 2);
+audioTrack.setTrackIndex(1);
+audioTrack.setRegions([audioRegion]);
 
 const storeState = {
-  tracks: [track],
+  tracks: [midiTrack, audioTrack],
   maxBars: 8,
   barWidthMultiplier: 1,
   reorderTracks: vi.fn(),
@@ -35,6 +42,8 @@ const storeState = {
     storeState.activeRegionId = regionId;
   }),
   pianoRollMode: 'midi-edit' as const,
+  requestedSheetMusicViewEnabled: false,
+  pianoRollViewRequestVersion: 0,
   openMidiPianoRoll: vi.fn(),
   openSpectrogramViewer: vi.fn(),
   openHybridMode: vi.fn(),
@@ -69,13 +78,16 @@ vi.mock('../stores/projectStore', () => ({
 vi.mock('../core/KGCore', () => ({
   KGCore: {
     instance: () => ({
-      addSelectedItems: (items: KGMidiRegion[]) => {
+      addSelectedItems: (items: Array<{ getId(): string }>) => {
         storeState.selectedRegionIds = items.map(item => item.getId());
       },
       clearSelectedItems: () => {
         storeState.selectedRegionIds = [];
       },
       executeCommand: vi.fn(),
+      getCurrentProject: () => ({
+        getTracks: () => storeState.tracks,
+      }),
     }),
   },
 }));
@@ -92,14 +104,26 @@ vi.mock('./track/TrackInfoPanel', () => ({
 
 vi.mock('./track/TrackGridPanel', () => ({
   default: ({ onRegionClick }: { onRegionClick?: RegionClickHandler }) => (
-    <button type="button" onClick={() => onRegionClick?.('region-1', { shiftKey: false })}>
-      select-region
-    </button>
+    <>
+      <button type="button" onClick={() => onRegionClick?.('region-1', { shiftKey: false })}>
+        select-midi-region
+      </button>
+      <button type="button" onClick={() => onRegionClick?.('region-2', { shiftKey: false })}>
+        select-second-midi-region
+      </button>
+      <button type="button" onClick={() => onRegionClick?.('audio-1', { shiftKey: false })}>
+        select-audio-region
+      </button>
+    </>
   ),
 }));
 
 vi.mock('./piano-roll/PianoRoll', () => ({
-  default: () => <div data-testid="piano-roll" />,
+  default: ({ onClose }: { onClose?: () => void }) => (
+    <div data-testid="piano-roll">
+      <button type="button" onClick={onClose}>close-piano-roll</button>
+    </div>
+  ),
 }));
 
 describe('MainContent', () => {
@@ -118,11 +142,65 @@ describe('MainContent', () => {
   it('updates activeRegionId when selecting a region with piano roll closed', () => {
     render(<MainContent />);
 
-    fireEvent.click(screen.getByRole('button', { name: 'select-region' }));
+    fireEvent.click(screen.getByRole('button', { name: 'select-midi-region' }));
 
     expect(storeState.activeRegionId).toBe('region-1');
     expect(storeState.setActiveRegionId).toHaveBeenCalledWith('region-1');
     expect(storeState.showPianoRoll).toBe(false);
     expect(storeState.openMidiPianoRoll).not.toHaveBeenCalled();
   });
+
+  it('keeps piano roll open and preserves activeRegionId when deselecting all regions', () => {
+    storeState.showPianoRoll = true;
+    storeState.activeRegionId = 'region-1';
+    storeState.selectedRegionIds = ['region-1'];
+
+    const { container } = render(<MainContent />);
+
+    fireEvent.click(container.firstChild as HTMLElement);
+
+    expect(storeState.selectedRegionIds).toEqual([]);
+    expect(storeState.showPianoRoll).toBe(true);
+    expect(storeState.activeRegionId).toBe('region-1');
+    expect(storeState.setShowPianoRoll).not.toHaveBeenCalledWith(false);
+    expect(storeState.setActiveRegionId).not.toHaveBeenCalledWith(null);
+  });
+
+  it('auto-switches the open piano roll when selecting another MIDI region', () => {
+    storeState.showPianoRoll = true;
+    storeState.activeRegionId = 'region-1';
+
+    render(<MainContent />);
+
+    fireEvent.click(screen.getByRole('button', { name: 'select-second-midi-region' }));
+
+    expect(storeState.activeRegionId).toBe('region-2');
+    expect(storeState.openMidiPianoRoll).toHaveBeenCalledWith('region-2');
+  });
+
+  it('preserves current cross-type behavior when selecting an audio region with the editor open', () => {
+    storeState.showPianoRoll = true;
+    storeState.activeRegionId = 'region-1';
+
+    render(<MainContent />);
+
+    fireEvent.click(screen.getByRole('button', { name: 'select-audio-region' }));
+
+    expect(storeState.activeRegionId).toBe('audio-1');
+    expect(storeState.openSpectrogramViewer).toHaveBeenCalledWith('audio-1');
+  });
+
+  it('explicit close still clears piano roll visibility and active region', () => {
+    storeState.showPianoRoll = true;
+    storeState.activeRegionId = 'region-1';
+
+    render(<MainContent />);
+
+    fireEvent.click(screen.getByRole('button', { name: 'close-piano-roll' }));
+
+    expect(storeState.showPianoRoll).toBe(false);
+    expect(storeState.activeRegionId).toBeNull();
+    expect(storeState.setShowPianoRoll).toHaveBeenCalledWith(false);
+    expect(storeState.setActiveRegionId).toHaveBeenCalledWith(null);
+  });
 });

src/components/MainContent.tsx

@@ -46,6 +46,8 @@ const MainContent: React.FC<MainContentProps> = ({
     setShowPianoRoll,
     setActiveRegionId,
     pianoRollMode,
+    requestedSheetMusicViewEnabled,
+    pianoRollViewRequestVersion,
     openMidiPianoRoll,
     openSpectrogramViewer,
     openHybridMode,
@@ -334,6 +336,21 @@ const MainContent: React.FC<MainContentProps> = ({
     setRegions(updatedRegions);
   }, [tracks, timeSignature]);
 
+  useEffect(() => {
+    if (!showPianoRoll || !activeRegionId) {
+      return;
+    }
+
+    const activeRegionStillExists = tracks.some(track =>
+      track.getRegions().some(region => region.getId() === activeRegionId)
+    );
+
+    if (!activeRegionStillExists) {
+      setShowPianoRoll(false);
+      setActiveRegionId(null);
+    }
+  }, [tracks, showPianoRoll, activeRegionId, setShowPianoRoll, setActiveRegionId]);
+
   // Apply auto-selection for newly created/imported regions after the regions state commits.
   useEffect(() => {
     const pendingRegionId = pendingAutoSelectionRegionIdRef.current;
@@ -587,7 +604,9 @@ const MainContent: React.FC<MainContentProps> = ({
       : null;
 
     setSelectedRegionId(lastSelectedRegionId);
-    setActiveRegionId(lastSelectedRegionId);
+    if (lastSelectedRegionId) {
+      setActiveRegionId(lastSelectedRegionId);
+    }
 
     if (DEBUG_MODE.MAIN_CONTENT) {
       console.log(`Selected regions: ${selectedRegions.map(selectedRegion => selectedRegion.getId()).join(', ')}`);
@@ -598,7 +617,6 @@ const MainContent: React.FC<MainContentProps> = ({
     }
 
     if (!lastSelectedRegionId) {
-      setShowPianoRoll(false);
       return;
     }
 
@@ -1052,6 +1070,8 @@ const MainContent: React.FC<MainContentProps> = ({
           onClose={handlePianoRollClose}
           regionId={activeRegionId}
           mode={pianoRollMode}
+          requestedSheetMusicViewEnabled={requestedSheetMusicViewEnabled}
+          pianoRollViewRequestVersion={pianoRollViewRequestVersion}
           audioRegion={(() => {
             // spectrogram mode: audio region IS the activeRegionId
             // hybrid mode: audio region is hybridAudioRegionId

src/components/Toolbar.css

@@ -7,6 +7,8 @@
   height: 50px;
   padding: 0 10px;
   border-bottom: 1px solid #3a3a3a;
+  position: relative;
+  z-index: 2000;
 }
 
 .toolbar-left,

src/components/Toolbar.test.tsx

@@ -1,5 +1,5 @@
 import React from 'react';
-import { fireEvent, render, screen } from '@testing-library/react';
+import { fireEvent, render, screen, waitFor } from '@testing-library/react';
 import { beforeEach, describe, expect, it, vi } from 'vitest';
 import Toolbar from './Toolbar';
 
@@ -52,13 +52,13 @@ const storeState = {
   setShowPianoRoll: vi.fn(),
   activeRegionId: null,
   setActiveRegionId: vi.fn(),
-  selectedRegionIds: [],
+  selectedRegionIds: [] as string[],
   selectedTrackId: null,
   playheadPosition: 0,
   refreshProjectState: vi.fn(),
   requestMainContentScroll: vi.fn(),
   requestPianoRollScroll: vi.fn(),
-  tracks: [],
+  tracks: [] as unknown[],
 };
 
 type StoreState = typeof storeState;
@@ -108,6 +108,14 @@ vi.mock('../core/state/KGMainContentState', () => ({ KGMainContentState: {} }));
 vi.mock('../util/regionDeleteUtil', () => ({ regionDeleteManager: { deleteSelectedRegions: vi.fn(() => false) } }));
 vi.mock('../core/commands/region/SplitRegionCommand', () => ({ SplitRegionCommand: class {} }));
 vi.mock('../core/commands/region/MergeMidiRegionsCommand', () => ({ MergeMidiRegionsCommand: class {} }));
+const regionEditUtilMocks = vi.hoisted(() => ({
+  splitSelectedRegionAtPlayheadMock: vi.fn(),
+  mergeSelectedMidiRegionsMock: vi.fn(),
+}));
+vi.mock('../util/regionEditUtil', () => ({
+  splitSelectedRegionAtPlayhead: regionEditUtilMocks.splitSelectedRegionAtPlayheadMock,
+  mergeSelectedMidiRegions: regionEditUtilMocks.mergeSelectedMidiRegionsMock,
+}));
 vi.mock('../util/copyPasteUtil', () => ({
   handleCopyOperation: vi.fn(() => false),
   handlePasteOperation: vi.fn(() => false),
@@ -135,6 +143,8 @@ vi.mock('../util/dialogUtil', () => ({
 
 describe('Toolbar settings side-panel behavior', () => {
   beforeEach(() => {
+    regionEditUtilMocks.splitSelectedRegionAtPlayheadMock.mockReset();
+    regionEditUtilMocks.mergeSelectedMidiRegionsMock.mockReset();
     storeState.toggleChatBox.mockClear();
     storeState.toggleKGOnePanel.mockClear();
     storeState.toggleEventListPanel.mockClear();
@@ -162,4 +172,25 @@ describe('Toolbar settings side-panel behavior', () => {
     expect(storeState.activateSidePanel).toHaveBeenCalledWith('eventList');
     expect(storeState.toggleEventListPanel).not.toHaveBeenCalled();
   });
+
+  it('routes the split toolbar button through the shared split helper', async () => {
+    storeState.selectedRegionIds = ['region-1'];
+    storeState.playheadPosition = 12;
+    regionEditUtilMocks.splitSelectedRegionAtPlayheadMock.mockResolvedValue('Split 1 note at beat 12.00');
+
+    render(<Toolbar />);
+    fireEvent.click(screen.getByTitle('Split Region at Playhead'));
+
+    await waitFor(() => {
+      expect(regionEditUtilMocks.splitSelectedRegionAtPlayheadMock).toHaveBeenCalledWith({
+        selectedRegionIds: ['region-1'],
+        playheadPosition: 12,
+        refreshProjectState: storeState.refreshProjectState,
+      });
+    });
+
+    await waitFor(() => {
+      expect(storeState.setStatus).toHaveBeenCalledWith('Split 1 note at beat 12.00');
+    });
+  });
 });

src/components/Toolbar.tsx

@@ -22,8 +22,6 @@ import { plainToInstance } from 'class-transformer';
 import { FaPencil, FaCopy, FaPaste, FaTrash, FaWandMagicSparkles, FaListUl } from 'react-icons/fa6';
 import { KGMainContentState } from '../core/state/KGMainContentState';
 import { regionDeleteManager } from '../util/regionDeleteUtil';
-import { SplitRegionCommand } from '../core/commands/region/SplitRegionCommand';
-import { MergeMidiRegionsCommand } from '../core/commands/region/MergeMidiRegionsCommand';
 import { handleCopyOperation, handlePasteOperation } from '../util/copyPasteUtil';
 import { convertProjectToMidi, convertMidiToProject } from '../util/midiUtil';
 import { KEY_SIGNATURE_MAP } from '../constants/coreConstants';
@@ -35,7 +33,7 @@ import OpenProjectModal from './common/OpenProjectModal';
 import { clearChatHistoryAndUI } from '../util/chatUtil';
 import PianoIcon from './common/icons/PianoIcon';
 import MetronomeIcon from './common/icons/MetronomeIcon';
-import { ConfigManager } from '../core/config/ConfigManager';
+import { mergeSelectedMidiRegions, splitSelectedRegionAtPlayhead } from '../util/regionEditUtil';
 import { showAlert, showChoice, showConfirm, showPrompt, showTimeSigPrompt } from '../util/dialogUtil';
 
 const Toolbar: React.FC = () => {
@@ -791,43 +789,19 @@ const Toolbar: React.FC = () => {
       console.log("Split button clicked");
     }
 
-    if (selectedRegionIds.length === 0) {
-      await showAlert("Please select a region to split.");
-      return;
-    }
-    if (selectedRegionIds.length > 1) {
-      await showAlert("Please select exactly one region to split.");
+    const status = await splitSelectedRegionAtPlayhead({
+      selectedRegionIds,
+      playheadPosition,
+      refreshProjectState,
+    });
+    if (!status) {
       return;
     }
 
-    const regionId = lastSelectedRegionId;
-    const tracks = KGCore.instance().getCurrentProject().getTracks();
-    let targetRegion = null;
-    for (const track of tracks) {
-      const found = track.getRegions().find(r => r.getId() === regionId);
-      if (found) { targetRegion = found; break; }
-    }
-
-    if (!targetRegion) {
-      await showAlert("Selected region not found.");
-      return;
-    }
-
-    const regionStart = targetRegion.getStartFromBeat();
-    const regionEnd = regionStart + targetRegion.getLength();
-
-    if (playheadPosition <= regionStart || playheadPosition >= regionEnd) {
-      await showAlert("The playhead is not inside the selected region. Move the playhead inside the region before splitting.");
-      return;
-    }
-
-    const command = new SplitRegionCommand(regionId, playheadPosition);
-    KGCore.instance().executeCommand(command);
-    refreshProjectState();
-    setStatus(`Split region at beat ${playheadPosition.toFixed(2)}`);
+    setStatus(status);
 
     if (DEBUG_MODE.TOOLBAR) {
-      console.log(`Split region ${regionId} at beat ${playheadPosition}`);
+      console.log(`Split selected region at beat ${playheadPosition}`);
     }
   };
 
@@ -836,92 +810,15 @@ const Toolbar: React.FC = () => {
       console.log('Merge button clicked');
     }
 
-    if (selectedRegionIds.length < 2) {
-      await showAlert('Please select at least two MIDI regions on the same track to merge.');
-      return;
-    }
-
-    const tracks = KGCore.instance().getCurrentProject().getTracks();
-    const selectedRegionIdSet = new Set(selectedRegionIds);
-    const selectedMidiRegions: KGMidiRegion[] = [];
-    let targetTrackId: string | null = null;
-
-    for (const track of tracks) {
-      for (const region of track.getRegions()) {
-        if (!selectedRegionIdSet.has(region.getId())) {
-          continue;
-        }
-
-        if (!(region instanceof KGMidiRegion)) {
-          await showAlert('Only MIDI regions can be merged. Please adjust your selection and try again.');
-          return;
-        }
-
-        const regionTrackId = track.getId().toString();
-        if (targetTrackId && targetTrackId !== regionTrackId) {
-          await showAlert('Please select only MIDI regions from a single track before merging.');
-          return;
-        }
-
-        targetTrackId = regionTrackId;
-        selectedMidiRegions.push(region);
-      }
-    }
-
-    if (selectedMidiRegions.length !== selectedRegionIds.length || !targetTrackId) {
-      await showAlert('Some selected regions could not be found. Please reselect the MIDI regions and try again.');
-      return;
-    }
-
-    const sortedSelectedRegions = [...selectedMidiRegions].sort((a, b) => {
-      const startDelta = a.getStartFromBeat() - b.getStartFromBeat();
-      if (startDelta !== 0) return startDelta;
-      return a.getLength() - b.getLength();
+    const status = await mergeSelectedMidiRegions({
+      selectedRegionIds,
+      refreshProjectState,
     });
-
-    let regionIdsToMerge = selectedRegionIds;
-    const firstSelectedRegion = sortedSelectedRegions[0];
-    const lastSelectedRegion = sortedSelectedRegions[sortedSelectedRegions.length - 1];
-    const spanStart = firstSelectedRegion.getStartFromBeat();
-    const spanEnd = lastSelectedRegion.getStartFromBeat() + lastSelectedRegion.getLength();
-
-    const targetTrack = tracks.find(track => track.getId().toString() === targetTrackId);
-    const inBetweenRegions = targetTrack
-      ?.getRegions()
-      .filter(region => (
-        region instanceof KGMidiRegion &&
-        !selectedRegionIdSet.has(region.getId()) &&
-        region.getStartFromBeat() >= spanStart &&
-        region.getStartFromBeat() <= spanEnd
-      )) ?? [];
-
-    if (inBetweenRegions.length > 0) {
-      const shouldIncludeInBetweenRegions = await showConfirm(
-        'There are additional MIDI regions between the first and last selected regions on this track. Would you like KGStudio to merge those as well?',
-        {
-          confirmLabel: 'Merge All In Between',
-          cancelLabel: 'Stop',
-        }
-      );
-
-      if (!shouldIncludeInBetweenRegions) {
-        return;
-      }
-
-      regionIdsToMerge = Array.from(new Set([
-        ...selectedRegionIds,
-        ...inBetweenRegions.map(region => region.getId()),
-      ]));
+    if (!status) {
+      return;
     }
 
-    try {
-      const command = new MergeMidiRegionsCommand(regionIdsToMerge);
-      KGCore.instance().executeCommand(command, { rethrow: true });
-      refreshProjectState();
-      setStatus(`Merged ${regionIdsToMerge.length} MIDI regions`);
-    } catch (error) {
-      await showAlert(error instanceof Error ? error.message : 'Unable to merge the selected MIDI regions.');
-    }
+    setStatus(status);
   };
 
   // Handle undo button click

src/components/interfaces.ts

@@ -18,6 +18,11 @@ export interface RegionUI {
   name: string;
 }
 
+export interface RegionPreviewContentStyle {
+  left: string;
+  width: string;
+}
+
 export interface RegionClickOptions {
   shiftKey: boolean;
 }

src/components/piano-roll/PianoKeys.test.tsx

@@ -0,0 +1,146 @@
+import React from 'react';
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { act, fireEvent, render, screen } from '@testing-library/react';
+import PianoKeys from './PianoKeys';
+import { createMockMidiNote, createMockMidiRegion, createMockMidiTrack } from '../../test/utils/mock-data';
+
+type TestLiveNoteActivityListener = (...args: [{ pitch: number; isNoteOn: boolean }]) => void;
+
+const storeState = {
+  tracks: [createMockMidiTrack({ id: 1 })],
+  playheadPosition: 0,
+  isPlaying: false,
+};
+
+const audioInterfaceMock = {
+  getIsInitialized: vi.fn(),
+  getIsAudioContextStarted: vi.fn(),
+  startAudioContext: vi.fn(),
+  triggerNoteAttack: vi.fn(),
+  releaseNote: vi.fn(),
+};
+
+let liveNoteActivityListener: TestLiveNoteActivityListener | null = null;
+const midiInputMock = {
+  addLiveNoteActivityListener: vi.fn((listener: TestLiveNoteActivityListener) => {
+    liveNoteActivityListener = listener;
+  }),
+  removeLiveNoteActivityListener: vi.fn((listener: TestLiveNoteActivityListener) => {
+    if (liveNoteActivityListener === listener) {
+      liveNoteActivityListener = null;
+    }
+  }),
+};
+
+vi.mock('../../stores/projectStore', () => ({
+  useProjectStore: (selector: (...args: [typeof storeState]) => unknown) => selector(storeState),
+}));
+
+vi.mock('../../core/audio-interface/KGAudioInterface', () => ({
+  KGAudioInterface: {
+    instance: () => audioInterfaceMock,
+  },
+}));
+
+vi.mock('../../core/midi-input/KGMidiInput', () => ({
+  KGMidiInput: {
+    instance: () => midiInputMock,
+  },
+}));
+
+describe('PianoKeys', () => {
+  const activeRegion = createMockMidiRegion({
+    trackId: '1',
+    notes: [createMockMidiNote({ id: 'note-c4', pitch: 60, startBeat: 0, endBeat: 2 })],
+  });
+
+  beforeEach(() => {
+    storeState.tracks = [createMockMidiTrack({ id: 1 })];
+    storeState.playheadPosition = 0;
+    storeState.isPlaying = false;
+    liveNoteActivityListener = null;
+    vi.clearAllMocks();
+    audioInterfaceMock.getIsInitialized.mockReturnValue(true);
+    audioInterfaceMock.getIsAudioContextStarted.mockReturnValue(true);
+    audioInterfaceMock.startAudioContext.mockResolvedValue(undefined);
+  });
+
+  it('shows dot and background feedback for mouse preview while held', () => {
+    const { container } = render(<PianoKeys activeRegion={activeRegion} />);
+    const key = container.querySelector('[data-note="C4"]') as HTMLElement;
+
+    fireEvent.mouseDown(key);
+
+    expect(key.className).toContain('visual-active');
+    expect(screen.getByTestId('piano-key-dot-C4')).toBeInTheDocument();
+
+    fireEvent.mouseUp(key);
+
+    expect(key.className).not.toContain('visual-active');
+    expect(screen.queryByTestId('piano-key-dot-C4')).not.toBeInTheDocument();
+  });
+
+  it('shows MIDI activity dot without background feedback', () => {
+    const { container } = render(<PianoKeys activeRegion={activeRegion} />);
+    const key = container.querySelector('[data-note="C4"]') as HTMLElement;
+    expect(midiInputMock.addLiveNoteActivityListener).toHaveBeenCalledTimes(1);
+
+    act(() => {
+      liveNoteActivityListener?.({ pitch: 60, isNoteOn: true });
+    });
+
+    expect(screen.getByTestId('piano-key-dot-C4')).toBeInTheDocument();
+    expect(key.className).not.toContain('visual-active');
+
+    act(() => {
+      liveNoteActivityListener?.({ pitch: 60, isNoteOn: false });
+    });
+
+    expect(screen.queryByTestId('piano-key-dot-C4')).not.toBeInTheDocument();
+  });
+
+  it('shows playback background feedback without dot for sounding notes in the active region', () => {
+    storeState.isPlaying = true;
+    storeState.playheadPosition = 1;
+
+    const { container } = render(<PianoKeys activeRegion={activeRegion} />);
+    const key = container.querySelector('[data-note="C4"]') as HTMLElement;
+
+    expect(key.className).toContain('playback-active');
+    expect(key.className).toContain('visual-active');
+    expect(screen.queryByTestId('piano-key-dot-C4')).not.toBeInTheDocument();
+  });
+
+  it('preserves source-specific feedback while mouse, MIDI, and playback overlap', () => {
+    storeState.isPlaying = true;
+    storeState.playheadPosition = 1;
+
+    const { container, rerender } = render(<PianoKeys activeRegion={activeRegion} />);
+    const key = container.querySelector('[data-note="C4"]') as HTMLElement;
+
+    fireEvent.mouseDown(key);
+    act(() => {
+      liveNoteActivityListener?.({ pitch: 60, isNoteOn: true });
+    });
+
+    expect(key.className).toContain('visual-active');
+    expect(screen.getByTestId('piano-key-dot-C4')).toBeInTheDocument();
+
+    storeState.isPlaying = false;
+    rerender(<PianoKeys activeRegion={activeRegion} />);
+
+    expect(key.className).toContain('visual-active');
+    expect(screen.getByTestId('piano-key-dot-C4')).toBeInTheDocument();
+
+    fireEvent.mouseUp(key);
+
+    expect(key.className).not.toContain('visual-active');
+    expect(screen.getByTestId('piano-key-dot-C4')).toBeInTheDocument();
+
+    act(() => {
+      liveNoteActivityListener?.({ pitch: 60, isNoteOn: false });
+    });
+
+    expect(screen.queryByTestId('piano-key-dot-C4')).not.toBeInTheDocument();
+  });
+});

src/components/piano-roll/PianoKeys.tsx

@@ -1,30 +1,94 @@
-import React, { useState, useRef } from 'react';
+import React, { useEffect, useMemo, useRef, useState } from 'react';
 import { KGMidiRegion } from '../../core/region/KGMidiRegion';
 import { KGAudioInterface } from '../../core/audio-interface/KGAudioInterface';
-import { noteNameToPitch, midiPercussionKeyMap, pitchToNoteNameString } from '../../util/midiUtil';
+import { noteNameToPitch, midiPercussionKeyMap } from '../../util/midiUtil';
 import { useProjectStore } from '../../stores/projectStore';
 import { KGMidiTrack } from '../../core/track/KGMidiTrack';
+import { KGMidiInput, type LiveMidiNoteActivityEvent } from '../../core/midi-input/KGMidiInput';
 
 interface PianoKeysProps {
   activeRegion: KGMidiRegion | null;
 }
 
+function incrementPitchCount(source: Map<number, number>, pitch: number): Map<number, number> {
+  const next = new Map(source);
+  next.set(pitch, (next.get(pitch) ?? 0) + 1);
+  return next;
+}
+
+function decrementPitchCount(source: Map<number, number>, pitch: number): Map<number, number> {
+  const next = new Map(source);
+  const current = next.get(pitch) ?? 0;
+
+  if (current <= 1) {
+    next.delete(pitch);
+  } else {
+    next.set(pitch, current - 1);
+  }
+
+  return next;
+}
+
 const PianoKeys: React.FC<PianoKeysProps> = ({ activeRegion }) => {
-  const [pressedKeys, setPressedKeys] = useState<Set<string>>(new Set());
-  const pressedKeysRef = useRef<Set<string>>(new Set());
-  const { tracks } = useProjectStore();
+  const [mouseActivePitches, setMouseActivePitches] = useState<Map<number, number>>(new Map());
+  const [midiActivePitches, setMidiActivePitches] = useState<Map<number, number>>(new Map());
+  const mouseActivePitchesRef = useRef<Map<number, number>>(new Map());
+  const tracks = useProjectStore(state => state.tracks);
+  const playheadPosition = useProjectStore(state => state.playheadPosition);
+  const isPlaying = useProjectStore(state => state.isPlaying);
 
   // Check if current active region belongs to a drum track
-  const isDrumTrack = React.useMemo(() => {
+  const isDrumTrack = useMemo(() => {
     if (!activeRegion) return false;
     const track = tracks.find(t => t.getId().toString() === activeRegion.getTrackId());
     return track instanceof KGMidiTrack && track.getInstrument() === 'standard';
   }, [activeRegion, tracks]);
 
+  const playbackActivePitches = useMemo(() => {
+    if (!activeRegion || !isPlaying) {
+      return new Set<number>();
+    }
+
+    const activePitches = new Set<number>();
+    const absolutePlayhead = playheadPosition;
+    const regionStartBeat = activeRegion.getStartFromBeat();
+
+    activeRegion.getNotes().forEach(note => {
+      const startBeat = regionStartBeat + note.getStartBeat();
+      const endBeat = regionStartBeat + note.getEndBeat();
+
+      if (absolutePlayhead >= startBeat && absolutePlayhead < endBeat) {
+        activePitches.add(note.getPitch());
+      }
+    });
+
+    return activePitches;
+  }, [activeRegion, isPlaying, playheadPosition]);
+
+  useEffect(() => {
+    const midiInput = KGMidiInput.instance();
+
+    const handleLiveNoteActivity = (event: LiveMidiNoteActivityEvent) => {
+      setMidiActivePitches(current => (
+        event.isNoteOn
+          ? incrementPitchCount(current, event.pitch)
+          : decrementPitchCount(current, event.pitch)
+      ));
+    };
+
+    midiInput.addLiveNoteActivityListener(handleLiveNoteActivity);
+
+    return () => {
+      midiInput.removeLiveNoteActivityListener(handleLiveNoteActivity);
+    };
+  }, []);
+
   // Handle mouse down on piano key
   const handleKeyMouseDown = (keyId: string) => {
+    const pitch = noteNameToPitch(keyId);
+
     // Prevent double pressing the same key
-    if (pressedKeysRef.current.has(keyId)) {
+    if ((mouseActivePitchesRef.current.get(pitch) ?? 0) > 0) {
       return;
     }
 
@@ -37,9 +101,6 @@ const PianoKeys: React.FC<PianoKeysProps> = ({ activeRegion }) => {
     const trackId = activeRegion.getTrackId();
     
     try {
-      // Convert note name to pitch (keyId is always a note name like "C4")
-      const pitch = noteNameToPitch(keyId);
-      
       // Get audio interface and start playing the note
       const audioInterface = KGAudioInterface.instance();
       if (audioInterface.getIsInitialized()) {
@@ -54,11 +115,9 @@ const PianoKeys: React.FC<PianoKeysProps> = ({ activeRegion }) => {
         if (audioInterface.getIsAudioContextStarted()) {
           audioInterface.triggerNoteAttack(trackId, pitch, 127);
           
-          // Update pressed keys state
-          const newPressedKeys = new Set(pressedKeysRef.current);
-          newPressedKeys.add(keyId);
-          pressedKeysRef.current = newPressedKeys;
-          setPressedKeys(newPressedKeys);
+          const nextMouseActivePitches = incrementPitchCount(mouseActivePitchesRef.current, pitch);
+          mouseActivePitchesRef.current = nextMouseActivePitches;
+          setMouseActivePitches(nextMouseActivePitches);
           
           console.log(`Started playing piano key: ${keyId} (pitch ${pitch})`);
         }
@@ -70,8 +129,10 @@ const PianoKeys: React.FC<PianoKeysProps> = ({ activeRegion }) => {
 
   // Handle mouse up on piano key
   const handleKeyMouseUp = (keyId: string) => {
+    const pitch = noteNameToPitch(keyId);
+
     // Only release if key was actually pressed
-    if (!pressedKeysRef.current.has(keyId)) {
+    if ((mouseActivePitchesRef.current.get(pitch) ?? 0) === 0) {
       return;
     }
 
@@ -83,19 +144,14 @@ const PianoKeys: React.FC<PianoKeysProps> = ({ activeRegion }) => {
     const trackId = activeRegion.getTrackId();
     
     try {
-      // Convert note name to pitch (keyId is always a note name like "C4")
-      const pitch = noteNameToPitch(keyId);
-      
       // Get audio interface and stop playing the note
       const audioInterface = KGAudioInterface.instance();
       if (audioInterface.getIsInitialized() && audioInterface.getIsAudioContextStarted()) {
         audioInterface.releaseNote(trackId, pitch);
         
-        // Update pressed keys state
-        const newPressedKeys = new Set(pressedKeysRef.current);
-        newPressedKeys.delete(keyId);
-        pressedKeysRef.current = newPressedKeys;
-        setPressedKeys(newPressedKeys);
+        const nextMouseActivePitches = decrementPitchCount(mouseActivePitchesRef.current, pitch);
+        mouseActivePitchesRef.current = nextMouseActivePitches;
+        setMouseActivePitches(nextMouseActivePitches);
         
         console.log(`Stopped playing piano key: ${keyId} (pitch ${pitch})`);
       }
@@ -123,14 +179,25 @@ const PianoKeys: React.FC<PianoKeysProps> = ({ activeRegion }) => {
         const note = notes[i];
         const isSharp = note.includes('#');
         const keyId = `${note}${octave}`;
-        const isPressed = pressedKeys.has(keyId);
-        const keyClass = `piano-key ${isSharp ? 'sharp' : 'natural'} ${isPressed ? 'pressed' : ''}`;
+        const pitch = noteNameToPitch(keyId);
+        const isMouseActive = (mouseActivePitches.get(pitch) ?? 0) > 0;
+        const isMidiActive = (midiActivePitches.get(pitch) ?? 0) > 0;
+        const isPlaybackActive = playbackActivePitches.has(pitch);
+        const showIndicator = isMouseActive || isMidiActive;
+        const hasBackgroundFeedback = isMouseActive || isPlaybackActive;
+        const keyClass = [
+          'piano-key',
+          isSharp ? 'sharp' : 'natural',
+          isMouseActive ? 'mouse-active' : '',
+          isMidiActive ? 'midi-active' : '',
+          isPlaybackActive ? 'playback-active' : '',
+          hasBackgroundFeedback ? 'visual-active' : '',
+        ].filter(Boolean).join(' ');
         const isC = note === 'C';
         
         // For drum tracks, show drum labels when available
         let labelContent = null;
         if (isDrumTrack) {
-          const pitch = noteNameToPitch(keyId);
           const drumInfo = midiPercussionKeyMap[pitch];
           if (drumInfo) {
             labelContent = <span className="key-label">{drumInfo.shortName}</span>;
@@ -153,6 +220,7 @@ const PianoKeys: React.FC<PianoKeysProps> = ({ activeRegion }) => {
             }}
           >
             {labelContent}
+            {showIndicator ? <span className="piano-key-activity-dot" data-testid={`piano-key-dot-${keyId}`} /> : null}
           </div>
         );
       }
@@ -175,4 +243,4 @@ const PianoKeys: React.FC<PianoKeysProps> = ({ activeRegion }) => {
   );
 };
 
-export default PianoKeys; 
+export default PianoKeys; 

src/components/piano-roll/PianoRoll.css

@@ -572,6 +572,8 @@
   display: flex;
   align-items: center;
   border-bottom: 1px solid #3a3a3a;
+  position: relative;
+  transition: background-color 0.08s ease;
 }
 
 .piano-key.natural {
@@ -584,16 +586,47 @@
   color: #e0e0e0;
 }
 
+.piano-key.natural.visual-active {
+  background-color: #b8b8b8;
+}
+
+.piano-key.sharp.visual-active {
+  background-color: #5a5a5a;
+}
+
 .key-label {
   font-size: 10px;
   padding-left: 5px;
+  padding-right: 18px;
+  position: relative;
+  z-index: 1;
+}
+
+.piano-key-activity-dot {
+  position: absolute;
+  right: 6px;
+  top: 50%;
+  width: 8px;
+  height: 8px;
+  margin-top: -4px;
+  border-radius: 999px;
+  background-color: #000;
+  border: 1px solid #000;
+}
+
+.piano-key.sharp .piano-key-activity-dot {
+  background-color: #fff;
+  border-color: #fff;
 }
 
 .piano-grid {
   width: 100%;
   height: 100%;
   position: relative;
-  background-size: var(--region-grid-beat-width) var(--region-piano-key-height), 100% 100%;
+  background-size:
+    var(--region-grid-bar-width) var(--region-piano-key-height),
+    var(--region-grid-beat-width) var(--region-piano-key-height),
+    100% 100%;
   /* background-image is now set dynamically via React inline styles in PianoGrid component */
 }
 

src/components/piano-roll/PianoRoll.test.ts

@@ -33,6 +33,7 @@ vi.mock('../../stores/projectStore', () => ({
 
 import {
   createPendingModeSwitchRequest,
+  getRegionStartScrollLeft,
   getRegionPlayheadRelation,
   getScrollLeftForViewportRequest,
 } from './PianoRoll';
@@ -57,6 +58,13 @@ describe('PianoRoll viewport switch helpers', () => {
     expect(getRegionPlayheadRelation(25, 16, 24)).toBe('after');
   });
 
+  it('uses the zoomed beat width when scrolling to a different region in piano-roll view', () => {
+    document.documentElement.style.setProperty('--region-grid-beat-width', '80px');
+    document.documentElement.style.setProperty('--region-grid-bar-width', 'calc(var(--region-grid-beat-width) * var(--time-signature-numerator))');
+
+    expect(getRegionStartScrollLeft(16)).toBe(1280);
+  });
+
   it('centers an in-region playhead when switching to region-scope sheet view', () => {
     const request = createPendingModeSwitchRequest({
       playheadBeat: 20,

src/components/piano-roll/PianoRoll.tsx

@@ -63,6 +63,8 @@ interface PianoRollProps {
   initialPosition?: { x: number; y: number };
   initialSize?: { width: number; height: number };
   mode?: 'midi-edit' | 'spectrogram' | 'hybrid';
+  requestedSheetMusicViewEnabled?: boolean;
+  pianoRollViewRequestVersion?: number;
   audioRegion?: KGAudioRegion;
   trackId?: string;
   projectName?: string;
@@ -74,6 +76,8 @@ const PianoRoll: React.FC<PianoRollProps> = ({
   initialPosition,
   initialSize,
   mode = 'midi-edit',
+  requestedSheetMusicViewEnabled = false,
+  pianoRollViewRequestVersion = 0,
   audioRegion,
   trackId,
   projectName,
@@ -92,7 +96,7 @@ const PianoRoll: React.FC<PianoRollProps> = ({
     useState<SpectrogramHeightResolution>(3);
 
   // Piano roll zoom (1x–8x); updates --region-grid-beat-width CSS variable
-  const [pianoRollZoom, setPianoRollZoom] = useState<number>(1);
+  const [pianoRollZoom, setPianoRollZoom] = useState<number>(() => KGPianoRollState.instance().getPianoRollZoom());
   const [automationEnabled, setAutomationEnabled] = useState(false);
   const [automationType, setAutomationType] = useState<PianoRollAutomationType>('pitch-bend');
   const [sheetMusicViewEnabled, setSheetMusicViewEnabled] = useState(false);
@@ -153,6 +157,7 @@ const PianoRoll: React.FC<PianoRollProps> = ({
   const pendingModeSwitchRequestRef = useRef<PendingModeSwitchRequest | null>(null);
   const previousSheetMusicViewEnabledRef = useRef<boolean>(false);
   const previousActiveRegionIdRef = useRef<string | null>(null);
+  const lastAppliedViewRequestVersionRef = useRef<number>(0);
 
   // Ref for storing the setNoteUpdateCounter function
   const triggerNoteUpdateRef = useRef<React.Dispatch<React.SetStateAction<number>> | null>(null);
@@ -268,6 +273,42 @@ const PianoRoll: React.FC<PianoRollProps> = ({
     }
   }, []); // Empty dependency array means this runs once on mount
 
+  useEffect(() => {
+    if (isSpectrogram) {
+      return;
+    }
+
+    if (pianoRollViewRequestVersion === 0 || lastAppliedViewRequestVersionRef.current === pianoRollViewRequestVersion) {
+      return;
+    }
+
+    lastAppliedViewRequestVersionRef.current = pianoRollViewRequestVersion;
+
+    if (activeRegion) {
+      pendingModeSwitchRequestRef.current = createPendingModeSwitchRequest({
+        playheadBeat: playheadPosition,
+        regionStartBeat: activeRegion.getStartFromBeat(),
+        regionEndBeat: activeRegion.getStartFromBeat() + activeRegion.getLength(),
+        sourceSheetMusicViewEnabled: sheetMusicViewEnabled,
+        destinationSheetMusicViewEnabled: requestedSheetMusicViewEnabled,
+        destinationSheetMusicTrackScopeEnabled: requestedSheetMusicViewEnabled && sheetMusicTrackScopeEnabled,
+      });
+    } else {
+      pendingModeSwitchRequestRef.current = null;
+    }
+
+    setSheetMusicViewEnabled(requestedSheetMusicViewEnabled);
+    KGPianoRollState.instance().setSheetMusicViewEnabled(requestedSheetMusicViewEnabled);
+  }, [
+    activeRegion,
+    isSpectrogram,
+    pianoRollViewRequestVersion,
+    playheadPosition,
+    requestedSheetMusicViewEnabled,
+    sheetMusicTrackScopeEnabled,
+    sheetMusicViewEnabled,
+  ]);
+
   useEffect(() => {
     let unsubscribe: (() => void) | undefined;
 
@@ -744,6 +785,8 @@ const PianoRoll: React.FC<PianoRollProps> = ({
       }
     }
 
+    KGPianoRollState.instance().setPianoRollZoom(nextZoom);
+    KGCore.instance().getCurrentProject().setPianoRollZoom(nextZoom);
     setPianoRollZoom(nextZoom);
   }, [pianoRollZoom]);
 
@@ -986,7 +1029,7 @@ const PianoRoll: React.FC<PianoRollProps> = ({
     };
   }, [pianoRollZoom]);
 
-  // Scroll horizontally to the active region's starting bar
+  // Scroll horizontally to the active region's starting position
   useEffect(() => {
     if (!pianoRollNoteScrollRef.current || !activeRegion) {
       previousActiveRegionIdRef.current = activeRegion?.getId() ?? null;
@@ -1002,27 +1045,17 @@ const PianoRoll: React.FC<PianoRollProps> = ({
       // Get the starting beat of the region
       const startBeat = activeRegion.getStartFromBeat();
 
-      // Get the time signature to calculate beats per bar
-      const beatsPerBar = timeSignature.numerator;
-
-      // Calculate the bar number (0-indexed)
-      const barNumber = Math.floor(startBeat / beatsPerBar);
-
       if (DEBUG_MODE.PIANO_ROLL) {
-        console.log(`Scrolling to region's starting bar: ${barNumber + 1} (startBeat: ${startBeat}, beatsPerBar: ${beatsPerBar})`);
+        console.log(`Scrolling to region's starting position: startBeat=${startBeat}`);
       }
 
-      // Calculate the pixel position (each bar is --region-grid-bar-width wide, which is 160px by default)
-      const barWidth = parseInt(getComputedStyle(document.documentElement).getPropertyValue('--region-grid-bar-width')) || 160;
-
-      // Calculate the scroll position to scroll to the starting bar
-      const scrollPosition = barNumber * barWidth;
+      const scrollPosition = getRegionStartScrollLeft(startBeat);
 
       // Scroll to the calculated position
       pianoRollNoteScrollRef.current.scrollLeft = Math.max(0, scrollPosition);
       previousActiveRegionIdRef.current = activeRegion.getId();
     }
-  }, [activeRegion, timeSignature]);
+  }, [activeRegion]);
 
   useLayoutEffect(() => {
     const request = pendingModeSwitchRequestRef.current;
@@ -1548,3 +1581,11 @@ export function getScrollLeftForViewportRequest({
     container,
   });
 }
+
+export function getRegionStartScrollLeft(startBeat: number): number {
+  const beatWidth = parseInt(
+    getComputedStyle(document.documentElement).getPropertyValue('--region-grid-beat-width')
+  ) || TOOLBAR_CONSTANTS.BASE_BAR_WIDTH;
+
+  return Math.max(0, startBeat * beatWidth);
+}

src/components/piano-roll/PianoRollContent.tsx

@@ -239,8 +239,8 @@ const PianoRollContent: React.FC<PianoRollContentProps> = ({
       
       const noteId = note.getId();
       
-      // Check if this note is being resized or dragged and has a temporary style
-      if ((resizingNoteId === noteId || draggingNoteId === noteId) && tempNoteStyles[noteId]) {
+      // Use preview geometry whenever this note has an active temporary style.
+      if (tempNoteStyles[noteId]) {
         // Use the temporary style for position and size
         const tempStyle = tempNoteStyles[noteId];
         
@@ -351,7 +351,6 @@ const PianoRollContent: React.FC<PianoRollContentProps> = ({
                   keySignature={sheetKeySignature}
                   instrument={sheetInstrument}
                   quantization={sheetQuantization}
-                  noteScrollRef={noteScrollRef}
                   onMetricsChange={onSheetMeasureMetricsChange ?? NOOP_SHEET_METRICS_CHANGE}
                 />
               ) : (

src/components/piano-roll/PianoRollToolbar.test.tsx

@@ -130,10 +130,23 @@ describe('PianoRollToolbar', () => {
     expect(screen.getByRole('button', { name: 'Sheet Music View' })).toBeInTheDocument();
     expect(screen.getByRole('button', { name: /16,48/i })).toBeInTheDocument();
     expect(screen.getByRole('button', { name: 'Show Entire Track' })).toBeInTheDocument();
+    expect(screen.queryByRole('button', { name: '1x' })).not.toBeInTheDocument();
     expect(screen.queryByRole('button', { name: 'Pointer Tool' })).not.toBeInTheDocument();
     expect(screen.queryByRole('button', { name: /Pitch Bend/i })).not.toBeInTheDocument();
   });
 
+  it('shows the zoom button outside sheet mode', () => {
+    render(
+      <PianoRollToolbar
+        {...baseProps}
+        sheetMusicViewEnabled={false}
+        mode="midi-edit"
+      />
+    );
+
+    expect(screen.getByRole('button', { name: '1x' })).toBeInTheDocument();
+  });
+
   it('toggles the full-track sheet scope button', () => {
     const onSheetMusicTrackScopeToggle = vi.fn();
 

src/components/piano-roll/PianoRollToolbar.tsx

@@ -256,28 +256,30 @@ const PianoRollToolbar: React.FC<PianoRollToolbarProps> = ({
           </div>
         )}
 
-        <div className="quant-dropdown-container" ref={zoomSliderRef}>
-          <button
-            className="quant-button"
-            onClick={() => setShowZoomSlider(!showZoomSlider)}
-            title="Zoom"
-          >
-            {zoom}x
-          </button>
-          {showZoomSlider && (
-            <div className="piano-roll-zoom-popup">
-              <input
-                type="range"
-                min="1"
-                max="8"
-                step="1"
-                value={zoom}
-                onChange={(e) => onZoomChange(parseInt(e.target.value))}
-              />
-              <span className="piano-roll-zoom-value">{zoom}x</span>
-            </div>
-          )}
-        </div>
+        {!sheetMusicViewEnabled && (
+          <div className="quant-dropdown-container" ref={zoomSliderRef}>
+            <button
+              className="quant-button"
+              onClick={() => setShowZoomSlider(!showZoomSlider)}
+              title="Zoom"
+            >
+              {zoom}x
+            </button>
+            {showZoomSlider && (
+              <div className="piano-roll-zoom-popup">
+                <input
+                  type="range"
+                  min="1"
+                  max="8"
+                  step="1"
+                  value={zoom}
+                  onChange={(e) => onZoomChange(parseInt(e.target.value))}
+                />
+                <span className="piano-roll-zoom-value">{zoom}x</span>
+              </div>
+            )}
+          </div>
+        )}
       </div>
     </div>
   );

src/components/piano-roll/PianoRollZoomPersistence.test.tsx

@@ -0,0 +1,162 @@
+import React from 'react';
+import { describe, expect, it, beforeEach, vi } from 'vitest';
+import { render } from '@testing-library/react';
+import PianoRoll from './PianoRoll';
+import { createMockMidiRegion, createMockMidiTrack } from '../../test/utils/mock-data';
+
+const pianoRollState = {
+  zoom: 1,
+  getCurrentSnap: vi.fn(() => 'NO SNAP'),
+  getActiveTool: vi.fn(() => 'pointer'),
+  getAutomationViewEnabled: vi.fn(() => false),
+  getCurrentAutomationType: vi.fn(() => 'pitch-bend'),
+  getPianoRollZoom: vi.fn(() => pianoRollState.zoom),
+  setPianoRollZoom: vi.fn((zoom: number) => {
+    pianoRollState.zoom = zoom;
+  }),
+  getSheetMusicViewEnabled: vi.fn(() => false),
+  getSheetMusicTrackScopeEnabled: vi.fn(() => false),
+  getSheetQuantization: vi.fn(() => '16,48'),
+  setSheetMusicViewEnabled: vi.fn(),
+  setActiveTool: vi.fn(),
+  setAutomationViewEnabled: vi.fn(),
+  setCurrentAutomationType: vi.fn(),
+  setSheetQuantization: vi.fn(),
+  setSheetMusicTrackScopeEnabled: vi.fn(),
+  setCurrentSnap: vi.fn(),
+  setCurrentSuitableChords: vi.fn(),
+  setCurrentSuitableChordsPitchClasses: vi.fn(),
+};
+
+const mockProject = {
+  setPianoRollZoom: vi.fn(),
+};
+
+const region = createMockMidiRegion({ id: 'region-1', trackId: '1', trackIndex: 0 });
+const track = createMockMidiTrack({ id: 1, regions: [region] });
+
+const storeState = {
+  maxBars: 8,
+  tracks: [track],
+  updateTrack: vi.fn(),
+  timeSignature: { numerator: 4, denominator: 4 },
+  showChatBox: false,
+  showKGOnePanel: false,
+  showEventListPanel: false,
+  showInstrumentSelection: false,
+  keySignature: 'C major',
+  selectedMode: 'ionian',
+  setSelectedMode: vi.fn(),
+  playheadPosition: 0,
+  isPlaying: false,
+  autoScrollEnabled: false,
+  bpm: 120,
+  pianoRollScrollRequest: null,
+  selectedNoteIds: [],
+  automationRedrawVersion: 0,
+};
+
+let latestToolbarProps: { zoom: number; onZoomChange: (value: number) => void } | null = null;
+
+vi.mock('../../stores/projectStore', () => ({
+  useProjectStore: Object.assign(
+    (selector?: (state: typeof storeState) => unknown) => selector ? selector(storeState) : storeState,
+    {
+      getState: () => ({ setAutoScrollEnabled: vi.fn() }),
+      setState: vi.fn(),
+    }
+  ),
+}));
+
+vi.mock('../../core/state/KGPianoRollState', () => ({
+  KGPianoRollState: {
+    instance: () => pianoRollState,
+    SNAP_OPTIONS: ['NO SNAP'],
+    QUANT_POS_OPTIONS: ['1/8'],
+    QUANT_LEN_OPTIONS: ['1/8'],
+  },
+}));
+
+vi.mock('../../core/KGCore', () => ({
+  KGCore: {
+    FUNCTIONAL_CHORDS_DATA: { ionian: { name: 'Ionian' } },
+    instance: () => ({
+      getCurrentProject: () => mockProject,
+      getSelectedItems: () => [],
+      executeCommand: vi.fn(),
+    }),
+  },
+}));
+
+vi.mock('../../core/config/ConfigManager', () => ({
+  ConfigManager: {
+    instance: () => ({
+      getIsInitialized: () => true,
+      get: vi.fn(),
+      addChangeListener: () => () => undefined,
+    }),
+  },
+}));
+
+vi.mock('../../util/scaleUtil', () => ({
+  getSuitableChords: vi.fn(() => ({})),
+  noteNameToPitchClass: vi.fn(),
+}));
+
+vi.mock('../../util/dialogUtil', () => ({
+  showAlert: vi.fn(),
+  showPrompt: vi.fn(),
+}));
+
+vi.mock('./PianoRollHeader', () => ({ default: () => <div data-testid="header" /> }));
+vi.mock('./NoteAttributeBar', () => ({ default: () => <div data-testid="note-attribute-bar" /> }));
+vi.mock('./PianoRollContent', () => ({ default: () => <div data-testid="content" /> }));
+vi.mock('./PianoRollToolbar', () => ({
+  default: (props: { zoom: number; onZoomChange: (value: number) => void }) => {
+    latestToolbarProps = props;
+    return <div data-testid="toolbar">{props.zoom}x</div>;
+  },
+}));
+
+describe('PianoRoll zoom persistence', () => {
+  beforeEach(() => {
+    latestToolbarProps = null;
+    pianoRollState.zoom = 1;
+    mockProject.setPianoRollZoom.mockReset();
+    pianoRollState.getPianoRollZoom.mockClear();
+    pianoRollState.setPianoRollZoom.mockClear();
+    document.documentElement.style.setProperty('--region-piano-key-width', '60px');
+    document.documentElement.style.setProperty('--region-grid-beat-width', '40px');
+  });
+
+  it('restores the previous zoom after closing and reopening the piano roll', () => {
+    const firstRender = render(
+      <PianoRoll
+        onClose={vi.fn()}
+        regionId="region-1"
+        initialPosition={{ x: 0, y: 0 }}
+        initialSize={{ width: 800, height: 400 }}
+      />
+    );
+
+    expect(latestToolbarProps?.zoom).toBe(1);
+
+    latestToolbarProps?.onZoomChange(3);
+
+    expect(pianoRollState.setPianoRollZoom).toHaveBeenCalledWith(3);
+    expect(mockProject.setPianoRollZoom).toHaveBeenCalledWith(3);
+
+    firstRender.unmount();
+
+    render(
+      <PianoRoll
+        onClose={vi.fn()}
+        regionId="region-1"
+        initialPosition={{ x: 0, y: 0 }}
+        initialSize={{ width: 800, height: 400 }}
+      />
+    );
+
+    expect(latestToolbarProps?.zoom).toBe(3);
+  });
+});

src/components/piano-roll/SheetMusicView.test.tsx

@@ -0,0 +1,302 @@
+import React from 'react';
+import { fireEvent, render, screen } from '@testing-library/react';
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import SheetMusicView from './SheetMusicView';
+import { getSheetPlayheadPixel, parseSheetQuantization } from './sheetNotation';
+import type { SheetMeasureMetric } from './sheetNotationTypes';
+import { createMockMidiNote, createMockMidiRegion } from '../../test/utils/mock-data';
+
+const setPlayheadPosition = vi.fn();
+const requestMainContentScroll = vi.fn();
+
+vi.mock('../../stores/projectStore', () => ({
+  useProjectStore: (selector: (state: {
+    playheadPosition: number;
+    setPlayheadPosition: typeof setPlayheadPosition;
+    requestMainContentScroll: typeof requestMainContentScroll;
+  }) => unknown) => selector({
+    playheadPosition: 0,
+    setPlayheadPosition,
+    requestMainContentScroll,
+  }),
+}));
+
+vi.mock('../common', () => ({
+  Playhead: ({ pixelPositionOverride }: { pixelPositionOverride?: number }) => (
+    <div data-testid="playhead" data-pixel={pixelPositionOverride ?? 0} />
+  ),
+}));
+
+vi.mock('vexflow', () => {
+  class MockRenderer {
+    static Backends = { SVG: 'svg' };
+
+    private readonly host: HTMLElement;
+
+    constructor(host: HTMLElement) {
+      this.host = host;
+    }
+
+    resize() {}
+
+    getContext() {
+      const svg = document.createElementNS('http://www.w3.org/2000/svg', 'svg');
+      this.host.appendChild(svg);
+      return {};
+    }
+  }
+
+  class MockStave {
+    constructor(
+      _x: number,
+      _y: number,
+      _width: number
+    ) {}
+
+    setBegBarType() {
+      return this;
+    }
+
+    setEndBarType() {
+      return this;
+    }
+
+    addClef() {
+      return this;
+    }
+
+    addKeySignature() {
+      return this;
+    }
+
+    addTimeSignature() {
+      return this;
+    }
+
+    setContext() {
+      return this;
+    }
+
+    draw() {
+      return this;
+    }
+  }
+
+  class MockStaveNote {
+    constructor(_options: unknown) {}
+
+    isRest() {
+      return false;
+    }
+
+    getTieLeftX() {
+      return 0;
+    }
+
+    getTieRightX() {
+      return 0;
+    }
+
+    getYs() {
+      return [0];
+    }
+  }
+
+  class MockVoice {
+    constructor(_options: unknown) {}
+
+    setStrict() {
+      return this;
+    }
+
+    addTickables() {
+      return this;
+    }
+
+    draw() {
+      return this;
+    }
+  }
+
+  class MockFormatter {
+    joinVoices() {
+      return this;
+    }
+
+    formatToStave() {
+      return this;
+    }
+  }
+
+  class MockBeam {
+    static generateBeams() {
+      return [];
+    }
+
+    setContext() {
+      return this;
+    }
+
+    draw() {
+      return this;
+    }
+  }
+
+  return {
+    Accidental: { applyAccidentals: vi.fn() },
+    BarlineType: { SINGLE: 1, NONE: 0 },
+    Beam: MockBeam,
+    Dot: { buildAndAttach: vi.fn() },
+    Formatter: MockFormatter,
+    Renderer: MockRenderer,
+    Stave: MockStave,
+    StaveNote: MockStaveNote,
+    Voice: MockVoice,
+  };
+});
+
+describe('SheetMusicView', () => {
+  const quantization = parseSheetQuantization('16,48');
+  const onMetricsChange = vi.fn();
+
+  const getLatestMetrics = (): SheetMeasureMetric[] => {
+    const latestCall = onMetricsChange.mock.calls.at(-1);
+    expect(latestCall).toBeDefined();
+    return latestCall?.[0] as SheetMeasureMetric[];
+  };
+
+  beforeEach(() => {
+    setPlayheadPosition.mockClear();
+    requestMainContentScroll.mockClear();
+    onMetricsChange.mockClear();
+  });
+
+  it('maps header clicks in region scope without adding scroll offset', () => {
+    const activeRegion = createMockMidiRegion({
+      startFromBeat: 16,
+      length: 8,
+      notes: [],
+    });
+
+    render(
+      <SheetMusicView
+        activeRegion={activeRegion}
+        midiRegions={[activeRegion]}
+        maxBars={8}
+        sheetMusicTrackScopeEnabled={false}
+        timeSignature={{ numerator: 4, denominator: 4 }}
+        keySignature="C major"
+        instrument="acoustic_grand_piano"
+        quantization={quantization}
+        onMetricsChange={onMetricsChange}
+      />
+    );
+
+    const header = document.querySelector('.sheet-music-header') as HTMLDivElement;
+    expect(header).not.toBeNull();
+    const metrics = getLatestMetrics();
+    const expectedLocalBeat = 6;
+    const headerPixel = getSheetPlayheadPixel(expectedLocalBeat, metrics);
+
+    Object.defineProperty(header, 'getBoundingClientRect', {
+      value: () => ({
+        left: 100,
+        top: 0,
+        right: 540,
+        bottom: 20,
+        width: 440,
+        height: 20,
+        x: 100,
+        y: 0,
+        toJSON: () => ({}),
+      }),
+    });
+
+    fireEvent.click(header, { clientX: 100 + headerPixel });
+
+    expect(setPlayheadPosition).toHaveBeenCalledTimes(1);
+    expect(requestMainContentScroll).toHaveBeenCalledTimes(1);
+    expect(setPlayheadPosition).toHaveBeenCalledWith(22);
+    expect(requestMainContentScroll).toHaveBeenCalledWith(22);
+  });
+
+  it('maps header clicks in track scope to absolute beats', () => {
+    const activeRegion = createMockMidiRegion({
+      startFromBeat: 16,
+      length: 8,
+      notes: [
+        createMockMidiNote({ id: 'note-1', startBeat: 0, endBeat: 1, pitch: 60 }),
+      ],
+    });
+
+    const anotherRegion = createMockMidiRegion({
+      id: 'region-2',
+      startFromBeat: 24,
+      length: 4,
+      notes: [
+        createMockMidiNote({ id: 'note-2', startBeat: 0, endBeat: 1, pitch: 67 }),
+      ],
+    });
+
+    render(
+      <SheetMusicView
+        activeRegion={activeRegion}
+        midiRegions={[activeRegion, anotherRegion]}
+        maxBars={8}
+        sheetMusicTrackScopeEnabled={true}
+        timeSignature={{ numerator: 4, denominator: 4 }}
+        keySignature="C major"
+        instrument="acoustic_grand_piano"
+        quantization={quantization}
+        onMetricsChange={onMetricsChange}
+      />
+    );
+
+    const header = document.querySelector('.sheet-music-header') as HTMLDivElement;
+    expect(header).not.toBeNull();
+    const metrics = getLatestMetrics();
+    const expectedBeat = 3;
+    const headerPixel = getSheetPlayheadPixel(expectedBeat, metrics);
+
+    Object.defineProperty(header, 'getBoundingClientRect', {
+      value: () => ({
+        left: 100,
+        top: 0,
+        right: 300,
+        bottom: 20,
+        width: 200,
+        height: 20,
+        x: 100,
+        y: 0,
+        toJSON: () => ({}),
+      }),
+    });
+
+    fireEvent.click(header, { clientX: 100 + headerPixel });
+
+    expect(setPlayheadPosition).toHaveBeenCalledTimes(1);
+    expect(requestMainContentScroll).toHaveBeenCalledTimes(1);
+    expect(setPlayheadPosition).toHaveBeenCalledWith(3);
+    expect(requestMainContentScroll).toHaveBeenCalledWith(3);
+  });
+
+  it('renders the playhead container', () => {
+    const activeRegion = createMockMidiRegion();
+
+    render(
+      <SheetMusicView
+        activeRegion={activeRegion}
+        midiRegions={[activeRegion]}
+        maxBars={4}
+        sheetMusicTrackScopeEnabled={false}
+        timeSignature={{ numerator: 4, denominator: 4 }}
+        keySignature="C major"
+        instrument="acoustic_grand_piano"
+        quantization={quantization}
+        onMetricsChange={onMetricsChange}
+      />
+    );
+
+    expect(screen.getByTestId('playhead')).toBeInTheDocument();
+  });
+});

src/components/piano-roll/SheetMusicView.tsx

@@ -26,7 +26,6 @@ interface SheetMusicViewProps {
   keySignature: KeySignature;
   instrument: InstrumentType;
   quantization: SheetQuantization;
-  noteScrollRef: React.MutableRefObject<HTMLDivElement | null>;
   onMetricsChange: (metrics: SheetMeasureMetric[]) => void;
 }
 
@@ -61,7 +60,6 @@ const SheetMusicView: React.FC<SheetMusicViewProps> = ({
   keySignature,
   instrument,
   quantization,
-  noteScrollRef,
   onMetricsChange,
 }) => {
   const setPlayheadPosition = useProjectStore(state => state.setPlayheadPosition);
@@ -245,7 +243,7 @@ const SheetMusicView: React.FC<SheetMusicViewProps> = ({
     }
 
     const rect = headerRef.current.getBoundingClientRect();
-    const relativeX = event.clientX - rect.left + (noteScrollRef.current?.scrollLeft ?? 0);
+    const relativeX = event.clientX - rect.left;
     const metric = metrics.find(candidate => (
       relativeX >= candidate.leftPx && relativeX <= candidate.leftPx + candidate.widthPx
     ));
@@ -365,7 +363,6 @@ const arePropsEqual = (previous: SheetMusicViewProps, next: SheetMusicViewProps)
     previous.quantization.raw === next.quantization.raw &&
     previous.timeSignature.numerator === next.timeSignature.numerator &&
     previous.timeSignature.denominator === next.timeSignature.denominator &&
-    previous.noteScrollRef === next.noteScrollRef &&
     previous.onMetricsChange === next.onMetricsChange
   );
 };

src/components/settings/sections/BehaviorSettings.tsx

@@ -14,6 +14,7 @@ const BehaviorSettings: React.FC = () => {
   const [midiAutomationInterpolationIntervalMs, setMidiAutomationInterpolationIntervalMs] = useState<number>(10);
   const [playbackDelay, setPlaybackDelay] = useState<string>('200');
   const [recordingOffset, setRecordingOffset] = useState<string>('0');
+  const [bounceStartsFromBeat1, setBounceStartsFromBeat1] = useState<boolean>(true);
   const [enableAudioCapture, setEnableAudioCapture] = useState<boolean>(false);
   const [lookaheadValidationErrors, setLookaheadValidationErrors] = useState<string[]>([]);
   const [playbackDelayValidationErrors, setPlaybackDelayValidationErrors] = useState<string[]>([]);
@@ -42,6 +43,7 @@ const BehaviorSettings: React.FC = () => {
       setPlaybackDelay(((playbackDelaySeconds * 1000).toFixed(0)));
       const recordingOffsetSeconds = (configManager.get('audio.recording_offset') as number) ?? 0;
       setRecordingOffset(((recordingOffsetSeconds * 1000).toFixed(0)));
+      setBounceStartsFromBeat1((configManager.get('audio.bounce_starts_from_beat_1') as boolean) ?? true);
       setEnableAudioCapture((configManager.get('audio.enable_audio_capture_for_screen_sharing') as boolean) ?? false);
     };
 
@@ -158,6 +160,12 @@ const BehaviorSettings: React.FC = () => {
     await configManager.set('audio.enable_audio_capture_for_screen_sharing', boolValue);
   };
 
+  const handleBounceStartsFromBeat1Change = async (value: string) => {
+    const boolValue = value === 'yes';
+    setBounceStartsFromBeat1(boolValue);
+    await configManager.set('audio.bounce_starts_from_beat_1', boolValue);
+  };
+
   return (
     <div className="settings-section">
       <div className="settings-section-header">
@@ -325,6 +333,23 @@ const BehaviorSettings: React.FC = () => {
             </div>
           </div>
 
+          <div className="settings-item">
+            <label className="settings-label">
+              Bounce Starts From Beat 1
+            </label>
+            <select
+              className="settings-select"
+              value={bounceStartsFromBeat1 ? 'yes' : 'no'}
+              onChange={(e) => handleBounceStartsFromBeat1Change(e.target.value)}
+            >
+              <option value="no">No</option>
+              <option value="yes">Yes</option>
+            </select>
+            <div className="settings-help" style={{ fontSize: '12px', color: '#888', marginTop: '4px' }}>
+              Yes includes leading silence from the start of the song up to the first rendered region when bouncing WAV/MP3. No trims that leading silence and starts bounce at the first rendered note or audio region.
+            </div>
+          </div>
+
           <div className="settings-item">
             <label className="settings-label">
               Capture Audio for Screen Sharing

src/components/settings/sections/GeneralSettings.test.tsx

@@ -3,6 +3,13 @@ import { beforeEach, describe, expect, it, vi } from 'vitest';
 import { fireEvent, render, screen, waitFor } from '@testing-library/react';
 import GeneralSettings from './GeneralSettings';
 
+const { localSeparatorModelCacheMock } = vi.hoisted(() => ({
+  localSeparatorModelCacheMock: {
+    delete: vi.fn().mockResolvedValue(undefined),
+    exists: vi.fn().mockResolvedValue(true),
+  },
+}));
+
 const configState = new Map<string, unknown>([
   ['general.llm_provider', 'local_browser'],
   ['general.persist_api_keys_non_localhost', false],
@@ -20,6 +27,8 @@ const configState = new Map<string, unknown>([
   ['general.openai_compatible.base_url', ''],
   ['general.openai_compatible.model', ''],
   ['general.local_browser.context_length', 65536],
+  ['general.local_browser.model_url', 'https://huggingface.co/notabilia/gemma-4-E4B-it-litert-lm/resolve/main/gemma-4-E4B-it-web.task'],
+  ['general.uvr5_web_runtime.mdx_net_model_url', 'https://huggingface.co/notabilia/uvr5-models/resolve/main/UVR-MDX-NET-Inst_HQ_3.onnx'],
   ['general.soundfont.base_url', 'https://cdn.jsdelivr.net/npm/soundfont-for-samplers/FluidR3_GM/'],
   ['general.kgone.enabled', false],
   ['general.kgone.base_url', 'http://127.0.0.1:8000'],
@@ -71,6 +80,10 @@ vi.mock('../../../util/localLLMModelManager', () => ({
   },
 }));
 
+vi.mock('../../../util/localSeparatorModelCache', () => ({
+  LocalSeparatorModelCache: localSeparatorModelCacheMock,
+}));
+
 describe('GeneralSettings', () => {
   beforeEach(() => {
     configState.set('general.local_browser.context_length', 65536);
@@ -91,6 +104,9 @@ describe('GeneralSettings', () => {
       secureContext: true,
       reason: null,
     };
+    localSeparatorModelCacheMock.delete.mockClear();
+    localSeparatorModelCacheMock.exists.mockClear();
+    localSeparatorModelCacheMock.exists.mockResolvedValue(true);
   });
 
   it('renders the local context length selector and VRAM hint', async () => {
@@ -119,6 +135,65 @@ describe('GeneralSettings', () => {
     });
   });
 
+  it('renders and persists local runtime download URLs', async () => {
+    render(<GeneralSettings />);
+
+    expect(await screen.findByDisplayValue('https://huggingface.co/notabilia/gemma-4-E4B-it-litert-lm/resolve/main/gemma-4-E4B-it-web.task')).toBeTruthy();
+    expect(screen.getByDisplayValue('https://huggingface.co/notabilia/uvr5-models/resolve/main/UVR-MDX-NET-Inst_HQ_3.onnx')).toBeTruthy();
+
+    const inputs = screen.getAllByRole('textbox');
+    const gemmaUrlInput = inputs.find(input =>
+      (input as HTMLInputElement).value.includes('gemma-4-E4B-it-web.task'),
+    ) as HTMLInputElement | undefined;
+    const uvr5UrlInput = inputs.find(input =>
+      (input as HTMLInputElement).value.includes('UVR-MDX-NET-Inst_HQ_3.onnx'),
+    ) as HTMLInputElement | undefined;
+
+    expect(gemmaUrlInput).toBeTruthy();
+    expect(uvr5UrlInput).toBeTruthy();
+
+    fireEvent.change(gemmaUrlInput!, { target: { value: 'https://example.com/gemma.task' } });
+    fireEvent.change(uvr5UrlInput!, { target: { value: 'https://example.com/uvr5.onnx' } });
+
+    await waitFor(() => {
+      expect(configManagerMock.set).toHaveBeenCalledWith('general.local_browser.model_url', 'https://example.com/gemma.task');
+      expect(configManagerMock.set).toHaveBeenCalledWith('general.uvr5_web_runtime.mdx_net_model_url', 'https://example.com/uvr5.onnx');
+    });
+  });
+
+  it('restores default download URLs and deletes the UVR5 model cache', async () => {
+    localSeparatorModelCacheMock.exists
+      .mockResolvedValueOnce(true)
+      .mockResolvedValueOnce(false);
+
+    render(<GeneralSettings />);
+
+    expect(await screen.findByText('UVR5 Web Runtime')).toBeTruthy();
+
+    const restoreLinks = screen.getAllByText('Restore default');
+    fireEvent.click(restoreLinks[0]);
+    fireEvent.click(restoreLinks[1]);
+    const uvr5DeleteButton = screen.getAllByRole('button', { name: 'Delete Cached Model' })[1];
+    expect(uvr5DeleteButton).not.toBeDisabled();
+    fireEvent.click(uvr5DeleteButton);
+
+    await waitFor(() => {
+      expect(configManagerMock.set).toHaveBeenCalledWith(
+        'general.local_browser.model_url',
+        'https://huggingface.co/notabilia/gemma-4-E4B-it-litert-lm/resolve/main/gemma-4-E4B-it-web.task',
+      );
+      expect(configManagerMock.set).toHaveBeenCalledWith(
+        'general.uvr5_web_runtime.mdx_net_model_url',
+        'https://huggingface.co/notabilia/uvr5-models/resolve/main/UVR-MDX-NET-Inst_HQ_3.onnx',
+      );
+      expect(localSeparatorModelCacheMock.delete).toHaveBeenCalled();
+    });
+
+    await waitFor(() => {
+      expect(screen.getAllByRole('button', { name: 'Delete Cached Model' })[1]).toBeDisabled();
+    });
+  });
+
   it('keeps local runtime available when runtime may fail on this host', async () => {
     localModelState.runtimeSupport = {
       supported: true,

src/components/settings/sections/GeneralSettings.tsx

@@ -1,15 +1,18 @@
 import React, { useState, useEffect, useCallback, useMemo } from 'react';
 import { ConfigManager } from '../../../core/config/ConfigManager';
 import { LocalLLMModelManager, type LocalLLMModelState } from '../../../util/localLLMModelManager';
+import { LocalSeparatorModelCache } from '../../../util/localSeparatorModelCache';
 import {
   formatLocalLLMContextLength,
   LOCAL_LLM_CONTEXT_LENGTH_OPTIONS,
+  LOCAL_LLM_DEFAULT_MODEL_URL,
   LOCAL_LLM_DEFAULT_CONTEXT_LENGTH,
   LOCAL_LLM_DISPLAY_NAME,
   LOCAL_LLM_PROVIDER_KEY,
   normalizeLocalLLMContextLength,
   type LocalLLMContextLength,
 } from '../../../util/localLLMConfig';
+import { LOCAL_SEPARATOR_DEFAULT_MODEL_URL } from '../../../util/localSeparatorConfig';
 
 const GeneralSettings: React.FC = () => {
   const [llmProvider, setLlmProvider] = useState<string>(LOCAL_LLM_PROVIDER_KEY);
@@ -34,6 +37,11 @@ const GeneralSettings: React.FC = () => {
   const [soundfontServerManaged, setSoundfontServerManaged] = useState<boolean>(false);
   const [localContextLength, setLocalContextLength] = useState<LocalLLMContextLength>(LOCAL_LLM_DEFAULT_CONTEXT_LENGTH);
   const [localModelState, setLocalModelState] = useState<LocalLLMModelState>(LocalLLMModelManager.getState());
+  const [localModelUrl, setLocalModelUrl] = useState<string>('');
+  const [uvr5ModelUrl, setUvr5ModelUrl] = useState<string>('');
+  const [isUvr5ModelCached, setIsUvr5ModelCached] = useState<boolean>(false);
+  const [isCheckingUvr5ModelCache, setIsCheckingUvr5ModelCache] = useState<boolean>(false);
+  const [isDeletingUvr5Model, setIsDeletingUvr5Model] = useState<boolean>(false);
 
   const configManager = ConfigManager.instance();
 
@@ -51,6 +59,18 @@ const GeneralSettings: React.FC = () => {
     }
   }, []);
 
+  const refreshUvr5ModelCacheState = useCallback(async () => {
+    setIsCheckingUvr5ModelCache(true);
+    try {
+      setIsUvr5ModelCached(await LocalSeparatorModelCache.exists());
+    } catch (error) {
+      console.error('Failed to check UVR5 cached model state:', error);
+      setIsUvr5ModelCached(false);
+    } finally {
+      setIsCheckingUvr5ModelCache(false);
+    }
+  }, []);
+
   // Load configuration values on component mount
   useEffect(() => {
     const loadConfig = async () => {
@@ -74,6 +94,8 @@ const GeneralSettings: React.FC = () => {
       setCompatibleBaseUrl((configManager.get('general.openai_compatible.base_url') as string) || '');
       setCompatibleModel((configManager.get('general.openai_compatible.model') as string) || '');
       setLocalContextLength(normalizeLocalLLMContextLength(configManager.get('general.local_browser.context_length')));
+      setLocalModelUrl((configManager.get('general.local_browser.model_url') as string) || LOCAL_LLM_DEFAULT_MODEL_URL);
+      setUvr5ModelUrl((configManager.get('general.uvr5_web_runtime.mdx_net_model_url') as string) || LOCAL_SEPARATOR_DEFAULT_MODEL_URL);
       setSoundfontBaseUrl((configManager.get('general.soundfont.base_url') as string) || '');
       setKgoneEnabled((configManager.get('general.kgone.enabled') as boolean) ?? false);
       setKgoneBaseUrl((configManager.get('general.kgone.base_url') as string) || '');
@@ -83,8 +105,9 @@ const GeneralSettings: React.FC = () => {
 
     loadConfig();
     const unsubscribe = LocalLLMModelManager.subscribe(setLocalModelState);
+    void refreshUvr5ModelCacheState();
     return unsubscribe;
-  }, [configManager]);
+  }, [configManager, refreshUvr5ModelCacheState]);
 
   // Debounced save function for text inputs
   const debouncedSave = useCallback((key: string, value: string) => {
@@ -212,6 +235,16 @@ const GeneralSettings: React.FC = () => {
     debouncedSave('general.kgone.base_url', value);
   };
 
+  const handleLocalModelUrlChange = (value: string) => {
+    setLocalModelUrl(value);
+    debouncedSave('general.local_browser.model_url', value);
+  };
+
+  const handleUvr5ModelUrlChange = (value: string) => {
+    setUvr5ModelUrl(value);
+    debouncedSave('general.uvr5_web_runtime.mdx_net_model_url', value);
+  };
+
   const handleDeleteLocalModel = async () => {
     try {
       await LocalLLMModelManager.deleteCachedModel();
@@ -220,6 +253,19 @@ const GeneralSettings: React.FC = () => {
     }
   };
 
+  const handleDeleteUvr5Model = async () => {
+    setIsDeletingUvr5Model(true);
+    try {
+      await LocalSeparatorModelCache.delete();
+      setIsUvr5ModelCached(false);
+    } catch (error) {
+      console.error('Failed to delete UVR5 cached model:', error);
+    } finally {
+      setIsDeletingUvr5Model(false);
+      await refreshUvr5ModelCacheState();
+    }
+  };
+
   const handleLocalContextLengthChange = async (value: string) => {
     const parsed = Number(value);
     const normalized = normalizeLocalLLMContextLength(parsed);
@@ -325,6 +371,32 @@ const GeneralSettings: React.FC = () => {
             </div>
           </div>
 
+          <div className="settings-item">
+            <label className="settings-label">
+              Download URL
+            </label>
+            <input
+              type="text"
+              className="settings-input"
+              placeholder={`e.g. ${LOCAL_LLM_DEFAULT_MODEL_URL}`}
+              value={localModelUrl}
+              onChange={(e) => handleLocalModelUrlChange(e.target.value)}
+            />
+            <div className="settings-help" style={{ fontSize: '12px', color: '#888', marginTop: '4px' }}>
+              Changing this URL may break downloads or point to an incompatible model file.{' '}
+              <a
+                href="#"
+                onClick={(e) => {
+                  e.preventDefault();
+                  handleLocalModelUrlChange(LOCAL_LLM_DEFAULT_MODEL_URL);
+                }}
+                style={{ color: '#5a9fd4', textDecoration: 'underline', cursor: 'pointer' }}
+              >
+                Restore default
+              </a>
+            </div>
+          </div>
+
           {!localModelState.isCached && !localModelState.isDownloading && localModelState.runtimeSupport.supported && (
             <div className="settings-help" style={{ fontSize: '12px', color: '#888', marginTop: '4px', marginBottom: '8px' }}>
               The local model downloads automatically the next time you chat with `Local LLM (Browser)`.
@@ -366,6 +438,47 @@ const GeneralSettings: React.FC = () => {
           </div>
         </div>
 
+        <div className="settings-group">
+          <h4>UVR5 Web Runtime</h4>
+
+          <div className="settings-item">
+            <label className="settings-label">
+              UVR-MDX-NET-Inst_HQ_3 Download URL
+            </label>
+            <input
+              type="text"
+              className="settings-input"
+              placeholder={`e.g. ${LOCAL_SEPARATOR_DEFAULT_MODEL_URL}`}
+              value={uvr5ModelUrl}
+              onChange={(e) => handleUvr5ModelUrlChange(e.target.value)}
+            />
+            <div className="settings-help" style={{ fontSize: '12px', color: '#888', marginTop: '4px' }}>
+              Changing this URL may break downloads or point to an incompatible model file.{' '}
+              <a
+                href="#"
+                onClick={(e) => {
+                  e.preventDefault();
+                  handleUvr5ModelUrlChange(LOCAL_SEPARATOR_DEFAULT_MODEL_URL);
+                }}
+                style={{ color: '#5a9fd4', textDecoration: 'underline', cursor: 'pointer' }}
+              >
+                Restore default
+              </a>
+            </div>
+          </div>
+
+          <div className="settings-item" style={{ marginTop: '12px' }}>
+            <button
+              type="button"
+              className="settings-btn settings-btn-danger"
+              onClick={() => void handleDeleteUvr5Model()}
+              disabled={isCheckingUvr5ModelCache || isDeletingUvr5Model || !isUvr5ModelCached}
+            >
+              {isDeletingUvr5Model ? 'Deleting...' : 'Delete Cached Model'}
+            </button>
+          </div>
+        </div>
+
         <div className="settings-group">
           <h4>OpenAI</h4>
 

src/components/track/Region.css

@@ -73,12 +73,26 @@
   background-color: #87CEFA; /* Light blue */
   width: 100%;
   position: relative; /* Allow overlayed controls */
+  overflow: hidden;
 }
 
 .region-content.audio-region-content {
   background-color: #90EE90; /* Light green for audio regions */
 }
 
+.region-preview-content {
+  position: absolute;
+  inset: 0 auto 0 0;
+  width: 100%;
+  height: 100%;
+}
+
+.region-preview-content canvas {
+  display: block;
+  width: 100%;
+  height: 100%;
+}
+
 /* Audio region overrides */
 .track-region.audio-region {
   background-color: #3a6b4a;

src/components/track/RegionItem.test.tsx

@@ -147,4 +147,35 @@ describe('RegionItem', () => {
     expect(context.stroke).toHaveBeenCalled();
     rectSpy.mockRestore();
   });
+
+  it('applies preview content clipping styles when provided', () => {
+    const { container } = renderRegion({
+      previewContentStyle: {
+        left: '-40px',
+        width: '120px',
+      },
+    });
+
+    const previewContent = container.querySelector('.region-preview-content');
+
+    expect(previewContent).toBeTruthy();
+    expect(previewContent).toHaveAttribute('data-preview-content-active', 'true');
+    expect(previewContent).toHaveStyle({
+      left: '-40px',
+      width: '120px',
+    });
+  });
+
+  it('uses the default preview content wrapper sizing for normal regions', () => {
+    const { container } = renderRegion();
+
+    const previewContent = container.querySelector('.region-preview-content');
+
+    expect(previewContent).toBeTruthy();
+    expect(previewContent).toHaveAttribute('data-preview-content-active', 'false');
+    expect(previewContent).not.toHaveStyle({
+      left: '-40px',
+      width: '120px',
+    });
+  });
 });

src/components/track/RegionItem.tsx

@@ -9,6 +9,7 @@ import { KGAudioRegion } from '../../core/region/KGAudioRegion';
 import { useProjectStore } from '../../stores/projectStore';
 import { KGMainContentState } from '../../core/state/KGMainContentState';
 import type { AudioRecordingPeak } from '../../core/audio-interface/KGAudioRecorder';
+import type { RegionPreviewContentStyle } from '../interfaces';
 
 const DRAG_START_THRESHOLD_PX = 4;
 
@@ -46,6 +47,7 @@ interface RegionItemProps {
   previewWaveformPeaks?: AudioRecordingPeak[];
   isPreview?: boolean;
   isAudioRegion?: boolean;
+  previewContentStyle?: RegionPreviewContentStyle;
 }
 
 const RegionItem: React.FC<RegionItemProps> = ({
@@ -73,6 +75,7 @@ const RegionItem: React.FC<RegionItemProps> = ({
   previewWaveformPeaks,
   isPreview = false,
   isAudioRegion = false,
+  previewContentStyle,
 }) => {
   // Get selection state and time signature from store
   const { selectedRegionIds, timeSignature, bpm } = useProjectStore();
@@ -98,24 +101,26 @@ const RegionItem: React.FC<RegionItemProps> = ({
 
   // Canvas ref for note visualization
   const canvasRef = useRef<HTMLCanvasElement | null>(null);
-  const regionContentRef = useRef<HTMLDivElement | null>(null);
+  const previewContentRef = useRef<HTMLDivElement | null>(null);
 
   // Function to render notes on canvas
   const renderNotesOnCanvas = () => {
-    if (!canvasRef.current || !regionContentRef.current || !midiRegion) return;
+    if (!canvasRef.current || !previewContentRef.current || !midiRegion) return;
 
     const canvas = canvasRef.current;
     const ctx = canvas.getContext('2d');
     if (!ctx) return;
 
     // Get the current dimensions of the region content
-    const contentRect = regionContentRef.current.getBoundingClientRect();
-    const width = contentRect.width;
-    const height = contentRect.height;
+    const contentRect = previewContentRef.current.getBoundingClientRect();
+    const width = Math.max(1, Math.round(contentRect.width));
+    const height = Math.max(1, Math.round(contentRect.height));
 
     // Set canvas size to match the region content
     canvas.width = width;
     canvas.height = height;
+    canvas.style.width = `${width}px`;
+    canvas.style.height = `${height}px`;
 
     // Clear the canvas
     ctx.clearRect(0, 0, width, height);
@@ -242,18 +247,20 @@ const RegionItem: React.FC<RegionItemProps> = ({
 
   // Function to render audio waveform on canvas
   const renderWaveformOnCanvas = () => {
-    if (!canvasRef.current || !regionContentRef.current || !audioBuffer) return;
+    if (!canvasRef.current || !previewContentRef.current || !audioBuffer) return;
 
     const canvas = canvasRef.current;
     const ctx = canvas.getContext('2d');
     if (!ctx) return;
 
-    const contentRect = regionContentRef.current.getBoundingClientRect();
-    const width = contentRect.width;
-    const height = contentRect.height;
+    const contentRect = previewContentRef.current.getBoundingClientRect();
+    const width = Math.max(1, Math.round(contentRect.width));
+    const height = Math.max(1, Math.round(contentRect.height));
 
     canvas.width = width;
     canvas.height = height;
+    canvas.style.width = `${width}px`;
+    canvas.style.height = `${height}px`;
 
     ctx.clearRect(0, 0, width, height);
 
@@ -311,18 +318,20 @@ const RegionItem: React.FC<RegionItemProps> = ({
   };
 
   const renderPreviewWaveformOnCanvas = () => {
-    if (!canvasRef.current || !regionContentRef.current || !previewWaveformPeaks || previewWaveformPeaks.length === 0) return;
+    if (!canvasRef.current || !previewContentRef.current || !previewWaveformPeaks || previewWaveformPeaks.length === 0) return;
 
     const canvas = canvasRef.current;
     const ctx = canvas.getContext('2d');
     if (!ctx) return;
 
-    const contentRect = regionContentRef.current.getBoundingClientRect();
-    const width = contentRect.width;
-    const height = contentRect.height;
+    const contentRect = previewContentRef.current.getBoundingClientRect();
+    const width = Math.max(1, Math.round(contentRect.width));
+    const height = Math.max(1, Math.round(contentRect.height));
 
     canvas.width = width;
     canvas.height = height;
+    canvas.style.width = `${width}px`;
+    canvas.style.height = `${height}px`;
     ctx.clearRect(0, 0, width, height);
 
     const centerY = height / 2;
@@ -374,11 +383,11 @@ const RegionItem: React.FC<RegionItemProps> = ({
     } else {
       renderNotesOnCanvas();
     }
-  }, [midiRegion, audioRegion, audioBuffer, previewWaveformPeaks, timeSignature, bpm, id, noteUpdateTrigger, barNumber, length]);
+  }, [midiRegion, audioRegion, audioBuffer, previewWaveformPeaks, timeSignature, bpm, id, noteUpdateTrigger, barNumber, length, previewContentStyle?.width]);
 
   // Re-render canvas when region content size changes
   useEffect(() => {
-    if (!regionContentRef.current) return;
+    if (!previewContentRef.current) return;
 
     const resizeObserver = new ResizeObserver(() => {
       if (previewWaveformPeaks && previewWaveformPeaks.length > 0) {
@@ -390,14 +399,14 @@ const RegionItem: React.FC<RegionItemProps> = ({
       }
     });
 
-    resizeObserver.observe(regionContentRef.current);
+    resizeObserver.observe(previewContentRef.current);
 
     return () => {
-      if (regionContentRef.current) {
-        resizeObserver.unobserve(regionContentRef.current);
+      if (previewContentRef.current) {
+        resizeObserver.unobserve(previewContentRef.current);
       }
     };
-  }, [midiRegion, audioRegion, audioBuffer, previewWaveformPeaks, timeSignature, bpm]);
+  }, [midiRegion, audioRegion, audioBuffer, previewWaveformPeaks, timeSignature, bpm, previewContentStyle?.width]);
 
   // Handle mouse movement to detect edge proximity
   const handleMouseMove = (e: React.MouseEvent<HTMLDivElement>) => {
@@ -668,7 +677,7 @@ const RegionItem: React.FC<RegionItemProps> = ({
       <div className="region-header">
         {name}
       </div>
-      <div className={`region-content${(audioRegion || isAudioRegion) ? ' audio-region-content' : ''}`} ref={regionContentRef}>
+      <div className={`region-content${(audioRegion || isAudioRegion) ? ' audio-region-content' : ''}`}>
         {!isPreview && <div className="region-left-buttons">
           {!audioRegion && (
             <button
@@ -771,7 +780,14 @@ const RegionItem: React.FC<RegionItemProps> = ({
             )}
           </div>
         </div>}
-        <canvas ref={canvasRef} />
+        <div
+          className="region-preview-content"
+          ref={previewContentRef}
+          style={previewContentStyle}
+          data-preview-content-active={previewContentStyle ? 'true' : 'false'}
+        >
+          <canvas ref={canvasRef} />
+        </div>
       </div>
     </div>
   );

src/components/track/TrackGridItem.test.tsx

@@ -1,41 +1,71 @@
-import React from 'react';
-import { beforeAll, describe, expect, it, vi } from 'vitest';
-import { render } from '@testing-library/react';
+import React, { useState } from 'react';
+import { act, render } from '@testing-library/react';
+import { beforeAll, beforeEach, describe, expect, it, vi } from 'vitest';
 import TrackGridItem from './TrackGridItem';
 import { KGAudioTrack } from '../../core/track/KGAudioTrack';
+import { KGMainContentState } from '../../core/state/KGMainContentState';
+import { createMockMidiRegion, createMockMidiTrack } from '../../test/utils/mock-data';
+import type { RegionPreviewContentStyle } from '../interfaces';
+
+const storeState = {
+  selectedRegionIds: [] as string[],
+  activeTrackAutomationTrackId: null as string | null,
+  activeTrackAutomationType: null,
+  trackAutomationRedrawVersion: 0,
+  recordingMode: 'audio' as 'audio' | 'midi' | null,
+  recordingTargetTrackIndex: 0,
+  recordingCommitStartBeatAbsolute: 4,
+  recordingAudioPreviewCurrentBeat: 8,
+  recordingAudioPreviewPeaks: [{ min: -0.5, max: 0.5 }],
+  recordingAudioPreviewFileName: 'Recording' as string | null,
+  timeSignature: { numerator: 4, denominator: 4 },
+};
+
+const regionItemProps = new Map<string, Record<string, unknown>>();
 
 vi.mock('../../stores/projectStore', () => ({
-  useProjectStore: (selector?: (state: {
-    selectedRegionIds: string[];
-    activeTrackAutomationTrackId: string | null;
-    activeTrackAutomationType: null;
-    trackAutomationRedrawVersion: number;
-    recordingMode: 'audio' | 'midi' | null;
-    recordingTargetTrackIndex: number | null;
-    recordingCommitStartBeatAbsolute: number;
-    recordingAudioPreviewCurrentBeat: number;
-    recordingAudioPreviewPeaks: Array<{ min: number; max: number }>;
-    recordingAudioPreviewFileName: string | null;
-    timeSignature: { numerator: number; denominator: number };
-  }) => unknown) => {
-    const state = {
-      selectedRegionIds: [],
-      activeTrackAutomationTrackId: null,
-      activeTrackAutomationType: null,
-      trackAutomationRedrawVersion: 0,
-      recordingMode: 'audio' as const,
-      recordingTargetTrackIndex: 0,
-      recordingCommitStartBeatAbsolute: 4,
-      recordingAudioPreviewCurrentBeat: 8,
-      recordingAudioPreviewPeaks: [{ min: -0.5, max: 0.5 }],
-      recordingAudioPreviewFileName: 'Recording',
-      timeSignature: { numerator: 4, denominator: 4 },
-    };
-    return selector ? selector(state) : state;
+  useProjectStore: (selector?: (state: typeof storeState) => unknown) => (
+    selector ? selector(storeState) : storeState
+  ),
+}));
+
+vi.mock('./RegionItem', () => ({
+  default: (props: Record<string, unknown>) => {
+    regionItemProps.set(props.id as string, props);
+    return (
+      <div
+        data-region-id={props.id as string}
+        data-preview-region={(props.isPreview as boolean | undefined) ? 'true' : 'false'}
+        style={props.style as React.CSSProperties}
+      />
+    );
   },
 }));
 
-describe('TrackGridItem recording preview', () => {
+vi.mock('./TrackAutomationLane', () => ({
+  default: () => null,
+}));
+
+vi.mock('../../core/audio-interface/KGAudioInterface', () => ({
+  KGAudioInterface: {
+    instance: () => ({
+      getAudioBuffer: () => undefined,
+    }),
+  },
+}));
+
+describe('TrackGridItem preview behavior', () => {
+  const getRegionItem = (regionId: string) => regionItemProps.get(regionId) as {
+    style: React.CSSProperties;
+    previewContentStyle?: RegionPreviewContentStyle;
+    onResizeStart?: (regionId: string, resizeAction: 'start' | 'end', initialX: number) => void;
+    onResize?: (regionId: string, resizeAction: 'start' | 'end', deltaX: number) => void;
+    onResizeEnd?: (regionId: string, resizeAction: 'start' | 'end') => void;
+    onDragStart?: (regionId: string, initialX: number, initialY: number) => void;
+    onDrag?: (regionId: string, deltaX: number, deltaY: number) => void;
+    onDragEnd?: (regionId: string) => void;
+  };
+
   beforeAll(() => {
     Object.defineProperty(HTMLCanvasElement.prototype, 'getContext', {
       value: vi.fn(() => ({
@@ -57,11 +87,102 @@ describe('TrackGridItem recording preview', () => {
     vi.stubGlobal('ResizeObserver', ResizeObserverMock);
   });
 
+  beforeEach(() => {
+    regionItemProps.clear();
+    storeState.selectedRegionIds = [];
+    storeState.activeTrackAutomationTrackId = null;
+    storeState.activeTrackAutomationType = null;
+    storeState.trackAutomationRedrawVersion = 0;
+    storeState.recordingMode = 'audio';
+    storeState.recordingTargetTrackIndex = 0;
+    storeState.recordingCommitStartBeatAbsolute = 4;
+    storeState.recordingAudioPreviewCurrentBeat = 8;
+    storeState.recordingAudioPreviewPeaks = [{ min: -0.5, max: 0.5 }];
+    storeState.recordingAudioPreviewFileName = 'Recording';
+    storeState.timeSignature = { numerator: 4, denominator: 4 };
+    KGMainContentState.instance().setActiveTool('pointer');
+    KGMainContentState.instance().setSnapping(true);
+  });
+
+  const createGridContainerRef = () => {
+    const gridElement = document.createElement('div');
+    Object.defineProperty(gridElement, 'clientWidth', { configurable: true, value: 800 });
+    Object.defineProperty(gridElement, 'clientHeight', { configurable: true, value: 240 });
+    return { current: gridElement };
+  };
+
+  const renderSharedPreviewHarness = (
+    selectedRegionIds: string[] = [],
+    regionOverrides: Array<{ id: string; trackId: string; trackIndex: number; barNumber: number; length: number; name: string }> = [
+      { id: 'region-a', trackId: '1', trackIndex: 0, barNumber: 1, length: 1, name: 'Region A' },
+      { id: 'region-b', trackId: '2', trackIndex: 1, barNumber: 3, length: 2, name: 'Region B' },
+    ],
+  ) => {
+    storeState.selectedRegionIds = selectedRegionIds;
+
+    const regionAData = regionOverrides.find(region => region.id === 'region-a')!;
+    const regionBData = regionOverrides.find(region => region.id === 'region-b')!;
+    const regionA = createMockMidiRegion({ id: 'region-a', trackId: '1', trackIndex: 0, startFromBeat: (regionAData.barNumber - 1) * 4, length: regionAData.length * 4 });
+    const regionB = createMockMidiRegion({ id: 'region-b', trackId: '2', trackIndex: 1, startFromBeat: (regionBData.barNumber - 1) * 4, length: regionBData.length * 4 });
+    const trackA = createMockMidiTrack({ id: 1, regions: [regionA] });
+    const trackB = createMockMidiTrack({ id: 2, regions: [regionB] });
+    trackA.setTrackIndex(0);
+    trackB.setTrackIndex(1);
+
+    const gridContainerRef = createGridContainerRef();
+    const baseProps = {
+      isDragging: false,
+      isDragOver: false,
+      regions: regionOverrides,
+      maxBars: 8,
+      selectedRegionId: null,
+      gridContainerRef,
+      onDoubleClick: vi.fn(),
+      onRegionResize: vi.fn(),
+      onRegionResizeEnd: vi.fn(),
+      onRegionDrag: vi.fn(),
+      onRegionDragEnd: vi.fn(),
+      allTracks: [trackA, trackB],
+    };
+
+    const SharedPreviewHarness = () => {
+      const [previewRegionStyles, setPreviewRegionStyles] = useState<Record<string, React.CSSProperties>>({});
+      const [previewRegionContentStyles, setPreviewRegionContentStyles] = useState<Record<string, RegionPreviewContentStyle>>({});
+
+      return (
+        <>
+          <TrackGridItem
+            track={trackA}
+            index={0}
+            previewRegionStyles={previewRegionStyles}
+            setPreviewRegionStyles={setPreviewRegionStyles}
+            previewRegionContentStyles={previewRegionContentStyles}
+            setPreviewRegionContentStyles={setPreviewRegionContentStyles}
+            {...baseProps}
+          />
+          <TrackGridItem
+            track={trackB}
+            index={1}
+            previewRegionStyles={previewRegionStyles}
+            setPreviewRegionStyles={setPreviewRegionStyles}
+            previewRegionContentStyles={previewRegionContentStyles}
+            setPreviewRegionContentStyles={setPreviewRegionContentStyles}
+            {...baseProps}
+          />
+        </>
+      );
+    };
+
+    render(<SharedPreviewHarness />);
+
+    return baseProps;
+  };
+
   it('renders a non-interactive preview region on the recording audio track', () => {
     const track = new KGAudioTrack('Audio Track', 1);
     track.setTrackIndex(0);
 
-    const view = render(
+    render(
       <TrackGridItem
         track={track}
         index={0}
@@ -70,12 +191,253 @@ describe('TrackGridItem recording preview', () => {
         regions={[]}
         maxBars={8}
         selectedRegionId={null}
-        gridContainerRef={{ current: document.createElement('div') }}
+        gridContainerRef={createGridContainerRef()}
         onDoubleClick={vi.fn()}
       />
     );
 
-    const previewRegion = view.container.querySelector('[data-preview-region="true"]');
-    expect(previewRegion).toBeTruthy();
+    expect(regionItemProps.get('audio-recording-preview')).toBeTruthy();
+  });
+
+  it('previews end resize for all selected regions across track rows', () => {
+    renderSharedPreviewHarness(['region-a', 'region-b']);
+
+    act(() => {
+      getRegionItem('region-a').onResizeStart?.('region-a', 'end', 0);
+      getRegionItem('region-a').onResize?.('region-a', 'end', 40);
+    });
+
+    expect(getRegionItem('region-a').style).toEqual({
+      left: '0px',
+      width: '140px',
+      position: 'absolute',
+    });
+    expect(getRegionItem('region-a').previewContentStyle).toEqual({
+      left: '0px',
+      width: '100px',
+    });
+    expect(getRegionItem('region-b').style).toEqual({
+      left: '200px',
+      width: '240px',
+      position: 'absolute',
+    });
+    expect(getRegionItem('region-b').previewContentStyle).toEqual({
+      left: '0px',
+      width: '200px',
+    });
+  });
+
+  it('previews start resize for all selected regions across track rows', () => {
+    renderSharedPreviewHarness(
+      ['region-a', 'region-b'],
+      [
+        { id: 'region-a', trackId: '1', trackIndex: 0, barNumber: 2, length: 2, name: 'Region A' },
+        { id: 'region-b', trackId: '2', trackIndex: 1, barNumber: 4, length: 3, name: 'Region B' },
+      ],
+    );
+
+    act(() => {
+      getRegionItem('region-a').onResizeStart?.('region-a', 'start', 0);
+      getRegionItem('region-a').onResize?.('region-a', 'start', 40);
+    });
+
+    expect(getRegionItem('region-a').style).toEqual({
+      left: '140px',
+      width: '160px',
+      position: 'absolute',
+    });
+    expect(getRegionItem('region-a').previewContentStyle).toEqual({
+      left: '-40px',
+      width: '200px',
+    });
+    expect(getRegionItem('region-b').style).toEqual({
+      left: '340px',
+      width: '260px',
+      position: 'absolute',
+    });
+    expect(getRegionItem('region-b').previewContentStyle).toEqual({
+      left: '-40px',
+      width: '300px',
+    });
+  });
+
+  it('keeps preview content fixed while shrinking from the end', () => {
+    renderSharedPreviewHarness(
+      ['region-a', 'region-b'],
+      [
+        { id: 'region-a', trackId: '1', trackIndex: 0, barNumber: 1, length: 2, name: 'Region A' },
+        { id: 'region-b', trackId: '2', trackIndex: 1, barNumber: 3, length: 3, name: 'Region B' },
+      ],
+    );
+
+    act(() => {
+      getRegionItem('region-a').onResizeStart?.('region-a', 'end', 0);
+      getRegionItem('region-a').onResize?.('region-a', 'end', -40);
+    });
+
+    expect(getRegionItem('region-a').style).toEqual({
+      left: '0px',
+      width: '160px',
+      position: 'absolute',
+    });
+    expect(getRegionItem('region-a').previewContentStyle).toEqual({
+      left: '0px',
+      width: '200px',
+    });
+    expect(getRegionItem('region-b').style).toEqual({
+      left: '200px',
+      width: '260px',
+      position: 'absolute',
+    });
+    expect(getRegionItem('region-b').previewContentStyle).toEqual({
+      left: '0px',
+      width: '300px',
+    });
+  });
+
+  it('shifts preview content right when extending from the start', () => {
+    renderSharedPreviewHarness(
+      ['region-a', 'region-b'],
+      [
+        { id: 'region-a', trackId: '1', trackIndex: 0, barNumber: 2, length: 2, name: 'Region A' },
+        { id: 'region-b', trackId: '2', trackIndex: 1, barNumber: 4, length: 3, name: 'Region B' },
+      ],
+    );
+
+    act(() => {
+      getRegionItem('region-a').onResizeStart?.('region-a', 'start', 0);
+      getRegionItem('region-a').onResize?.('region-a', 'start', -40);
+    });
+
+    expect(getRegionItem('region-a').style).toEqual({
+      left: '60px',
+      width: '240px',
+      position: 'absolute',
+    });
+    expect(getRegionItem('region-a').previewContentStyle).toEqual({
+      left: '40px',
+      width: '200px',
+    });
+    expect(getRegionItem('region-b').style).toEqual({
+      left: '260px',
+      width: '340px',
+      position: 'absolute',
+    });
+    expect(getRegionItem('region-b').previewContentStyle).toEqual({
+      left: '40px',
+      width: '300px',
+    });
+  });
+
+  it('previews drag movement for all selected regions across track rows', () => {
+    renderSharedPreviewHarness(['region-a', 'region-b']);
+
+    act(() => {
+      getRegionItem('region-a').onDragStart?.('region-a', 0, 0);
+      getRegionItem('region-a').onDrag?.('region-a', 50, 60);
+    });
+
+    expect(getRegionItem('region-a').style).toEqual({
+      left: '50px',
+      width: '100px',
+      position: 'absolute',
+      zIndex: 100,
+      transform: 'translateY(0px)',
+    });
+    expect(getRegionItem('region-b').style).toEqual({
+      left: '250px',
+      width: '200px',
+      position: 'absolute',
+      zIndex: 100,
+      transform: 'translateY(0px)',
+    });
+  });
+
+  it('previews only the grabbed region when it is outside the current selection', () => {
+    renderSharedPreviewHarness(['region-a', 'region-b']);
+
+    const regionC = {
+      id: 'region-c',
+      trackId: '1',
+      trackIndex: 0,
+      barNumber: 5,
+      length: 1,
+      name: 'Region C',
+    };
+
+    const track = createMockMidiTrack({ id: 1, regions: [createMockMidiRegion({ id: 'region-c', trackId: '1', trackIndex: 0, startFromBeat: 16, length: 4 })] });
+    track.setTrackIndex(0);
+    const gridContainerRef = createGridContainerRef();
+
+    render(
+      <TrackGridItem
+        track={track}
+        index={0}
+        isDragging={false}
+        isDragOver={false}
+        regions={[regionC]}
+        maxBars={8}
+        selectedRegionId={null}
+        gridContainerRef={gridContainerRef}
+        onDoubleClick={vi.fn()}
+      />
+    );
+
+    act(() => {
+      getRegionItem('region-c').onDragStart?.('region-c', 0, 0);
+      getRegionItem('region-c').onDrag?.('region-c', 50, 60);
+    });
+
+    expect(getRegionItem('region-c').style).toEqual({
+      left: '450px',
+      width: '100px',
+      position: 'absolute',
+      zIndex: 100,
+      transform: 'translateY(60px)',
+    });
+  });
+
+  it('clears preview styles for the full cohort after drag and resize end', () => {
+    const { onRegionResizeEnd, onRegionDragEnd } = renderSharedPreviewHarness(['region-a', 'region-b']);
+
+    act(() => {
+      getRegionItem('region-a').onResizeStart?.('region-a', 'end', 0);
+      getRegionItem('region-a').onResize?.('region-a', 'end', 40);
+      getRegionItem('region-a').onResizeEnd?.('region-a', 'end');
+    });
+
+    expect(getRegionItem('region-a').style).toEqual({
+      left: '0px',
+      width: '100px',
+      position: 'absolute',
+    });
+    expect(getRegionItem('region-a').previewContentStyle).toBeUndefined();
+    expect(getRegionItem('region-b').style).toEqual({
+      left: '200px',
+      width: '200px',
+      position: 'absolute',
+    });
+    expect(getRegionItem('region-b').previewContentStyle).toBeUndefined();
+    expect(onRegionResizeEnd).toHaveBeenCalledWith('region-a', 1, 1);
+
+    act(() => {
+      getRegionItem('region-a').onDragStart?.('region-a', 0, 0);
+      getRegionItem('region-a').onDrag?.('region-a', 50, 60);
+      getRegionItem('region-a').onDragEnd?.('region-a');
+    });
+
+    expect(getRegionItem('region-a').style).toEqual({
+      left: '0px',
+      width: '100px',
+      position: 'absolute',
+    });
+    expect(getRegionItem('region-a').previewContentStyle).toBeUndefined();
+    expect(getRegionItem('region-b').style).toEqual({
+      left: '200px',
+      width: '200px',
+      position: 'absolute',
+    });
+    expect(getRegionItem('region-b').previewContentStyle).toBeUndefined();
+    expect(onRegionDragEnd).toHaveBeenCalledWith('region-a', 2, 0);
   });
 });

src/components/track/TrackGridItem.tsx

@@ -5,12 +5,29 @@ import { KGAudioRegion } from '../../core/region/KGAudioRegion';
 import { KGAudioInterface } from '../../core/audio-interface/KGAudioInterface';
 import RegionItem from './RegionItem';
 import TrackAutomationLane from './TrackAutomationLane';
-import type { RegionClickOptions, RegionUI, ResizeAction } from '../interfaces';
+import type { RegionClickOptions, RegionPreviewContentStyle, RegionUI, ResizeAction } from '../interfaces';
 import { REGION_CONSTANTS, DEBUG_MODE } from '../../constants';
 import { KGMainContentState } from '../../core/state/KGMainContentState';
 import { isModifierKeyPressed } from '../../util/osUtil';
 import { useProjectStore } from '../../stores/projectStore';
 
+interface RegionResizePreviewBaseline {
+  regionId: string;
+  originalBarNumber: number;
+  originalLength: number;
+  originalLeft: number;
+  originalWidth: number;
+  originalContentWidth: number;
+}
+
+interface RegionDragPreviewBaseline {
+  regionId: string;
+  originalBarNumber: number;
+  originalTrackIndex: number;
+  originalLeft: number;
+  originalWidth: number;
+}
+
 interface TrackGridItemProps {
   track: KGTrack;
   index: number;
@@ -35,6 +52,10 @@ interface TrackGridItemProps {
   onOpenHybrid?: (regionId: string) => void;
   allTracks?: KGTrack[]; // Added to access all tracks for drag operations
   onKGOneClipDrop?: (e: React.DragEvent<HTMLDivElement>, trackIndex: number) => void;
+  previewRegionStyles?: Record<string, React.CSSProperties>;
+  setPreviewRegionStyles?: React.Dispatch<React.SetStateAction<Record<string, React.CSSProperties>>>;
+  previewRegionContentStyles?: Record<string, RegionPreviewContentStyle>;
+  setPreviewRegionContentStyles?: React.Dispatch<React.SetStateAction<Record<string, RegionPreviewContentStyle>>>;
 }
 
 const TrackGridItem: React.FC<TrackGridItemProps> = ({
@@ -61,6 +82,10 @@ const TrackGridItem: React.FC<TrackGridItemProps> = ({
   onOpenHybrid,
   allTracks,
   onKGOneClipDrop,
+  previewRegionStyles,
+  setPreviewRegionStyles,
+  previewRegionContentStyles,
+  setPreviewRegionContentStyles,
 }) => {
   const selectedRegionIds = useProjectStore(state => state.selectedRegionIds);
   const activeTrackAutomationTrackId = useProjectStore(state => state.activeTrackAutomationTrackId);
@@ -76,7 +101,8 @@ const TrackGridItem: React.FC<TrackGridItemProps> = ({
   const [containerWidth, setContainerWidth] = useState(0);
   const [resizingRegion, setResizingRegion] = useState<string | null>(null);
   const [draggingRegion, setDraggingRegion] = useState<string | null>(null);
-  const [tempRegionStyles, setTempRegionStyles] = useState<Record<string, React.CSSProperties>>({});
+  const [localTempRegionStyles, setLocalTempRegionStyles] = useState<Record<string, React.CSSProperties>>({});
+  const [localPreviewRegionContentStyles, setLocalPreviewRegionContentStyles] = useState<Record<string, RegionPreviewContentStyle>>({});
   const [isModifierPressed, setIsModifierPressed] = useState(false);
   
   // Refs for resize operations
@@ -86,14 +112,71 @@ const TrackGridItem: React.FC<TrackGridItemProps> = ({
   const currentResizeRegion = useRef<RegionUI | null>(null);
   const initialBarNumberRef = useRef<number | null>(null);
   const initialLengthRef = useRef<number | null>(null);
+  const resizePreviewBaselinesRef = useRef<RegionResizePreviewBaseline[]>([]);
+  const resizePreviewRegionIdsRef = useRef<string[]>([]);
   
   // Refs for drag operations
   const currentDragLeft = useRef<number | null>(null);
   const currentDragTop = useRef<number | null>(null);
   const currentDragRegion = useRef<RegionUI | null>(null);
+  const dragPreviewBaselinesRef = useRef<RegionDragPreviewBaseline[]>([]);
+  const dragPreviewRegionIdsRef = useRef<string[]>([]);
   const trackElementRef = useRef<HTMLDivElement | null>(null);
 
   const isBulkRegionEdit = (regionId: string) => selectedRegionIds.length > 1 && selectedRegionIds.includes(regionId);
+  const tempRegionStyles = previewRegionStyles ?? localTempRegionStyles;
+  const setTempRegionStyles = setPreviewRegionStyles ?? setLocalTempRegionStyles;
+  const tempPreviewRegionContentStyles = previewRegionContentStyles ?? localPreviewRegionContentStyles;
+  const setTempPreviewRegionContentStyles = setPreviewRegionContentStyles ?? setLocalPreviewRegionContentStyles;
+
+  const getPreviewRegionIds = (regionId: string) => (
+    selectedRegionIds.length > 1 && selectedRegionIds.includes(regionId)
+      ? selectedRegionIds
+      : [regionId]
+  );
+
+  const clearTempRegionStyles = (regionIds?: string[]) => {
+    if (!regionIds || regionIds.length === 0) {
+      setTempRegionStyles({});
+      return;
+    }
+
+    setTempRegionStyles(prev => {
+      const updated = { ...prev };
+      regionIds.forEach(id => {
+        delete updated[id];
+      });
+      return updated;
+    });
+  };
+
+  const clearTempPreviewRegionContentStyles = (regionIds?: string[]) => {
+    if (!regionIds || regionIds.length === 0) {
+      setTempPreviewRegionContentStyles({});
+      return;
+    }
+
+    setTempPreviewRegionContentStyles(prev => {
+      const updated = { ...prev };
+      regionIds.forEach(id => {
+        delete updated[id];
+      });
+      return updated;
+    });
+  };
+
+  const getMeasuredRegionContentWidth = (regionId: string, fallbackWidth: number) => {
+    const regionElement = Array.from(document.querySelectorAll<HTMLElement>('[data-region-id]'))
+      .find(element => element.getAttribute('data-region-id') === regionId);
+    const regionContentElement = regionElement?.querySelector<HTMLElement>('.region-content');
+    const measuredWidth = regionContentElement?.getBoundingClientRect().width;
+
+    if (!measuredWidth || Number.isNaN(measuredWidth)) {
+      return fallbackWidth;
+    }
+
+    return measuredWidth;
+  };
 
   // Update container width when the grid container changes size
   useEffect(() => {
@@ -144,7 +227,7 @@ const TrackGridItem: React.FC<TrackGridItemProps> = ({
   // Calculate region position and style
   const getRegionStyle = (region: RegionUI) => {
     // Check if there's a temporary style for this region during resize or drag
-    if ((resizingRegion === region.id || draggingRegion === region.id) && tempRegionStyles[region.id]) {
+    if (tempRegionStyles[region.id]) {
       return tempRegionStyles[region.id];
     }
     
@@ -195,17 +278,42 @@ const TrackGridItem: React.FC<TrackGridItemProps> = ({
     // Store the initial width and left position
     currentResizeWidth.current = region.length * barWidth;
     currentResizeLeft.current = (region.barNumber - 1) * barWidth;
-    
-    // Set initial style to current position/size
-    const initialStyle = {
-      left: `${currentResizeLeft.current}px`,
-      width: `${currentResizeWidth.current}px`,
-      position: 'absolute' as const, // Fixed: Use const assertion
-    };
-    
+
+    const previewRegionIds = getPreviewRegionIds(regionId);
+    resizePreviewBaselinesRef.current = previewRegionIds
+      .map(id => regions.find(candidate => candidate.id === id))
+      .filter((candidate): candidate is RegionUI => candidate !== undefined)
+      .map(candidate => ({
+        regionId: candidate.id,
+        originalBarNumber: candidate.barNumber,
+        originalLength: candidate.length,
+        originalLeft: (candidate.barNumber - 1) * barWidth,
+        originalWidth: candidate.length * barWidth,
+        originalContentWidth: getMeasuredRegionContentWidth(candidate.id, candidate.length * barWidth),
+      }));
+    resizePreviewRegionIdsRef.current = resizePreviewBaselinesRef.current.map(baseline => baseline.regionId);
+
     setTempRegionStyles(prev => ({
       ...prev,
-      [regionId]: initialStyle
+      ...Object.fromEntries(resizePreviewBaselinesRef.current.map(baseline => [
+        baseline.regionId,
+        {
+          left: `${baseline.originalLeft}px`,
+          width: `${baseline.originalWidth}px`,
+          position: 'absolute' as const,
+        },
+      ])),
+    }));
+
+    setTempPreviewRegionContentStyles(prev => ({
+      ...prev,
+      ...Object.fromEntries(resizePreviewBaselinesRef.current.map(baseline => [
+        baseline.regionId,
+        {
+          left: '0px',
+          width: `${baseline.originalContentWidth}px`,
+        },
+      ])),
     }));
   };
 
@@ -254,16 +362,40 @@ const TrackGridItem: React.FC<TrackGridItemProps> = ({
       console.log(`RESIZE: regionId=${regionId}, action=${resizeAction}, deltaX=${deltaX}, newBarNumber=${newBarNumber}, newLength=${newLength}`);
     }
     
-    // Update the temporary style for this region
-    const newStyle = {
-      left: `${newLeft}px`,
-      width: `${newWidth}px`,
-      position: 'absolute' as const, // Fixed: Use const assertion
-    };
-    
+    const leftDelta = newLeft - originalLeft;
+    const widthDelta = newWidth - originalWidth;
+    const previewBaselines = resizePreviewBaselinesRef.current.length > 0
+      ? resizePreviewBaselinesRef.current
+      : [{
+        regionId,
+        originalBarNumber: region.barNumber,
+        originalLength: region.length,
+        originalLeft,
+        originalWidth,
+        originalContentWidth: getMeasuredRegionContentWidth(regionId, originalWidth),
+      }];
+
     setTempRegionStyles(prev => ({
       ...prev,
-      [regionId]: newStyle
+      ...Object.fromEntries(previewBaselines.map(baseline => [
+        baseline.regionId,
+        {
+          left: `${resizeAction === 'start' ? baseline.originalLeft + leftDelta : baseline.originalLeft}px`,
+          width: `${baseline.originalWidth + widthDelta}px`,
+          position: 'absolute' as const,
+        },
+      ])),
+    }));
+
+    setTempPreviewRegionContentStyles(prev => ({
+      ...prev,
+      ...Object.fromEntries(previewBaselines.map(baseline => [
+        baseline.regionId,
+        {
+          left: `${resizeAction === 'start' ? -(newLeft - originalLeft) : 0}px`,
+          width: `${baseline.originalContentWidth}px`,
+        },
+      ])),
     }));
     
     // Notify parent about resize
@@ -328,16 +460,15 @@ const TrackGridItem: React.FC<TrackGridItemProps> = ({
     
     // Clear resizing state
     setResizingRegion(null);
-    setTempRegionStyles(prev => {
-      const updated = { ...prev };
-      delete updated[regionId];
-      return updated;
-    });
+    clearTempRegionStyles(resizePreviewRegionIdsRef.current);
+    clearTempPreviewRegionContentStyles(resizePreviewRegionIdsRef.current);
     currentResizeWidth.current = null;
     currentResizeLeft.current = null;
     currentResizeRegion.current = null;
     initialBarNumberRef.current = null;
     initialLengthRef.current = null;
+    resizePreviewBaselinesRef.current = [];
+    resizePreviewRegionIdsRef.current = [];
     
     // Notify parent about resize end with rounded values
     if (onRegionResizeEnd) {
@@ -378,18 +509,32 @@ const TrackGridItem: React.FC<TrackGridItemProps> = ({
     // Store the initial position
     currentDragLeft.current = left;
     currentDragTop.current = 0; // Initially at the top of the current track
-    
-    // Set initial style
-    const initialStyle = {
-      left: `${left}px`,
-      width: `${width}px`,
-      position: 'absolute' as const, // Fixed: Use const assertion
-      zIndex: 100, // Bring to front during drag
-    };
-    
+
+    const previewRegionIds = getPreviewRegionIds(regionId);
+    dragPreviewBaselinesRef.current = previewRegionIds
+      .map(id => regions.find(candidate => candidate.id === id))
+      .filter((candidate): candidate is RegionUI => candidate !== undefined)
+      .map(candidate => ({
+        regionId: candidate.id,
+        originalBarNumber: candidate.barNumber,
+        originalTrackIndex: candidate.trackIndex,
+        originalLeft: (candidate.barNumber - 1) * barWidth,
+        originalWidth: candidate.length * barWidth,
+      }));
+    dragPreviewRegionIdsRef.current = dragPreviewBaselinesRef.current.map(baseline => baseline.regionId);
+
     setTempRegionStyles(prev => ({
       ...prev,
-      [regionId]: initialStyle
+      ...Object.fromEntries(dragPreviewBaselinesRef.current.map(baseline => [
+        baseline.regionId,
+        {
+          left: `${baseline.originalLeft}px`,
+          width: `${baseline.originalWidth}px`,
+          position: 'absolute' as const,
+          zIndex: 100,
+          transform: 'translateY(0px)',
+        },
+      ])),
     }));
   };
 
@@ -425,18 +570,29 @@ const TrackGridItem: React.FC<TrackGridItemProps> = ({
       console.log(`DRAG: regionId=${regionId}, deltaX=${deltaX}, deltaY=${deltaY}, newBarNumber=${newBarNumber}`);
     }
     
-    // Update the temporary style for this region
-    const newStyle = {
-      left: `${newLeft}px`,
-      width: `${region.length * barWidth}px`,
-      position: 'absolute' as const,
-      zIndex: 100, // Keep on top during drag
-      transform: `translateY(${appliedDeltaY}px)`,
-    };
-    
+    const leftDelta = newLeft - initialLeft;
+    const previewBaselines = dragPreviewBaselinesRef.current.length > 0
+      ? dragPreviewBaselinesRef.current
+      : [{
+        regionId,
+        originalBarNumber: region.barNumber,
+        originalTrackIndex: region.trackIndex,
+        originalLeft: initialLeft,
+        originalWidth: region.length * barWidth,
+      }];
+
     setTempRegionStyles(prev => ({
       ...prev,
-      [regionId]: newStyle
+      ...Object.fromEntries(previewBaselines.map(baseline => [
+        baseline.regionId,
+        {
+          left: `${baseline.originalLeft + leftDelta}px`,
+          width: `${baseline.originalWidth}px`,
+          position: 'absolute' as const,
+          zIndex: 100,
+          transform: `translateY(${appliedDeltaY}px)`,
+        },
+      ])),
     }));
     
     // We'll calculate the track index on drag end, but still notify parent about the drag
@@ -506,14 +662,13 @@ const TrackGridItem: React.FC<TrackGridItemProps> = ({
     
     // Clear dragging state
     setDraggingRegion(null);
-    setTempRegionStyles(prev => {
-      const updated = { ...prev };
-      delete updated[regionId];
-      return updated;
-    });
+    clearTempRegionStyles(dragPreviewRegionIdsRef.current);
+    clearTempPreviewRegionContentStyles(dragPreviewRegionIdsRef.current);
     currentDragLeft.current = null;
     currentDragTop.current = null;
     currentDragRegion.current = null;
+    dragPreviewBaselinesRef.current = [];
+    dragPreviewRegionIdsRef.current = [];
     
     if (finalBarNumber === region.barNumber && finalTrackIndex === region.trackIndex) {
       if (DEBUG_MODE.TRACK_GRID_ITEM) {
@@ -572,7 +727,7 @@ const TrackGridItem: React.FC<TrackGridItemProps> = ({
       }}
       onClick={(e) => {
         if (!isAutomationActive) {
-          onClick && onClick(e, index);
+          onClick?.(e, index);
         }
       }}
       ref={trackElementRef}
@@ -640,6 +795,7 @@ const TrackGridItem: React.FC<TrackGridItemProps> = ({
             midiRegion={midiRegion}
             audioRegion={audioRegion}
             audioBuffer={audioBuffer}
+            previewContentStyle={tempPreviewRegionContentStyles[region.id]}
           />
         );
       })}

src/components/track/TrackGridPanel.tsx

@@ -4,7 +4,7 @@ import { KGMidiRegion } from '../../core/region/KGMidiRegion';
 import TrackGridItem from './TrackGridItem';
 import { Playhead, FileImportModal } from '../common';
 import SelectionBox from '../piano-roll/SelectionBox';
-import type { RegionClickOptions, RegionUI } from '../interfaces';
+import type { RegionClickOptions, RegionPreviewContentStyle, RegionUI } from '../interfaces';
 import { DEBUG_MODE, PIANO_ROLL_CONSTANTS, REGION_CONSTANTS } from '../../constants';
 import { KGMainContentState } from '../../core/state/KGMainContentState';
 import { isModifierKeyPressed } from '../../util/osUtil';
@@ -66,6 +66,8 @@ const TrackGridPanel: React.FC<TrackGridPanelProps> = ({
   const refreshProjectState = useProjectStore(state => state.refreshProjectState);
   const gridContainerRef = useRef<HTMLDivElement>(null);
   const [showAudioImportModal, setShowAudioImportModal] = useState(false);
+  const [previewRegionStyles, setPreviewRegionStyles] = useState<Record<string, React.CSSProperties>>({});
+  const [previewRegionContentStyles, setPreviewRegionContentStyles] = useState<Record<string, RegionPreviewContentStyle>>({});
   const pendingAudioImportRef = useRef<{ barNumber: number; trackIndex: number } | null>(null);
   const isLassoSelectingRef = useRef(false);
   const isLassoShiftPressedRef = useRef(false);
@@ -907,6 +909,10 @@ const TrackGridPanel: React.FC<TrackGridPanelProps> = ({
           onOpenHybrid={onOpenHybrid}
           allTracks={tracks}
           onKGOneClipDrop={handleExternalDrop}
+          previewRegionStyles={previewRegionStyles}
+          setPreviewRegionStyles={setPreviewRegionStyles}
+          previewRegionContentStyles={previewRegionContentStyles}
+          setPreviewRegionContentStyles={setPreviewRegionContentStyles}
         />
       ))}
 

src/components/track/TrackInfoItem.tsx

@@ -96,8 +96,8 @@ const TrackInfoItem: React.FC<TrackInfoItemProps> = ({
   const volumeInputRef = useRef<HTMLInputElement>(null);
   // Local flag to track slider interaction; not used for rendering
   const isAdjustingVolumeRef = useRef(false);
-  const [muted, setMuted] = useState(false);
-  const [solo, setSolo] = useState(false);
+  const [muted, setMuted] = useState(track.getMuted());
+  const [solo, setSolo] = useState(track.getSolo());
 
   // Close dropdown when clicking outside
   useEffect(() => {
@@ -135,11 +135,10 @@ const TrackInfoItem: React.FC<TrackInfoItemProps> = ({
     setVolume(track.getVolume());
   }, [allTracks, track]);
 
-  // Sync mute/solo UI with audio interface state on track/project changes
+  // Sync mute/solo UI with the track model on track/project changes
   useEffect(() => {
-    const audioInterface = KGAudioInterface.instance();
-    setMuted(audioInterface.getTrackMuted(track.getId().toString()));
-    setSolo(audioInterface.getTrackSolo(track.getId().toString()));
+    setMuted(track.getMuted());
+    setSolo(track.getSolo());
   }, [allTracks, track]);
   
   // Handle track name edit within the component
@@ -252,22 +251,20 @@ const TrackInfoItem: React.FC<TrackInfoItemProps> = ({
     e.stopPropagation();
     const next = !muted;
     setMuted(next);
-    try {
-      KGAudioInterface.instance().setTrackMute(track.getId().toString(), next);
-    } catch (err) {
+    useProjectStore.getState().updateTrackProperties(track.getId(), { muted: next }).catch(err => {
+      setMuted(track.getMuted());
       console.error('Failed to toggle mute:', err);
-    }
+    });
   };
 
   const handleToggleSolo = (e: React.MouseEvent<HTMLButtonElement>) => {
     e.stopPropagation();
     const next = !solo;
     setSolo(next);
-    try {
-      KGAudioInterface.instance().setTrackSolo(track.getId().toString(), next);
-    } catch (err) {
+    useProjectStore.getState().updateTrackProperties(track.getId(), { solo: next }).catch(err => {
+      setSolo(track.getSolo());
       console.error('Failed to toggle solo:', err);
-    }
+    });
   };
 
   // Handle track click

src/constants/coreConstants.ts

@@ -104,7 +104,7 @@ export const OPFS_CONSTANTS = {
 
 export const CONFIG_UPGRADER_CONSTANTS = {
   VERSION_KEY: '__config_version',
-  CURRENT_VERSION: 3,
+  CURRENT_VERSION: 4,
 };
 
 export const URL_CONSTANTS = {

src/core/KGProject.ts

@@ -49,11 +49,15 @@ export class KGProject {
   @WithDefault(1)
   private barWidthMultiplier: number = 1;
 
+  @Expose()
+  @WithDefault(1)
+  private pianoRollZoom: number = 1;
+
   @Expose()
   @WithDefault(0)
   private projectStructureVersion: number = 0;
 
-  public static readonly CURRENT_PROJECT_STRUCTURE_VERSION: number = 10;
+  public static readonly CURRENT_PROJECT_STRUCTURE_VERSION: number = 12;
   
   @Expose()
   @Type(() => KGTrack, {
@@ -69,7 +73,7 @@ export class KGProject {
   private tracks: KGTrack[] = [];
 
   // Constructor
-  constructor(name: string = RESERVED_PROJECT_NAME, maxBars: number = 32, currentBars: number = 0, bpm: number = 125, timeSignature: TimeSignature = { numerator: 4, denominator: 4 }, keySignature: KeySignature = "C major", selectedMode: string = "ionian", isLooping: boolean = false, loopingRange: [number, number] = [0, 0], barWidthMultiplier: number = 1, tracks: KGTrack[] = [], projectStructureVersion: number = KGProject.CURRENT_PROJECT_STRUCTURE_VERSION) {
+  constructor(name: string = RESERVED_PROJECT_NAME, maxBars: number = 32, currentBars: number = 0, bpm: number = 125, timeSignature: TimeSignature = { numerator: 4, denominator: 4 }, keySignature: KeySignature = "C major", selectedMode: string = "ionian", isLooping: boolean = false, loopingRange: [number, number] = [0, 0], barWidthMultiplier: number = 1, tracks: KGTrack[] = [], projectStructureVersion: number = KGProject.CURRENT_PROJECT_STRUCTURE_VERSION, pianoRollZoom: number = 1) {
     this.name = name;
     this.maxBars = maxBars;
     this.currentBars = currentBars;
@@ -82,6 +86,7 @@ export class KGProject {
     this.barWidthMultiplier = barWidthMultiplier;
     this.tracks = tracks;
     this.projectStructureVersion = projectStructureVersion;
+    this.pianoRollZoom = pianoRollZoom;
   }
 
   // Getters
@@ -181,4 +186,12 @@ export class KGProject {
   public setBarWidthMultiplier(barWidthMultiplier: number): void {
     this.barWidthMultiplier = barWidthMultiplier;
   }
+
+  public getPianoRollZoom(): number {
+    return this.pianoRollZoom;
+  }
+
+  public setPianoRollZoom(pianoRollZoom: number): void {
+    this.pianoRollZoom = pianoRollZoom;
+  }
 }

src/core/audio-interface/KGAudioInterface.ts

@@ -242,11 +242,13 @@ export class KGAudioInterface {
       console.log(`Creating audio bus for track ${trackId} with instrument ${instrumentType}`);
       
       // Create new audio bus
-      // Initialize with track's stored volume if available
+      // Initialize with track's stored mix state if available
       const project = KGCore.instance().getCurrentProject();
       const track = project.getTracks().find(t => t.getId().toString() === trackId);
       const initialVolume = track ? track.getVolume() : AUDIO_INTERFACE_CONSTANTS.DEFAULT_TRACK_VOLUME;
-      const audioBus = await KGAudioBus.create(instrumentType, initialVolume, 0);
+      const initialMuted = track ? track.getMuted() : false;
+      const initialSolo = track ? track.getSolo() : false;
+      const audioBus = await KGAudioBus.create(instrumentType, initialVolume, 0, initialMuted, initialSolo);
       
       // Connect to master gain if available, otherwise to destination
       if (this.masterGain) {
@@ -299,7 +301,11 @@ export class KGAudioInterface {
 
     try {
       console.log(`Creating audio player bus for track ${trackId}`);
-      const playerBus = await KGAudioPlayerBus.create(volume, 0);
+      const project = KGCore.instance().getCurrentProject();
+      const track = project.getTracks().find(t => t.getId().toString() === trackId);
+      const initialMuted = track ? track.getMuted() : false;
+      const initialSolo = track ? track.getSolo() : false;
+      const playerBus = await KGAudioPlayerBus.create(volume, 0, initialMuted, initialSolo);
 
       if (this.masterGain) {
         playerBus.connect(this.masterGain);

src/core/audio-interface/KGOfflineRenderer.test.ts

@@ -1,7 +1,37 @@
-import { describe, it, expect } from 'vitest';
-import { applyOfflinePitchBendAutomation, encodeWav, getOfflineTrackGain, getOfflineTrackVolumeDb } from './KGOfflineRenderer';
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { createMockMidiNote, createMockMidiRegion, createMockMidiTrack, createMockProject } from '../../test/utils/mock-data';
 import { bakeMidiAutomationPointsInWindow } from '../../util/midiAutomationUtil';
 
+const { offlineMock, configGetMock } = vi.hoisted(() => ({
+  offlineMock: vi.fn(),
+  configGetMock: vi.fn(),
+}));
+
+vi.mock('tone', () => ({
+  Offline: offlineMock,
+}));
+
+vi.mock('./KGAudioInterface', () => ({
+  KGAudioInterface: {
+    instance: vi.fn(() => ({
+      getTrackVolume: vi.fn().mockReturnValue(0),
+      getTrackMuted: vi.fn().mockReturnValue(false),
+      getTrackSolo: vi.fn().mockReturnValue(false),
+      getAudioBuffer: vi.fn().mockReturnValue(null),
+    })),
+  },
+}));
+
+vi.mock('../config/ConfigManager', () => ({
+  ConfigManager: {
+    instance: vi.fn(() => ({
+      get: configGetMock,
+    })),
+  },
+}));
+
+import { KGOfflineRenderer, applyOfflinePitchBendAutomation, encodeWav, getOfflineTrackGain, getOfflineTrackVolumeDb } from './KGOfflineRenderer';
+
 /**
  * Create a minimal AudioBuffer-like object for testing.
  * In the jsdom test environment, AudioBuffer is not available,
@@ -228,3 +258,77 @@ describe('offline pitch bend automation', () => {
     expect(calls[0][1]).toBe(0.26);
   });
 });
+
+describe('renderToBuffer bounce range', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    configGetMock.mockImplementation((key: string) => {
+      if (key === 'audio.bounce_starts_from_beat_1') return true;
+      if (key === 'audio.midi_automation_interpolation_interval_ms') return 10;
+      return null;
+    });
+    offlineMock.mockResolvedValue({
+      duration: 0,
+      numberOfChannels: 2,
+      get: vi.fn(),
+    });
+    ;(KGOfflineRenderer as unknown as { _instance: KGOfflineRenderer | null })._instance = null;
+  });
+
+  it('starts non-looping bounce at beat 0 when configured to include leading silence', async () => {
+    const region = createMockMidiRegion({
+      startFromBeat: 8,
+      notes: [createMockMidiNote({ startBeat: 0, endBeat: 4 })],
+    });
+    const track = createMockMidiTrack({ id: 1, regions: [region] });
+    const project = createMockProject({ bpm: 120, tracks: [track] });
+
+    await KGOfflineRenderer.instance().renderToBuffer(project, { tailSeconds: 0 });
+
+    expect(offlineMock).toHaveBeenCalledWith(expect.any(Function), 6, 2, 44100);
+  });
+
+  it('starts non-looping bounce at first content when the setting is disabled', async () => {
+    configGetMock.mockImplementation((key: string) => {
+      if (key === 'audio.bounce_starts_from_beat_1') return false;
+      if (key === 'audio.midi_automation_interpolation_interval_ms') return 10;
+      return null;
+    });
+
+    const region = createMockMidiRegion({
+      startFromBeat: 8,
+      notes: [createMockMidiNote({ startBeat: 0, endBeat: 4 })],
+    });
+    const track = createMockMidiTrack({ id: 1, regions: [region] });
+    const project = createMockProject({ bpm: 120, tracks: [track] });
+
+    await KGOfflineRenderer.instance().renderToBuffer(project, { tailSeconds: 0 });
+
+    expect(offlineMock).toHaveBeenCalledWith(expect.any(Function), 2, 2, 44100);
+  });
+
+  it('keeps looping bounce bounds regardless of the beat-1 setting', async () => {
+    configGetMock.mockImplementation((key: string) => {
+      if (key === 'audio.bounce_starts_from_beat_1') return false;
+      if (key === 'audio.midi_automation_interpolation_interval_ms') return 10;
+      return null;
+    });
+
+    const project = createMockProject({ bpm: 120, tracks: [] });
+    project.setIsLooping(true);
+    project.setLoopingRange([2, 5]);
+
+    await KGOfflineRenderer.instance().renderToBuffer(project, { tailSeconds: 0 });
+
+    expect(offlineMock).toHaveBeenCalledWith(expect.any(Function), 8, 2, 44100);
+  });
+
+  it('falls back to the full project length when there is no renderable content', async () => {
+    const project = createMockProject({ bpm: 120, tracks: [] });
+    project.setMaxBars(16);
+
+    await KGOfflineRenderer.instance().renderToBuffer(project, { tailSeconds: 0 });
+
+    expect(offlineMock).toHaveBeenCalledWith(expect.any(Function), 32, 2, 44100);
+  });
+});

src/core/audio-interface/KGOfflineRenderer.ts

@@ -99,6 +99,7 @@ export class KGOfflineRenderer {
 
     let renderStartBeat = 0;
     let renderEndBeat: number;
+    const bounceStartsFromBeat1 = (ConfigManager.instance().get('audio.bounce_starts_from_beat_1') as boolean) ?? true;
 
     const isLooping = project.getIsLooping();
     // Looping range is determined up-front; non-looping range is computed
@@ -272,7 +273,7 @@ export class KGOfflineRenderer {
       }
 
       if (contentEnd > 0) {
-        renderStartBeat = contentStart;
+        renderStartBeat = bounceStartsFromBeat1 ? 0 : contentStart;
         renderEndBeat = contentEnd;
       }
       // else: no content found, keep the full project range as fallback

src/core/commands/index.ts

@@ -41,6 +41,7 @@ export { DeleteNotesCommand, DeleteNoteCommand } from './note/DeleteNotesCommand
 export { ResizeNotesCommand } from './note/ResizeNotesCommand';
 export { MoveNotesCommand } from './note/MoveNotesCommand';
 export { PasteNotesCommand } from './note/PasteNotesCommand';
+export { SplitSelectedNotesCommand } from './note/SplitSelectedNotesCommand';
 export { UpdateNotePropertiesCommand } from './note/UpdateNotePropertiesCommand';
 export { UpdatePitchBendPropertiesCommand } from './note/UpdatePitchBendPropertiesCommand';
 export { UpdateControllerEventPropertiesCommand } from './note/UpdateControllerEventPropertiesCommand';

src/core/commands/note/SplitSelectedNotesCommand.test.ts

@@ -0,0 +1,131 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import type { Selectable } from '../../../components/interfaces';
+import { KGCore } from '../../KGCore';
+import { KGMidiNote } from '../../midi/KGMidiNote';
+import { SplitSelectedNotesCommand } from './SplitSelectedNotesCommand';
+import { createMockMidiNote, createMockMidiRegion, createMockMidiTrack, createMockProject } from '../../../test/utils/mock-data';
+
+vi.mock('../../KGCore', () => ({
+  KGCore: {
+    instance: vi.fn(),
+  }
+}));
+
+interface MockCore {
+  getCurrentProject: ReturnType<typeof vi.fn>;
+  getSelectedItems: ReturnType<typeof vi.fn>;
+  clearSelectedItems: ReturnType<typeof vi.fn>;
+  addSelectedItems: ReturnType<typeof vi.fn>;
+}
+
+describe('SplitSelectedNotesCommand', () => {
+  let mockCore: MockCore;
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockCore = {
+      getCurrentProject: vi.fn(),
+      getSelectedItems: vi.fn(),
+      clearSelectedItems: vi.fn(),
+      addSelectedItems: vi.fn(),
+    };
+
+    vi.mocked(KGCore.instance).mockReturnValue(mockCore as unknown as KGCore);
+  });
+
+  it('splits one selected note into two halves and selects both results', () => {
+    const splitNote = createMockMidiNote({ id: 'note-a', startBeat: 1, endBeat: 5, pitch: 64, velocity: 90 });
+    splitNote.select();
+    const region = createMockMidiRegion({
+      id: 'region-1',
+      trackId: '1',
+      trackIndex: 0,
+      notes: [splitNote],
+    });
+    const track = createMockMidiTrack({ id: 1, regions: [region] });
+    mockCore.getCurrentProject.mockReturnValue(createMockProject({ tracks: [track] }));
+    mockCore.getSelectedItems.mockReturnValue([splitNote]);
+
+    const command = new SplitSelectedNotesCommand(region.getId(), [splitNote.getId()], 3);
+
+    command.execute();
+
+    const notes = region.getNotes();
+    expect(notes).toHaveLength(2);
+    expect(notes.map(note => [note.getStartBeat(), note.getEndBeat(), note.getPitch(), note.getVelocity()])).toEqual([
+      [1, 3, 64, 90],
+      [3, 5, 64, 90],
+    ]);
+    expect(notes.every(note => note.isSelected())).toBe(true);
+    expect(command.getSplitCount()).toBe(1);
+    expect(mockCore.clearSelectedItems).toHaveBeenCalledTimes(1);
+    expect(mockCore.addSelectedItems).toHaveBeenCalledTimes(1);
+    expect((mockCore.addSelectedItems.mock.calls[0][0] as Selectable[])).toHaveLength(2);
+  });
+
+  it('splits multiple selected notes in one undoable operation and leaves uncrossed selected notes unchanged', () => {
+    const noteA = createMockMidiNote({ id: 'note-a', startBeat: 0, endBeat: 4, pitch: 60, velocity: 70 });
+    const noteB = createMockMidiNote({ id: 'note-b', startBeat: 1, endBeat: 2, pitch: 62, velocity: 75 });
+    const noteC = createMockMidiNote({ id: 'note-c', startBeat: 2, endBeat: 5, pitch: 65, velocity: 80 });
+    [noteA, noteB, noteC].forEach(note => note.select());
+    const region = createMockMidiRegion({
+      id: 'region-1',
+      trackId: '1',
+      trackIndex: 0,
+      notes: [noteA, noteB, noteC],
+    });
+    const track = createMockMidiTrack({ id: 1, regions: [region] });
+    mockCore.getCurrentProject.mockReturnValue(createMockProject({ tracks: [track] }));
+    mockCore.getSelectedItems.mockReturnValue([noteA, noteB, noteC]);
+
+    const command = new SplitSelectedNotesCommand(region.getId(), [noteA.getId(), noteB.getId(), noteC.getId()], 3);
+
+    command.execute();
+
+    const notes = region.getNotes();
+    expect(notes).toHaveLength(5);
+    expect(notes.map(note => [note.getStartBeat(), note.getEndBeat(), note.getPitch()])).toEqual([
+      [0, 3, 60],
+      [3, 4, 60],
+      [1, 2, 62],
+      [2, 3, 65],
+      [3, 5, 65],
+    ]);
+    expect(command.getSplitCount()).toBe(2);
+    expect(command.getUnchangedSelectedNoteIds()).toEqual(['note-b']);
+
+    command.undo();
+    expect(region.getNotes()).toEqual([noteA, noteB, noteC]);
+    expect(mockCore.clearSelectedItems).toHaveBeenCalledTimes(2);
+    expect(mockCore.addSelectedItems).toHaveBeenLastCalledWith([noteA, noteB, noteC]);
+
+    command.execute();
+    expect(region.getNotes().map(note => [note.getStartBeat(), note.getEndBeat(), note.getPitch()])).toEqual([
+      [0, 3, 60],
+      [3, 4, 60],
+      [1, 2, 62],
+      [2, 3, 65],
+      [3, 5, 65],
+    ]);
+  });
+
+  it('throws when no selected note crosses the playhead', () => {
+    const noteA = new KGMidiNote('note-a', 0, 1, 60, 100);
+    const noteB = new KGMidiNote('note-b', 4, 5, 62, 100);
+    const region = createMockMidiRegion({
+      id: 'region-1',
+      trackId: '1',
+      trackIndex: 0,
+      notes: [noteA, noteB],
+    });
+    const track = createMockMidiTrack({ id: 1, regions: [region] });
+    mockCore.getCurrentProject.mockReturnValue(createMockProject({ tracks: [track] }));
+    mockCore.getSelectedItems.mockReturnValue([noteA, noteB]);
+
+    const command = new SplitSelectedNotesCommand(region.getId(), [noteA.getId(), noteB.getId()], 3);
+
+    expect(() => command.execute()).toThrow(
+      'The playhead is not inside any selected note. Move the playhead inside a selected note before splitting.'
+    );
+  });
+});

src/core/commands/note/SplitSelectedNotesCommand.ts

@@ -0,0 +1,164 @@
+import type { Selectable } from '../../../components/interfaces';
+import { generateUniqueId } from '../../../util/miscUtil';
+import { KGCore } from '../../KGCore';
+import { KGMidiNote } from '../../midi/KGMidiNote';
+import { KGMidiRegion } from '../../region/KGMidiRegion';
+import { KGCommand } from '../KGCommand';
+
+interface SplitNoteRecord {
+  originalNoteId: string;
+  leftNoteId: string;
+  rightNoteId: string;
+  startBeat: number;
+  endBeat: number;
+  pitch: number;
+  velocity: number;
+}
+
+export class SplitSelectedNotesCommand extends KGCommand {
+  private readonly regionId: string;
+  private readonly selectedNoteIds: string[];
+  private readonly splitAtBeat: number;
+
+  private targetRegion: KGMidiRegion | null = null;
+  private originalNotes: KGMidiNote[] = [];
+  private originalSelectedItems: Selectable[] = [];
+  private splitNoteRecords: SplitNoteRecord[] = [];
+  private unchangedSelectedNoteIds: string[] = [];
+
+  constructor(regionId: string, selectedNoteIds: string[], splitAtBeat: number) {
+    super();
+    this.regionId = regionId;
+    this.selectedNoteIds = [...selectedNoteIds];
+    this.splitAtBeat = splitAtBeat;
+  }
+
+  execute(): void {
+    const core = KGCore.instance();
+    const targetRegion = this.resolveTargetRegion();
+    const noteIdSet = new Set(this.selectedNoteIds);
+    const currentNotes = targetRegion.getNotes();
+    const selectedNotes = currentNotes.filter(note => noteIdSet.has(note.getId()));
+
+    if (selectedNotes.length === 0) {
+      throw new Error('No selected notes were found in the active MIDI region.');
+    }
+
+    if (this.originalSelectedItems.length === 0) {
+      this.originalSelectedItems = [...core.getSelectedItems()];
+    }
+
+    if (this.originalNotes.length === 0) {
+      this.originalNotes = [...currentNotes];
+    }
+
+    if (this.splitNoteRecords.length === 0) {
+      this.splitNoteRecords = selectedNotes
+        .filter(note => note.getStartBeat() < this.splitAtBeat && this.splitAtBeat < note.getEndBeat())
+        .map(note => ({
+          originalNoteId: note.getId(),
+          leftNoteId: generateUniqueId('KGMidiNote'),
+          rightNoteId: generateUniqueId('KGMidiNote'),
+          startBeat: note.getStartBeat(),
+          endBeat: note.getEndBeat(),
+          pitch: note.getPitch(),
+          velocity: note.getVelocity(),
+        }));
+    }
+
+    if (this.splitNoteRecords.length === 0) {
+      throw new Error('The playhead is not inside any selected note. Move the playhead inside a selected note before splitting.');
+    }
+
+    const splitRecordByOriginalId = new Map(
+      this.splitNoteRecords.map(record => [record.originalNoteId, record])
+    );
+    this.unchangedSelectedNoteIds = selectedNotes
+      .filter(note => !splitRecordByOriginalId.has(note.getId()))
+      .map(note => note.getId());
+
+    const nextNotes: KGMidiNote[] = [];
+    const nextSelectedNotes: KGMidiNote[] = [];
+
+    for (const note of currentNotes) {
+      const splitRecord = splitRecordByOriginalId.get(note.getId());
+      if (!splitRecord) {
+        note.deselect();
+        nextNotes.push(note);
+
+        if (noteIdSet.has(note.getId())) {
+          note.select();
+          nextSelectedNotes.push(note);
+        }
+        continue;
+      }
+
+      const leftNote = new KGMidiNote(
+        splitRecord.leftNoteId,
+        splitRecord.startBeat,
+        this.splitAtBeat,
+        splitRecord.pitch,
+        splitRecord.velocity
+      );
+      const rightNote = new KGMidiNote(
+        splitRecord.rightNoteId,
+        this.splitAtBeat,
+        splitRecord.endBeat,
+        splitRecord.pitch,
+        splitRecord.velocity
+      );
+
+      leftNote.select();
+      rightNote.select();
+      nextNotes.push(leftNote, rightNote);
+      nextSelectedNotes.push(leftNote, rightNote);
+    }
+
+    targetRegion.setNotes(nextNotes);
+    core.clearSelectedItems();
+    core.addSelectedItems(nextSelectedNotes);
+  }
+
+  undo(): void {
+    if (!this.targetRegion) {
+      throw new Error('Cannot undo: split was never executed');
+    }
+
+    const core = KGCore.instance();
+    this.targetRegion.setNotes([...this.originalNotes]);
+    this.originalNotes.forEach(note => note.deselect());
+    this.originalSelectedItems.forEach(item => item.select());
+    core.clearSelectedItems();
+    core.addSelectedItems(this.originalSelectedItems);
+  }
+
+  getDescription(): string {
+    const splitCount = this.splitNoteRecords.length || this.selectedNoteIds.length;
+    return splitCount === 1 ? 'Split note' : `Split ${splitCount} notes`;
+  }
+
+  public getSplitCount(): number {
+    return this.splitNoteRecords.length;
+  }
+
+  public getUnchangedSelectedNoteIds(): string[] {
+    return [...this.unchangedSelectedNoteIds];
+  }
+
+  private resolveTargetRegion(): KGMidiRegion {
+    if (this.targetRegion) {
+      return this.targetRegion;
+    }
+
+    const tracks = KGCore.instance().getCurrentProject().getTracks();
+    for (const track of tracks) {
+      const region = track.getRegions().find(candidate => candidate.getId() === this.regionId);
+      if (region instanceof KGMidiRegion) {
+        this.targetRegion = region;
+        return region;
+      }
+    }
+
+    throw new Error(`MIDI region with ID ${this.regionId} not found`);
+  }
+}

src/core/commands/track/UpdateTrackCommand.test.ts

@@ -0,0 +1,87 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { KGCore } from '../../KGCore';
+import { KGProject } from '../../KGProject';
+import { KGAudioInterface } from '../../audio-interface/KGAudioInterface';
+import { KGTrack } from '../../track/KGTrack';
+import { UpdateTrackCommand } from './UpdateTrackCommand';
+
+vi.mock('../../KGCore', () => ({
+  KGCore: {
+    instance: vi.fn(),
+  },
+}));
+
+vi.mock('../../audio-interface/KGAudioInterface', () => ({
+  KGAudioInterface: {
+    instance: vi.fn(),
+  },
+}));
+
+describe('UpdateTrackCommand', () => {
+  let track: KGTrack;
+  let project: KGProject;
+  const mockCore = {
+    getCurrentProject: vi.fn(),
+  };
+  const mockAudioInterface = {
+    setTrackVolume: vi.fn(),
+    setTrackInstrument: vi.fn(),
+    setTrackMute: vi.fn(),
+    setTrackSolo: vi.fn(),
+  };
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+    track = new KGTrack('Track 1', 1);
+    project = new KGProject('Test', 32, 0, 125, undefined, undefined, undefined, undefined, undefined, 1, [track], 11);
+    mockCore.getCurrentProject.mockReturnValue(project);
+    vi.mocked(KGCore.instance).mockReturnValue(mockCore as unknown as KGCore);
+    vi.mocked(KGAudioInterface.instance).mockReturnValue(mockAudioInterface as unknown as KGAudioInterface);
+  });
+
+  it('updates muted state and propagates to the audio interface', () => {
+    const command = new UpdateTrackCommand(1, { muted: true });
+
+    command.execute();
+
+    expect(track.getMuted()).toBe(true);
+    expect(mockAudioInterface.setTrackMute).toHaveBeenCalledWith('1', true);
+    expect(command.getChangedProperties()).toEqual(new Set(['muted']));
+  });
+
+  it('updates solo state and propagates to the audio interface', () => {
+    const command = new UpdateTrackCommand(1, { solo: true });
+
+    command.execute();
+
+    expect(track.getSolo()).toBe(true);
+    expect(mockAudioInterface.setTrackSolo).toHaveBeenCalledWith('1', true);
+    expect(command.getChangedProperties()).toEqual(new Set(['solo']));
+  });
+
+  it('restores muted and solo state on undo', () => {
+    track.setMuted(true);
+    track.setSolo(true);
+    const command = new UpdateTrackCommand(1, { muted: false, solo: false });
+
+    command.execute();
+    command.undo();
+
+    expect(track.getMuted()).toBe(true);
+    expect(track.getSolo()).toBe(true);
+    expect(mockAudioInterface.setTrackMute).toHaveBeenLastCalledWith('1', true);
+    expect(mockAudioInterface.setTrackSolo).toHaveBeenLastCalledWith('1', true);
+  });
+
+  it('treats unchanged mute and solo values as no-ops', () => {
+    const command = new UpdateTrackCommand(1, { muted: false, solo: false });
+
+    command.execute();
+
+    expect(track.getMuted()).toBe(false);
+    expect(track.getSolo()).toBe(false);
+    expect(mockAudioInterface.setTrackMute).not.toHaveBeenCalled();
+    expect(mockAudioInterface.setTrackSolo).not.toHaveBeenCalled();
+    expect(command.getChangedProperties()).toEqual(new Set());
+  });
+});

src/core/commands/track/UpdateTrackCommand.ts

@@ -12,6 +12,8 @@ export interface TrackUpdateProperties {
   instrument?: InstrumentType; // Only applies to MIDI tracks
   type?: TrackType;
   volume?: number;
+  muted?: boolean;
+  solo?: boolean;
 }
 
 /**
@@ -47,6 +49,8 @@ export class UpdateTrackCommand extends KGCommand {
       name: this.targetTrack.getName(),
       type: this.targetTrack.getType(),
       volume: this.targetTrack.getVolume(),
+      muted: this.targetTrack.getMuted(),
+      solo: this.targetTrack.getSolo(),
     };
 
     // Store original instrument if it's a MIDI track
@@ -103,6 +107,32 @@ export class UpdateTrackCommand extends KGCommand {
       updatedProperties.push(`volume: ${originalVolume} → ${newVolume}`);
     }
 
+    if (this.newProperties.muted !== undefined && this.newProperties.muted !== this.originalProperties.muted) {
+      const newMuted = this.newProperties.muted;
+      const originalMuted = this.originalProperties.muted;
+
+      this.targetTrack.setMuted(newMuted);
+
+      const audioInterface = KGAudioInterface.instance();
+      audioInterface.setTrackMute(this.trackId.toString(), newMuted);
+
+      this.changedProperties.add('muted');
+      updatedProperties.push(`muted: ${originalMuted} → ${newMuted}`);
+    }
+
+    if (this.newProperties.solo !== undefined && this.newProperties.solo !== this.originalProperties.solo) {
+      const newSolo = this.newProperties.solo;
+      const originalSolo = this.originalProperties.solo;
+
+      this.targetTrack.setSolo(newSolo);
+
+      const audioInterface = KGAudioInterface.instance();
+      audioInterface.setTrackSolo(this.trackId.toString(), newSolo);
+
+      this.changedProperties.add('solo');
+      updatedProperties.push(`solo: ${originalSolo} → ${newSolo}`);
+    }
+
     if (updatedProperties.length > 0) {
       console.log(`Updated track ${this.trackId}: ${updatedProperties.join(', ')}`);
     } else {
@@ -156,6 +186,24 @@ export class UpdateTrackCommand extends KGCommand {
       restoredProperties.push(`volume: ${this.originalProperties.volume}`);
     }
 
+    if (this.changedProperties.has('muted') && this.originalProperties.muted !== undefined) {
+      this.targetTrack.setMuted(this.originalProperties.muted);
+
+      const audioInterface = KGAudioInterface.instance();
+      audioInterface.setTrackMute(this.trackId.toString(), this.originalProperties.muted);
+
+      restoredProperties.push(`muted: ${this.originalProperties.muted}`);
+    }
+
+    if (this.changedProperties.has('solo') && this.originalProperties.solo !== undefined) {
+      this.targetTrack.setSolo(this.originalProperties.solo);
+
+      const audioInterface = KGAudioInterface.instance();
+      audioInterface.setTrackSolo(this.trackId.toString(), this.originalProperties.solo);
+
+      restoredProperties.push(`solo: ${this.originalProperties.solo}`);
+    }
+
     console.log(`Restored track ${this.trackId}: ${restoredProperties.join(', ')}`);
   }
 
@@ -175,6 +223,12 @@ export class UpdateTrackCommand extends KGCommand {
     if (this.newProperties.volume !== undefined) {
       updatedProps.push('volume');
     }
+    if (this.newProperties.muted !== undefined) {
+      updatedProps.push('muted');
+    }
+    if (this.newProperties.solo !== undefined) {
+      updatedProps.push('solo');
+    }
 
     if (updatedProps.length === 1) {
       return `Update track "${trackName}" ${updatedProps[0]}`;
@@ -219,4 +273,4 @@ export class UpdateTrackCommand extends KGCommand {
   public getChangedProperties(): Set<keyof TrackUpdateProperties> {
     return new Set(this.changedProperties);
   }
-}
+}

src/core/config-upgrader/KGConfigUpgrader.ts

@@ -3,6 +3,7 @@ import { CONFIG_UPGRADER_CONSTANTS } from '../../constants/coreConstants';
 import { upgradeConfigToV1 } from './upgradeConfigToV1';
 import { upgradeConfigToV2 } from './upgradeConfigToV2';
 import { upgradeConfigToV3 } from './upgradeConfigToV3';
+import { upgradeConfigToV4 } from './upgradeConfigToV4';
 
 /**
  * KGConfigUpgrader — Orchestrates app-level migrations (e.g., storage backend changes).
@@ -43,6 +44,10 @@ export class KGConfigUpgrader {
           await upgradeConfigToV3();
           break;
         }
+        case 4: {
+          await upgradeConfigToV4();
+          break;
+        }
         default: {
           throw new Error(`No config upgrader found for version ${nextVersion}`);
         }

src/core/config-upgrader/upgradeConfigToV4.test.ts

@@ -0,0 +1,64 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+const getRawMock = vi.fn();
+const saveRawMock = vi.fn();
+
+vi.mock('../io/KGConfigStorage', () => ({
+  KGConfigStorage: {
+    getInstance: vi.fn(() => ({
+      getRaw: getRawMock,
+      saveRaw: saveRawMock,
+    })),
+  },
+}));
+
+import { upgradeConfigToV4 } from './upgradeConfigToV4';
+
+describe('upgradeConfigToV4', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it('creates the audio config and defaults bounce_starts_from_beat_1 to true when audio is missing', async () => {
+    const config: Record<string, unknown> = {
+      general: {},
+    };
+    getRawMock.mockResolvedValue(config);
+
+    await upgradeConfigToV4();
+
+    expect(config.audio).toEqual({ bounce_starts_from_beat_1: true });
+    expect(saveRawMock).toHaveBeenCalledWith('userConfig', config);
+  });
+
+  it('defaults bounce_starts_from_beat_1 to true when the key is missing', async () => {
+    const config: Record<string, unknown> = {
+      audio: {
+        playback_delay: 0.2,
+      },
+    };
+    getRawMock.mockResolvedValue(config);
+
+    await upgradeConfigToV4();
+
+    expect(config.audio).toEqual({
+      playback_delay: 0.2,
+      bounce_starts_from_beat_1: true,
+    });
+    expect(saveRawMock).toHaveBeenCalledWith('userConfig', config);
+  });
+
+  it('preserves an explicit false value', async () => {
+    const config: Record<string, unknown> = {
+      audio: {
+        bounce_starts_from_beat_1: false,
+      },
+    };
+    getRawMock.mockResolvedValue(config);
+
+    await upgradeConfigToV4();
+
+    expect(saveRawMock).not.toHaveBeenCalled();
+    expect((config.audio as Record<string, unknown>).bounce_starts_from_beat_1).toBe(false);
+  });
+});

src/core/config-upgrader/upgradeConfigToV4.ts

@@ -0,0 +1,30 @@
+import { KGConfigStorage } from '../io/KGConfigStorage';
+
+const CONFIG_KEY = 'userConfig';
+
+export async function upgradeConfigToV4(): Promise<void> {
+  const storage = KGConfigStorage.getInstance();
+  const rawConfig = await storage.getRaw(CONFIG_KEY);
+  if (!rawConfig || typeof rawConfig !== 'object') {
+    return;
+  }
+
+  const config = rawConfig as Record<string, unknown>;
+  const audio = config.audio;
+
+  if (!audio || typeof audio !== 'object') {
+    config.audio = {
+      bounce_starts_from_beat_1: true,
+    };
+    await storage.saveRaw(CONFIG_KEY, config);
+    return;
+  }
+
+  const audioRecord = audio as Record<string, unknown>;
+  if ('bounce_starts_from_beat_1' in audioRecord) {
+    return;
+  }
+
+  audioRecord.bounce_starts_from_beat_1 = true;
+  await storage.saveRaw(CONFIG_KEY, config);
+}

src/core/config/ConfigManager.ts

@@ -10,6 +10,10 @@ interface AppConfig {
     persist_api_keys_non_localhost: boolean;
     local_browser: {
       context_length: 32768 | 65536 | 131072;
+      model_url: string;
+    };
+    uvr5_web_runtime: {
+      mdx_net_model_url: string;
     };
     openai: {
       api_key: string;
@@ -47,6 +51,7 @@ interface AppConfig {
       hold_to_create_region: string;
       play: string;
       loop: string;
+      record: string;
       undo: string;
       redo: string;
       select_all: string;
@@ -54,6 +59,8 @@ interface AppConfig {
       cut: string;
       paste: string;
       save: string;
+      split_region: string;
+      merge_regions: string;
     };
     piano_roll: {
       switch: string;
@@ -80,6 +87,7 @@ interface AppConfig {
     default_open: boolean;
   };
   audio: {
+    bounce_starts_from_beat_1: boolean;
     enable_audio_capture_for_screen_sharing: boolean;
     input_device_id: string;
     lookahead_time: number;
@@ -212,7 +220,11 @@ export class ConfigManager {
             model: ''
           },
           local_browser: {
-            context_length: 32768
+            context_length: 32768,
+            model_url: 'https://huggingface.co/notabilia/gemma-4-E4B-it-litert-lm/resolve/main/gemma-4-E4B-it-web.task'
+          },
+          uvr5_web_runtime: {
+            mdx_net_model_url: 'https://huggingface.co/notabilia/uvr5-models/resolve/main/UVR-MDX-NET-Inst_HQ_3.onnx'
           },
           soundfont: {
             base_url: 'https://cdn.jsdelivr.net/npm/soundfont-for-samplers/FluidR3_GM/'
@@ -227,13 +239,16 @@ export class ConfigManager {
             hold_to_create_region: 'ctrl',
             play: 'space',
             loop: 'c',
+            record: 'r',
             undo: 'ctrl+z',
             redo: 'ctrl+shift+z',
             select_all: 'ctrl+a',
             copy: 'ctrl+c',
             cut: 'ctrl+x',
             paste: 'ctrl+v',
-            save: 'ctrl+s'
+            save: 'ctrl+s',
+            split_region: 'ctrl+t',
+            merge_regions: 'ctrl+j'
           },
           piano_roll: {
             switch: 'tab', 
@@ -260,6 +275,7 @@ export class ConfigManager {
           default_open: true
         },
         audio: {
+          bounce_starts_from_beat_1: true,
           enable_audio_capture_for_screen_sharing: false,
           input_device_id: 'default',
           lookahead_time: 0.05,

src/core/io/KGProjectStorage.test.ts

@@ -1,6 +1,7 @@
 import { describe, it, expect, beforeEach, vi } from 'vitest';
 import { KGProjectStorage, DuplicateEntryError } from './KGProjectStorage';
 import { KGProject } from '../KGProject';
+import { KGTrack } from '../track/KGTrack';
 
 // --- OPFS mock infrastructure ---
 
@@ -104,6 +105,13 @@ describe('KGProjectStorage', () => {
     return new KGProject(name, 16, 0, 120);
   }
 
+  it('defaults both zoom levels to 1 on a fresh project', () => {
+    const project = new KGProject();
+
+    expect(project.getBarWidthMultiplier()).toBe(1);
+    expect(project.getPianoRollZoom()).toBe(1);
+  });
+
   it('initializes and creates the projects directory', async () => {
     // The projects directory should exist after init
     const projects = await mockRoot.getDirectoryHandle('projects');
@@ -121,6 +129,36 @@ describe('KGProjectStorage', () => {
     expect(loaded!.getBpm()).toBe(120);
   });
 
+  it('preserves track mute and solo state when saving and loading', async () => {
+    const track = new KGTrack('Track 1', 1);
+    track.setMuted(true);
+    track.setSolo(true);
+    const project = new KGProject('My Song', 16, 0, 120, undefined, undefined, undefined, undefined, undefined, 1, [track], 11);
+
+    await storage.save('My Song', project);
+
+    const loaded = await storage.load('My Song');
+
+    expect(loaded).not.toBeNull();
+    expect(loaded!.getTracks()).toHaveLength(1);
+    expect(loaded!.getTracks()[0].getMuted()).toBe(true);
+    expect(loaded!.getTracks()[0].getSolo()).toBe(true);
+  });
+
+  it('preserves both main-grid and piano-roll zoom levels when saving and loading', async () => {
+    const project = createTestProject('Zoom Song');
+    project.setBarWidthMultiplier(3);
+    project.setPianoRollZoom(5);
+
+    await storage.save('Zoom Song', project);
+
+    const loaded = await storage.load('Zoom Song');
+
+    expect(loaded).not.toBeNull();
+    expect(loaded!.getBarWidthMultiplier()).toBe(3);
+    expect(loaded!.getPianoRollZoom()).toBe(5);
+  });
+
   it('creates meta.json and media/ directory on save', async () => {
     const project = createTestProject('My Song');
     await storage.save('My Song', project);

src/core/io/LocalSeparatorModelCache.test.ts

@@ -1,5 +1,9 @@
 import { beforeEach, describe, expect, it, vi } from 'vitest';
 import { LocalSeparatorModelCache } from '../../util/localSeparatorModelCache';
+import {
+  LOCAL_SEPARATOR_MODEL_EXPECTED_SIZE_BYTES,
+  LOCAL_SEPARATOR_MODEL_FILENAME,
+} from '../../util/localSeparatorConfig';
 
 class MockWritableFileStream {
   private readonly handle: MockFileSystemFileHandle;
@@ -109,47 +113,81 @@ vi.stubGlobal('navigator', {
 });
 
 describe('LocalSeparatorModelCache', () => {
+  const makeModelBytes = (fill: number): Uint8Array => new Uint8Array(LOCAL_SEPARATOR_MODEL_EXPECTED_SIZE_BYTES).fill(fill);
+
   beforeEach(() => {
     mockRoot.clear();
     vi.restoreAllMocks();
   });
 
   it('downloads and stores a model in OPFS cache', async () => {
+    const bytes = makeModelBytes(1);
+    vi.stubGlobal('fetch', vi.fn(async () => new Response(bytes, {
+      status: 200,
+      headers: { 'Content-Length': String(LOCAL_SEPARATOR_MODEL_EXPECTED_SIZE_BYTES) },
+    })));
+
+    await LocalSeparatorModelCache.download('https://example.com/model.onnx');
+
+    expect(await LocalSeparatorModelCache.exists()).toBe(true);
+    const buffer = await LocalSeparatorModelCache.getArrayBuffer();
+    expect(buffer.byteLength).toBe(LOCAL_SEPARATOR_MODEL_EXPECTED_SIZE_BYTES);
+    expect(new Uint8Array(buffer)[0]).toBe(1);
+  });
+
+  it('replaces a broken cached file on redownload', async () => {
+    vi.stubGlobal('fetch', vi.fn(async () => new Response(makeModelBytes(1), {
+      status: 200,
+      headers: { 'Content-Length': String(LOCAL_SEPARATOR_MODEL_EXPECTED_SIZE_BYTES) },
+    })));
+    await LocalSeparatorModelCache.download('https://example.com/model.onnx');
+
+    vi.stubGlobal('fetch', vi.fn(async () => new Response(makeModelBytes(9), {
+      status: 200,
+      headers: { 'Content-Length': String(LOCAL_SEPARATOR_MODEL_EXPECTED_SIZE_BYTES) },
+    })));
+    await LocalSeparatorModelCache.download('https://example.com/model.onnx');
+
+    const buffer = await LocalSeparatorModelCache.getArrayBuffer();
+    expect(buffer.byteLength).toBe(LOCAL_SEPARATOR_MODEL_EXPECTED_SIZE_BYTES);
+    expect(new Uint8Array(buffer)[0]).toBe(9);
+  });
+
+  it('deletes the cached model file', async () => {
+    vi.stubGlobal('fetch', vi.fn(async () => new Response(makeModelBytes(2), {
+      status: 200,
+      headers: { 'Content-Length': String(LOCAL_SEPARATOR_MODEL_EXPECTED_SIZE_BYTES) },
+    })));
+    await LocalSeparatorModelCache.download('https://example.com/model.onnx');
+
+    await LocalSeparatorModelCache.delete();
+
+    expect(await LocalSeparatorModelCache.exists()).toBe(false);
+  });
+
+  it('rejects and deletes a cached file when the size is wrong', async () => {
+    const dir = await navigator.storage.getDirectory();
+    const modelsDir = await dir.getDirectoryHandle('models', { create: true });
+    const fileHandle = await modelsDir.getFileHandle(LOCAL_SEPARATOR_MODEL_FILENAME, { create: true });
+    const fileWritable = await fileHandle.createWritable();
+    await fileWritable.write(new Uint8Array([1, 2, 3]));
+    await fileWritable.close();
+
+    const sizeHandle = await modelsDir.getFileHandle(`${LOCAL_SEPARATOR_MODEL_FILENAME}.size`, { create: true });
+    const sizeWritable = await sizeHandle.createWritable();
+    await sizeWritable.write(String(3));
+    await sizeWritable.close();
+
+    expect(await LocalSeparatorModelCache.exists()).toBe(false);
+  });
+
+  it('fails a download when the final size does not match the expected model size', async () => {
     vi.stubGlobal('fetch', vi.fn(async () => new Response(new Uint8Array([1, 2, 3]), {
       status: 200,
       headers: { 'Content-Length': '3' },
     })));
 
-    await LocalSeparatorModelCache.download('https://example.com/model.onnx', 'model.onnx');
-
-    expect(await LocalSeparatorModelCache.exists('model.onnx')).toBe(true);
-    const buffer = await LocalSeparatorModelCache.getArrayBuffer('model.onnx');
-    expect(Array.from(new Uint8Array(buffer))).toEqual([1, 2, 3]);
-  });
-
-  it('replaces a broken cached file on redownload', async () => {
-    vi.stubGlobal('fetch', vi.fn(async () => new Response(new Uint8Array([1]), {
-      status: 200,
-      headers: { 'Content-Length': '1' },
-    })));
-    await LocalSeparatorModelCache.download('https://example.com/model.onnx', 'model.onnx');
-
-    vi.stubGlobal('fetch', vi.fn(async () => new Response(new Uint8Array([9, 8, 7, 6]), {
-      status: 200,
-      headers: { 'Content-Length': '4' },
-    })));
-    await LocalSeparatorModelCache.download('https://example.com/model.onnx', 'model.onnx');
-
-    const buffer = await LocalSeparatorModelCache.getArrayBuffer('model.onnx');
-    expect(Array.from(new Uint8Array(buffer))).toEqual([9, 8, 7, 6]);
-  });
-
-  it('deletes the cached model file', async () => {
-    vi.stubGlobal('fetch', vi.fn(async () => new Response(new Uint8Array([1, 2]), { status: 200 })));
-    await LocalSeparatorModelCache.download('https://example.com/model.onnx', 'model.onnx');
-
-    await LocalSeparatorModelCache.delete('model.onnx');
-
-    expect(await LocalSeparatorModelCache.exists('model.onnx')).toBe(false);
+    await expect(LocalSeparatorModelCache.download('https://example.com/model.onnx')).rejects.toThrow(/size mismatch/i);
+    expect(await LocalSeparatorModelCache.exists()).toBe(false);
   });
 });

src/core/midi-input/KGMidiInput.test.ts

@@ -2,6 +2,9 @@ import { beforeEach, describe, expect, it, vi } from 'vitest';
 import { KGAudioTrack } from '../track/KGAudioTrack';
 import { KGMidiTrack } from '../track/KGMidiTrack';
 
+type TestMidiEvent = { data: Uint8Array };
+type TestLiveNoteActivityListener = (...args: [{ pitch: number; isNoteOn: boolean }]) => void;
+
 const { getStateMock, audioInterfaceMock } = vi.hoisted(() => ({
   getStateMock: vi.fn(),
   audioInterfaceMock: {
@@ -45,19 +48,40 @@ describe('KGMidiInput pitch bend', () => {
 
   it('routes live MIDI note on/off through the live monitoring path', () => {
     const midiInput = KGMidiInput.instance() as unknown as {
-      handleMIDIMessage: (event: { data: Uint8Array }) => void;
+      handleMIDIMessage: (...args: [TestMidiEvent]) => void;
+      addLiveNoteActivityListener: (...args: [TestLiveNoteActivityListener]) => void;
     };
+    const listener = vi.fn();
+
+    midiInput.addLiveNoteActivityListener(listener);
 
     midiInput.handleMIDIMessage({ data: new Uint8Array([0x90, 60, 100]) });
     midiInput.handleMIDIMessage({ data: new Uint8Array([0x80, 60, 0]) });
 
     expect(audioInterfaceMock.triggerLiveMidiNoteAttack).toHaveBeenCalledWith('1', 60, 100);
     expect(audioInterfaceMock.releaseLiveMidiNote).toHaveBeenCalledWith('1', 60);
+    expect(listener).toHaveBeenNthCalledWith(1, { pitch: 60, isNoteOn: true });
+    expect(listener).toHaveBeenNthCalledWith(2, { pitch: 60, isNoteOn: false });
+  });
+
+  it('stops notifying removed live note activity listeners', () => {
+    const midiInput = KGMidiInput.instance() as unknown as {
+      handleMIDIMessage: (...args: [TestMidiEvent]) => void;
+      addLiveNoteActivityListener: (...args: [TestLiveNoteActivityListener]) => void;
+      removeLiveNoteActivityListener: (...args: [TestLiveNoteActivityListener]) => void;
+    };
+    const listener = vi.fn();
+
+    midiInput.addLiveNoteActivityListener(listener);
+    midiInput.removeLiveNoteActivityListener(listener);
+    midiInput.handleMIDIMessage({ data: new Uint8Array([0x90, 60, 100]) });
+
+    expect(listener).not.toHaveBeenCalled();
   });
 
   it('latches live note ownership to the note-on track', () => {
     const midiInput = KGMidiInput.instance() as unknown as {
-      handleMIDIMessage: (event: { data: Uint8Array }) => void;
+      handleMIDIMessage: (...args: [TestMidiEvent]) => void;
     };
 
     midiInput.handleMIDIMessage({ data: new Uint8Array([0x90, 60, 100]) });
@@ -73,7 +97,7 @@ describe('KGMidiInput pitch bend', () => {
 
   it('normalizes MIDI pitch bend and forwards it to the selected track', () => {
     const midiInput = KGMidiInput.instance() as unknown as {
-      handleMIDIMessage: (event: { data: Uint8Array }) => void;
+      handleMIDIMessage: (...args: [TestMidiEvent]) => void;
     };
 
     midiInput.handleMIDIMessage({ data: new Uint8Array([0xe0, 0x00, 0x40]) });
@@ -87,7 +111,7 @@ describe('KGMidiInput pitch bend', () => {
 
   it('maps supported CC messages to live expression and sustain for standard pedals', () => {
     const midiInput = KGMidiInput.instance() as unknown as {
-      handleMIDIMessage: (event: { data: Uint8Array }) => void;
+      handleMIDIMessage: (...args: [TestMidiEvent]) => void;
     };
 
     midiInput.handleMIDIMessage({ data: new Uint8Array([0xb0, 0x01, 0x20]) });
@@ -106,7 +130,7 @@ describe('KGMidiInput pitch bend', () => {
 
   it('calibrates inverted sustain pedals from the first observed CC64 message', () => {
     const midiInput = KGMidiInput.instance() as unknown as {
-      handleMIDIMessage: (event: { data: Uint8Array }) => void;
+      handleMIDIMessage: (...args: [TestMidiEvent]) => void;
     };
 
     midiInput.handleMIDIMessage({ data: new Uint8Array([0xb0, 0x40, 0x00]) });
@@ -123,7 +147,7 @@ describe('KGMidiInput pitch bend', () => {
     });
 
     const midiInput = KGMidiInput.instance() as unknown as {
-      handleMIDIMessage: (event: { data: Uint8Array }) => void;
+      handleMIDIMessage: (...args: [TestMidiEvent]) => void;
     };
 
     midiInput.handleMIDIMessage({ data: new Uint8Array([0x90, 60, 100]) });

src/core/midi-input/KGMidiInput.ts

@@ -2,6 +2,13 @@ import { KGAudioInterface } from '../audio-interface/KGAudioInterface';
 import { useProjectStore } from '../../stores/projectStore';
 import { KGMidiTrack } from '../track/KGMidiTrack';
 
+export interface LiveMidiNoteActivityEvent {
+  pitch: number;
+  isNoteOn: boolean;
+}
+
+type LiveNoteActivityListener = (...args: [LiveMidiNoteActivityEvent]) => void;
+
 /**
  * KGMidiInput - MIDI input manager for the DAW
  * Implements the singleton pattern for global MIDI device management
@@ -32,6 +39,7 @@ export class KGMidiInput {
   private onRecordControlChange: ((controller: number, value: number) => void) | null = null;
   private liveNoteTrackOwnership: Map<number, string[]> = new Map();
   private sustainPolarityInverted: boolean | null = null;
+  private liveNoteActivityListeners: LiveNoteActivityListener[] = [];
 
   // Private constructor to prevent direct instantiation
   private constructor() {
@@ -178,12 +186,14 @@ export class KGMidiInput {
     // Note On: command = 0x90 (144)
     if (command === 0x90 && velocity > 0) {
       console.log(`MIDI Note On: pitch=${pitch}, velocity=${velocity}, channel=${channel}`);
+      this.emitLiveNoteActivity({ pitch, isNoteOn: true });
       this.triggerNoteOn(pitch, velocity);
       this.onRecordNoteOn?.(pitch, velocity);
     }
     // Note Off: command = 0x80 (128) or Note On with velocity 0
     else if (command === 0x80 || (command === 0x90 && velocity === 0)) {
       console.log(`MIDI Note Off: pitch=${pitch}, channel=${channel}`);
+      this.emitLiveNoteActivity({ pitch, isNoteOn: false });
       this.triggerNoteOff(pitch);
       this.onRecordNoteOff?.(pitch);
     }
@@ -354,6 +364,16 @@ export class KGMidiInput {
     return this.sustainPolarityInverted ? !rawPressed : rawPressed;
   }
 
+  private emitLiveNoteActivity(event: LiveMidiNoteActivityEvent): void {
+    for (const listener of this.liveNoteActivityListeners) {
+      try {
+        listener(event);
+      } catch {
+        // Swallow listener errors to avoid disrupting MIDI handling.
+      }
+    }
+  }
+
   /**
    * Clean up MIDI resources
    */
@@ -374,6 +394,7 @@ export class KGMidiInput {
       this.isInitialized = false;
       this.liveNoteTrackOwnership.clear();
       this.sustainPolarityInverted = null;
+      this.liveNoteActivityListeners = [];
 
       console.log("MIDI resources disposed successfully");
     } catch (error) {
@@ -395,6 +416,14 @@ export class KGMidiInput {
     this.onRecordControlChange = onControlChange;
   }
 
+  public addLiveNoteActivityListener(listener: LiveNoteActivityListener): void {
+    this.liveNoteActivityListeners.push(listener);
+  }
+
+  public removeLiveNoteActivityListener(listener: LiveNoteActivityListener): void {
+    this.liveNoteActivityListeners = this.liveNoteActivityListeners.filter(current => current !== listener);
+  }
+
   // ===== GETTERS =====
 
   public getIsInitialized(): boolean {

src/core/project-upgrader/KGProjectUpgrader.ts

@@ -9,6 +9,8 @@ import { upgradeToV7 } from './upgradeToV7';
 import { upgradeToV8 } from './upgradeToV8';
 import { upgradeToV9 } from './upgradeToV9';
 import { upgradeToV10 } from './upgradeToV10';
+import { upgradeToV11 } from './upgradeToV11';
+import { upgradeToV12 } from './upgradeToV12';
 
 /**
  * Upgrade the given project to the latest structure version, one version at a time.
@@ -68,6 +70,14 @@ export function upgradeProjectToLatest(project: KGProject): KGProject {
         workingProject = upgradeToV10(workingProject);
         break;
       }
+      case 11: {
+        workingProject = upgradeToV11(workingProject);
+        break;
+      }
+      case 12: {
+        workingProject = upgradeToV12(workingProject);
+        break;
+      }
       default: {
         // If an upgrader is missing, throw to prevent loading incompatible structures
         throw new Error(`No upgrader found for project structure version ${nextVersion}`);

src/core/project-upgrader/upgradeToV10.test.ts

@@ -26,7 +26,7 @@ describe('upgradeToV10', () => {
 
     const upgraded = upgradeProjectToLatest(project);
 
-    expect(upgraded.getProjectStructureVersion()).toBe(10);
+    expect(upgraded.getProjectStructureVersion()).toBe(KGProject.CURRENT_PROJECT_STRUCTURE_VERSION);
     expect(upgraded.getTracks()[0].getVolumeAutomation()).toHaveLength(1);
   });
 });

src/core/project-upgrader/upgradeToV11.test.ts

@@ -0,0 +1,45 @@
+import { describe, expect, it } from 'vitest';
+import { KGProject } from '../KGProject';
+import { KGTrack } from '../track/KGTrack';
+import { upgradeProjectToLatest } from './KGProjectUpgrader';
+import { upgradeToV11 } from './upgradeToV11';
+
+describe('upgradeToV11', () => {
+  it('initializes missing mute and solo flags on legacy tracks', () => {
+    const track = new KGTrack('Legacy Track', 1);
+    delete (track as unknown as { muted?: unknown }).muted;
+    delete (track as unknown as { solo?: unknown }).solo;
+    const project = new KGProject('Test', 32, 0, 125, undefined, undefined, undefined, undefined, undefined, 1, [track], 10);
+
+    upgradeToV11(project);
+
+    expect(track.getMuted()).toBe(false);
+    expect(track.getSolo()).toBe(false);
+    expect(project.getProjectStructureVersion()).toBe(11);
+  });
+
+  it('preserves existing mute and solo flags', () => {
+    const track = new KGTrack('Legacy Track', 1);
+    track.setMuted(true);
+    track.setSolo(true);
+    const project = new KGProject('Test', 32, 0, 125, undefined, undefined, undefined, undefined, undefined, 1, [track], 10);
+
+    upgradeToV11(project);
+
+    expect(track.getMuted()).toBe(true);
+    expect(track.getSolo()).toBe(true);
+  });
+
+  it('upgrades legacy projects through the main upgrader path', () => {
+    const track = new KGTrack('Legacy Track', 1);
+    delete (track as unknown as { muted?: unknown }).muted;
+    delete (track as unknown as { solo?: unknown }).solo;
+    const project = new KGProject('Test', 32, 0, 125, undefined, undefined, undefined, undefined, undefined, 1, [track], 10);
+
+    const upgraded = upgradeProjectToLatest(project);
+
+    expect(upgraded.getProjectStructureVersion()).toBe(KGProject.CURRENT_PROJECT_STRUCTURE_VERSION);
+    expect(upgraded.getTracks()[0].getMuted()).toBe(false);
+    expect(upgraded.getTracks()[0].getSolo()).toBe(false);
+  });
+});

src/core/project-upgrader/upgradeToV11.ts

@@ -0,0 +1,21 @@
+import { KGProject } from '../KGProject';
+
+export function upgradeToV11(project: KGProject): KGProject {
+  try {
+    for (const track of project.getTracks()) {
+      const muted = (track as unknown as { muted?: unknown }).muted;
+      if (typeof muted !== 'boolean') {
+        track.setMuted(false);
+      }
+
+      const solo = (track as unknown as { solo?: unknown }).solo;
+      if (typeof solo !== 'boolean') {
+        track.setSolo(false);
+      }
+    }
+  } finally {
+    project.setProjectStructureVersion(11);
+  }
+
+  return project;
+}

src/core/project-upgrader/upgradeToV12.test.ts

@@ -0,0 +1,39 @@
+import { describe, expect, it } from 'vitest';
+import { KGProject } from '../KGProject';
+import { upgradeProjectToLatest } from './KGProjectUpgrader';
+import { upgradeToV12 } from './upgradeToV12';
+
+describe('upgradeToV12', () => {
+  it('initializes missing zoom levels to 1', () => {
+    const project = new KGProject('Legacy', 32, 0, 125, undefined, undefined, undefined, undefined, [0, 0], 1, [], 11);
+    delete (project as unknown as { barWidthMultiplier?: unknown }).barWidthMultiplier;
+    delete (project as unknown as { pianoRollZoom?: unknown }).pianoRollZoom;
+
+    upgradeToV12(project);
+
+    expect(project.getBarWidthMultiplier()).toBe(1);
+    expect(project.getPianoRollZoom()).toBe(1);
+    expect(project.getProjectStructureVersion()).toBe(12);
+  });
+
+  it('preserves existing zoom levels', () => {
+    const project = new KGProject('Legacy', 32, 0, 125, undefined, undefined, undefined, undefined, [0, 0], 4, [], 11, 6);
+
+    upgradeToV12(project);
+
+    expect(project.getBarWidthMultiplier()).toBe(4);
+    expect(project.getPianoRollZoom()).toBe(6);
+  });
+
+  it('upgrades legacy projects through the main upgrader path', () => {
+    const project = new KGProject('Legacy', 32, 0, 125, undefined, undefined, undefined, undefined, [0, 0], 1, [], 11);
+    delete (project as unknown as { barWidthMultiplier?: unknown }).barWidthMultiplier;
+    delete (project as unknown as { pianoRollZoom?: unknown }).pianoRollZoom;
+
+    const upgraded = upgradeProjectToLatest(project);
+
+    expect(upgraded.getProjectStructureVersion()).toBe(KGProject.CURRENT_PROJECT_STRUCTURE_VERSION);
+    expect(upgraded.getBarWidthMultiplier()).toBe(1);
+    expect(upgraded.getPianoRollZoom()).toBe(1);
+  });
+});

src/core/project-upgrader/upgradeToV12.ts

@@ -0,0 +1,23 @@
+import { KGProject } from '../KGProject';
+
+function isValidZoomLevel(value: unknown): value is number {
+  return typeof value === 'number' && Number.isFinite(value) && value > 0;
+}
+
+export function upgradeToV12(project: KGProject): KGProject {
+  try {
+    const barWidthMultiplier = project.getBarWidthMultiplier?.();
+    if (!isValidZoomLevel(barWidthMultiplier)) {
+      project.setBarWidthMultiplier(1);
+    }
+
+    const pianoRollZoom = project.getPianoRollZoom?.();
+    if (!isValidZoomLevel(pianoRollZoom)) {
+      project.setPianoRollZoom(1);
+    }
+  } finally {
+    project.setProjectStructureVersion(12);
+  }
+
+  return project;
+}

src/core/project-upgrader/upgradeToV8.test.ts

@@ -57,7 +57,7 @@ describe('upgradeToV8', () => {
 
     const upgraded = upgradeProjectToLatest(project);
 
-    expect(upgraded.getProjectStructureVersion()).toBe(10);
+    expect(upgraded.getProjectStructureVersion()).toBe(KGProject.CURRENT_PROJECT_STRUCTURE_VERSION);
     expect((upgraded.getTracks()[0].getRegions()[0] as KGMidiRegion).getPitchBends()).toEqual([]);
     expect((upgraded.getTracks()[0].getRegions()[0] as KGMidiRegion).getControllerEventsByType()).toHaveLength(128);
   });

src/core/state/KGPianoRollState.ts

@@ -15,6 +15,7 @@ export class KGPianoRollState {
   private currentMode: string = "ionian"; // Default mode
   private automationViewEnabled: boolean = false;
   private currentAutomationType: string = "pitch-bend";
+  private pianoRollZoom: number = 1;
   private sheetMusicViewEnabled: boolean = false;
   private sheetMusicTrackScopeEnabled: boolean = false;
   private sheetQuantization: string = '16,48';
@@ -86,6 +87,14 @@ export class KGPianoRollState {
     this.currentAutomationType = type;
   }
 
+  public getPianoRollZoom(): number {
+    return this.pianoRollZoom;
+  }
+
+  public setPianoRollZoom(zoom: number): void {
+    this.pianoRollZoom = zoom;
+  }
+
   public getSheetMusicViewEnabled(): boolean {
     return this.sheetMusicViewEnabled;
   }

src/core/track/KGTrack.ts

@@ -37,6 +37,14 @@ export class KGTrack {
   @Expose()
   @WithDefault(AUDIO_INTERFACE_CONSTANTS.DEFAULT_TRACK_VOLUME)
   protected volume: number = AUDIO_INTERFACE_CONSTANTS.DEFAULT_TRACK_VOLUME;
+
+  @Expose()
+  @WithDefault(false)
+  protected muted: boolean = false;
+
+  @Expose()
+  @WithDefault(false)
+  protected solo: boolean = false;
   
   @Expose()
   @Type(() => KGRegion, {
@@ -91,6 +99,14 @@ export class KGTrack {
     return this.volume;
   }
 
+  public getMuted(): boolean {
+    return this.muted;
+  }
+
+  public getSolo(): boolean {
+    return this.solo;
+  }
+
   // Setters  
   public setName(name: string): void {
     this.name = name;
@@ -120,6 +136,14 @@ export class KGTrack {
     );
   }
 
+  public setMuted(muted: boolean): void {
+    this.muted = muted;
+  }
+
+  public setSolo(solo: boolean): void {
+    this.solo = solo;
+  }
+
   public setRegions(regions: KGRegion[]): void {
     this.regions = regions;
   }

src/hooks/useGlobalKeyboardHandler.test.tsx

@@ -0,0 +1,280 @@
+import React from 'react';
+import { fireEvent, render, waitFor } from '@testing-library/react';
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { useGlobalKeyboardHandler } from './useGlobalKeyboardHandler';
+import { showAlert } from '../util/dialogUtil';
+
+const regionEditUtilMocks = vi.hoisted(() => ({
+  splitSelectedRegionAtPlayhead: vi.fn(),
+  mergeSelectedMidiRegions: vi.fn(),
+}));
+
+const MockMidiRegion = vi.hoisted(() => class {
+  private readonly id: string;
+
+  constructor(id: string) {
+    this.id = id;
+  }
+
+  getId() {
+    return this.id;
+  }
+});
+
+let mockTracks: Array<{ getRegions: () => Array<{ getId: () => string }> }> = [];
+
+const storeState = {
+  undo: vi.fn(),
+  redo: vi.fn(),
+  setStatus: vi.fn(),
+  isPlaying: false,
+  startPlaying: vi.fn(),
+  stopTransport: vi.fn(),
+  toggleLoop: vi.fn(),
+  projectName: 'Test Project',
+  savedProjectName: 'Test Project',
+  setSavedProjectName: vi.fn(),
+  setProjectName: vi.fn(),
+  isRecording: false,
+  startRecording: vi.fn(),
+  stopRecording: vi.fn(),
+  activeRegionId: null,
+  selectedRegionIds: ['region-a', 'region-b'],
+  selectedNoteIds: [],
+  setActiveRegionId: vi.fn(),
+  setShowPianoRoll: vi.fn(),
+  showPianoRoll: false,
+  openMidiPianoRoll: vi.fn(),
+  openMidiPianoRollWithSheetMusicView: vi.fn(),
+  openSpectrogramViewer: vi.fn(),
+  playheadPosition: 12,
+  refreshProjectState: vi.fn(),
+  pianoRollMode: 'midi-edit' as const,
+};
+
+type StoreState = typeof storeState;
+
+vi.mock('../stores/projectStore', () => ({
+  useProjectStore: (selector?: unknown) => (
+    selector ? (selector as (state: StoreState) => unknown)(storeState) : storeState
+  ),
+}));
+
+vi.mock('../util/osUtil', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('../util/osUtil')>();
+  return {
+    ...actual,
+    matchesKeyboardShortcut: (event: KeyboardEvent, shortcut: string) => {
+      if (shortcut === 'ctrl+t') {
+        return event.ctrlKey && event.key.toLowerCase() === 't';
+      }
+      if (shortcut === 'ctrl+j') {
+        return event.ctrlKey && event.key.toLowerCase() === 'j';
+      }
+      return false;
+    },
+  };
+});
+
+vi.mock('../util/copyPasteUtil', () => ({
+  handleCopyOperation: vi.fn(() => false),
+  handlePasteOperation: vi.fn(() => false),
+}));
+
+vi.mock('../util/saveUtil', () => ({
+  saveProject: vi.fn(),
+}));
+
+vi.mock('../core/config/ConfigManager', () => ({
+  ConfigManager: {
+    instance: () => ({
+      getIsInitialized: () => true,
+      get: (path: string) => {
+        const shortcuts: Record<string, string> = {
+          'hotkeys.main.undo': 'ctrl+z',
+          'hotkeys.main.redo': 'ctrl+shift+z',
+          'hotkeys.main.copy': 'ctrl+c',
+          'hotkeys.main.paste': 'ctrl+v',
+          'hotkeys.main.select_all': 'ctrl+a',
+          'hotkeys.main.play': 'space',
+          'hotkeys.main.loop': 'c',
+          'hotkeys.main.save': 'ctrl+s',
+          'hotkeys.main.record': 'r',
+          'hotkeys.main.split_region': 'ctrl+t',
+          'hotkeys.main.merge_regions': 'ctrl+j',
+        };
+        return shortcuts[path];
+      },
+    }),
+  },
+}));
+
+vi.mock('../util/selectionUtil', () => ({
+  selectAllNotesInActiveRegion: vi.fn(),
+}));
+
+vi.mock('../core/KGCore', () => ({
+  KGCore: {
+    instance: () => ({
+      getCurrentProject: () => ({
+        getTracks: () => mockTracks,
+      }),
+    }),
+  },
+}));
+
+vi.mock('../core/midi-input/KGMidiInput', () => ({
+  KGMidiInput: {
+    instance: () => ({
+      getConnectedInputCount: () => 0,
+    }),
+  },
+}));
+
+vi.mock('../core/region/KGMidiRegion', () => ({
+  KGMidiRegion: MockMidiRegion,
+}));
+
+vi.mock('../core/track/KGAudioTrack', () => ({
+  KGAudioTrack: class {},
+}));
+
+vi.mock('../util/regionEditUtil', () => ({
+  splitSelectedRegionAtPlayhead: regionEditUtilMocks.splitSelectedRegionAtPlayhead,
+  mergeSelectedMidiRegions: regionEditUtilMocks.mergeSelectedMidiRegions,
+}));
+
+vi.mock('../util/dialogUtil', () => ({
+  showAlert: vi.fn(),
+}));
+
+const HookHarness = () => {
+  useGlobalKeyboardHandler();
+  return null;
+};
+
+describe('useGlobalKeyboardHandler region shortcuts', () => {
+  beforeEach(() => {
+    mockTracks = [];
+    regionEditUtilMocks.splitSelectedRegionAtPlayhead.mockReset();
+    regionEditUtilMocks.mergeSelectedMidiRegions.mockReset();
+    storeState.setStatus.mockClear();
+    storeState.setShowPianoRoll.mockClear();
+    storeState.showPianoRoll = false;
+    storeState.activeRegionId = null;
+    storeState.selectedRegionIds = ['region-a', 'region-b'];
+    storeState.openMidiPianoRoll.mockClear();
+    storeState.openMidiPianoRollWithSheetMusicView.mockClear();
+    storeState.openSpectrogramViewer.mockClear();
+    vi.mocked(showAlert).mockClear();
+  });
+
+  it('triggers split on Ctrl+T', async () => {
+    regionEditUtilMocks.splitSelectedRegionAtPlayhead.mockResolvedValue('Split 1 note at beat 12.00');
+
+    render(<HookHarness />);
+    fireEvent.keyDown(document.body, { key: 't', ctrlKey: true });
+
+    await waitFor(() => {
+      expect(regionEditUtilMocks.splitSelectedRegionAtPlayhead).toHaveBeenCalledWith({
+        selectedRegionIds: ['region-a', 'region-b'],
+        playheadPosition: 12,
+        refreshProjectState: storeState.refreshProjectState,
+      });
+    });
+
+    await waitFor(() => {
+      expect(storeState.setStatus).toHaveBeenCalledWith('Split 1 note at beat 12.00');
+    });
+  });
+
+  it('triggers merge on Ctrl+J', async () => {
+    regionEditUtilMocks.mergeSelectedMidiRegions.mockResolvedValue('Merged 2 MIDI regions');
+
+    render(<HookHarness />);
+    fireEvent.keyDown(document.body, { key: 'j', ctrlKey: true });
+
+    await waitFor(() => {
+      expect(regionEditUtilMocks.mergeSelectedMidiRegions).toHaveBeenCalledWith({
+        selectedRegionIds: ['region-a', 'region-b'],
+        refreshProjectState: storeState.refreshProjectState,
+      });
+    });
+
+    await waitFor(() => {
+      expect(storeState.setStatus).toHaveBeenCalledWith('Merged 2 MIDI regions');
+    });
+  });
+
+  it('opens a selected MIDI region in piano roll view on E', () => {
+    mockTracks = [
+      {
+        getRegions: () => [new MockMidiRegion('region-b')],
+      },
+    ];
+
+    render(<HookHarness />);
+    fireEvent.keyDown(document.body, { key: 'e' });
+
+    expect(storeState.openMidiPianoRollWithSheetMusicView).toHaveBeenCalledWith('region-b', false);
+    expect(storeState.openSpectrogramViewer).not.toHaveBeenCalled();
+  });
+
+  it('opens a selected MIDI region in sheet music view on N', () => {
+    mockTracks = [
+      {
+        getRegions: () => [new MockMidiRegion('region-b')],
+      },
+    ];
+
+    render(<HookHarness />);
+    fireEvent.keyDown(document.body, { key: 'n' });
+
+    expect(storeState.openMidiPianoRollWithSheetMusicView).toHaveBeenCalledWith('region-b', true);
+    expect(storeState.setShowPianoRoll).not.toHaveBeenCalled();
+  });
+
+  it('shows the editor alert on N when no region is selected', () => {
+    storeState.selectedRegionIds = [];
+
+    render(<HookHarness />);
+    fireEvent.keyDown(document.body, { key: 'n' });
+
+    expect(showAlert).toHaveBeenCalledWith('Please select a region to open the editor.');
+    expect(storeState.openMidiPianoRollWithSheetMusicView).not.toHaveBeenCalled();
+  });
+
+  it('does not open spectrogram on N for an audio region', () => {
+    mockTracks = [
+      {
+        getRegions: () => [
+          {
+            getId: () => 'region-b',
+          },
+        ],
+      },
+    ];
+
+    render(<HookHarness />);
+    fireEvent.keyDown(document.body, { key: 'n' });
+
+    expect(showAlert).toHaveBeenCalledWith('Sheet music view is only available for MIDI regions.');
+    expect(storeState.openSpectrogramViewer).not.toHaveBeenCalled();
+    expect(storeState.openMidiPianoRollWithSheetMusicView).not.toHaveBeenCalled();
+  });
+
+  it('does not close the editor when N is pressed while piano roll is already open', () => {
+    storeState.showPianoRoll = true;
+    mockTracks = [
+      {
+        getRegions: () => [new MockMidiRegion('region-b')],
+      },
+    ];
+
+    render(<HookHarness />);
+    fireEvent.keyDown(document.body, { key: 'n' });
+
+    expect(storeState.setShowPianoRoll).not.toHaveBeenCalled();
+    expect(storeState.openMidiPianoRollWithSheetMusicView).toHaveBeenCalledWith('region-b', true);
+  });
+});

src/hooks/useGlobalKeyboardHandler.ts

@@ -9,6 +9,7 @@ import { KGCore } from '../core/KGCore';
 import { KGMidiInput } from '../core/midi-input/KGMidiInput';
 import { KGMidiRegion } from '../core/region/KGMidiRegion';
 import { KGAudioTrack } from '../core/track/KGAudioTrack';
+import { mergeSelectedMidiRegions, splitSelectedRegionAtPlayhead } from '../util/regionEditUtil';
 import { showAlert } from '../util/dialogUtil';
 
 /**
@@ -16,14 +17,14 @@ import { showAlert } from '../util/dialogUtil';
  * Handles keyboard shortcuts defined in the configuration
  */
 export const useGlobalKeyboardHandler = () => {
-  const { undo, redo, setStatus, isPlaying, startPlaying, stopTransport, toggleLoop, projectName, savedProjectName, setSavedProjectName, setProjectName, isRecording, startRecording, stopRecording, activeRegionId, selectedRegionIds, setActiveRegionId, setShowPianoRoll, showPianoRoll, openMidiPianoRoll, openSpectrogramViewer } = useProjectStore();
+  const { undo, redo, setStatus, isPlaying, startPlaying, stopTransport, toggleLoop, projectName, savedProjectName, setSavedProjectName, setProjectName, isRecording, startRecording, stopRecording, activeRegionId, selectedRegionIds, setActiveRegionId, setShowPianoRoll, showPianoRoll, openMidiPianoRollWithSheetMusicView, openSpectrogramViewer, playheadPosition, refreshProjectState } = useProjectStore();
   const lastSelectedRegionId = selectedRegionIds[selectedRegionIds.length - 1] ?? null;
 
   useEffect(() => {
     const handleKeyDown = (event: KeyboardEvent) => {
       // Skip if user is typing in an input field
-      const target = event.target as HTMLElement;
-      if (target && (
+      const target = event.target;
+      if (target instanceof HTMLElement && (
         target.tagName === 'INPUT' || 
         target.tagName === 'TEXTAREA' || 
         target.contentEditable === 'true' ||
@@ -71,6 +72,8 @@ export const useGlobalKeyboardHandler = () => {
       const loopShortcut = configManager.get('hotkeys.main.loop') as string;
       const saveShortcut = configManager.get('hotkeys.main.save') as string;
       const recordShortcut = configManager.get('hotkeys.main.record') as string;
+      const splitShortcut = configManager.get('hotkeys.main.split_region') as string;
+      const mergeShortcut = configManager.get('hotkeys.main.merge_regions') as string;
 
       // Check for undo shortcut
       if (undoShortcut && matchesKeyboardShortcut(event, undoShortcut)) {
@@ -202,6 +205,33 @@ export const useGlobalKeyboardHandler = () => {
         return;
       }
 
+      if (splitShortcut && matchesKeyboardShortcut(event, splitShortcut)) {
+        event.preventDefault();
+        void splitSelectedRegionAtPlayhead({
+          selectedRegionIds,
+          playheadPosition,
+          refreshProjectState,
+        }).then(status => {
+          if (status) {
+            setStatus(status);
+          }
+        });
+        return;
+      }
+
+      if (mergeShortcut && matchesKeyboardShortcut(event, mergeShortcut)) {
+        event.preventDefault();
+        void mergeSelectedMidiRegions({
+          selectedRegionIds,
+          refreshProjectState,
+        }).then(status => {
+          if (status) {
+            setStatus(status);
+          }
+        });
+        return;
+      }
+
       // Check for edit/view shortcut (E) — open piano roll for MIDI, spectrogram for audio
       if (event.key.toLowerCase() === 'e' && !event.ctrlKey && !event.metaKey && !event.altKey && !event.shiftKey) {
         event.preventDefault();
@@ -226,13 +256,40 @@ export const useGlobalKeyboardHandler = () => {
           }
         }
         if (foundMidi) {
-          openMidiPianoRoll(candidateId);
+          openMidiPianoRollWithSheetMusicView(candidateId, false);
         } else if (foundAudio) {
           openSpectrogramViewer(candidateId);
         }
         return;
       }
 
+      // Check for sheet music shortcut (N) — open piano roll for MIDI in sheet music view
+      if (event.key.toLowerCase() === 'n' && !event.ctrlKey && !event.metaKey && !event.altKey && !event.shiftKey) {
+        event.preventDefault();
+        const candidateId = activeRegionId ?? lastSelectedRegionId;
+        if (!candidateId) {
+          void showAlert('Please select a region to open the editor.');
+          return;
+        }
+        const tracks = KGCore.instance().getCurrentProject().getTracks();
+        let foundMidi = false;
+        let foundAudio = false;
+        for (const track of tracks) {
+          const region = track.getRegions().find(r => r.getId() === candidateId);
+          if (region) {
+            if (region instanceof KGMidiRegion) foundMidi = true;
+            else foundAudio = true;
+            break;
+          }
+        }
+        if (foundMidi) {
+          openMidiPianoRollWithSheetMusicView(candidateId, true);
+        } else if (foundAudio) {
+          void showAlert('Sheet music view is only available for MIDI regions.');
+        }
+        return;
+      }
+
       // Check for save shortcut
       if (saveShortcut && matchesKeyboardShortcut(event, saveShortcut)) {
         event.preventDefault();
@@ -272,11 +329,14 @@ export const useGlobalKeyboardHandler = () => {
     startRecording,
     stopRecording,
     activeRegionId,
+    selectedRegionIds,
     lastSelectedRegionId,
     setActiveRegionId,
     setShowPianoRoll,
     showPianoRoll,
-    openMidiPianoRoll,
+    openMidiPianoRollWithSheetMusicView,
     openSpectrogramViewer,
+    playheadPosition,
+    refreshProjectState,
   ]); // Include dependencies for store actions
 };

src/hooks/useNoteOperations.test.ts

@@ -0,0 +1,431 @@
+import { act, renderHook } from '@testing-library/react';
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+type SelectableItem = { getId: () => string };
+
+const coreState = {
+  selectedItems: [] as SelectableItem[],
+  selectionChangedCallbacks: [] as Array<() => void>,
+  executeCommand: vi.fn(),
+};
+
+const projectStoreState = {
+  selectedNoteIds: [] as string[],
+  clearAllSelections: () => {
+    coreState.selectedItems = [];
+    syncSelectionFromCore();
+  },
+  bumpAutomationRedrawVersion: vi.fn(),
+  syncSelectionFromCore: () => {
+    syncSelectionFromCore();
+  },
+};
+
+const syncSelectionFromCore = () => {
+  projectStoreState.selectedNoteIds = coreState.selectedItems.map(item => item.getId());
+  coreState.selectionChangedCallbacks.forEach(callback => callback());
+};
+
+vi.mock('../core/KGCore', () => ({
+  KGCore: {
+    instance: () => ({
+      getSelectedItems: () => coreState.selectedItems,
+      addSelectedItem: (item: SelectableItem) => {
+        coreState.selectedItems = coreState.selectedItems.filter(selectedItem => selectedItem.getId() !== item.getId());
+        coreState.selectedItems.push(item);
+        syncSelectionFromCore();
+      },
+      addSelectedItems: (items: SelectableItem[]) => {
+        const incomingIds = new Set(items.map(item => item.getId()));
+        coreState.selectedItems = coreState.selectedItems.filter(item => !incomingIds.has(item.getId()));
+        coreState.selectedItems.push(...items);
+        syncSelectionFromCore();
+      },
+      clearSelectedItems: () => {
+        coreState.selectedItems = [];
+        syncSelectionFromCore();
+      },
+      executeCommand: (...args: unknown[]) => coreState.executeCommand(...args),
+      onSelectionChanged: (callback: () => void) => {
+        coreState.selectionChangedCallbacks.push(callback);
+      },
+    }),
+  },
+}));
+
+vi.mock('../stores/projectStore', () => ({
+  useProjectStore: Object.assign(vi.fn(), {
+    getState: () => projectStoreState,
+  }),
+}));
+
+import { useNoteOperations } from './useNoteOperations';
+import { KGCore } from '../core/KGCore';
+import { KGPianoRollState } from '../core/state/KGPianoRollState';
+import { MoveNotesCommand, ResizeNotesCommand } from '../core/commands';
+import { useProjectStore } from '../stores/projectStore';
+import { createMockMidiNote, createMockMidiRegion, createMockMidiTrack } from '../test/utils/mock-data';
+
+describe('useNoteOperations', () => {
+  beforeEach(() => {
+    vi.restoreAllMocks();
+    coreState.selectedItems = [];
+    coreState.selectionChangedCallbacks = [];
+    coreState.executeCommand = vi.fn();
+    projectStoreState.selectedNoteIds = [];
+    projectStoreState.bumpAutomationRedrawVersion.mockReset();
+
+    vi.spyOn(window, 'getComputedStyle').mockReturnValue({
+      getPropertyValue: (property: string) => {
+        if (property === '--region-grid-beat-width') {
+          return '40';
+        }
+
+        if (property === '--region-piano-key-height') {
+          return '20';
+        }
+
+        return '';
+      },
+    } as CSSStyleDeclaration);
+
+    KGPianoRollState.instance().setActiveTool('pointer');
+    KGPianoRollState.instance().setCurrentSnap('1/4');
+  });
+
+  const renderNoteOperations = (activeRegion: ReturnType<typeof createMockMidiRegion>, track = createMockMidiTrack({ id: 1, regions: [activeRegion] }), updateTrack = vi.fn()) => {
+    const hook = renderHook(() => useNoteOperations({
+      activeRegion,
+      timeSignature: { numerator: 4, denominator: 4 },
+      updateTrack,
+      tracks: [track],
+      pianoGridRef: { current: null },
+    }));
+
+    return { ...hook, track, updateTrack };
+  };
+
+  it('selects the grabbed note before resizing when it was not part of the current selection', () => {
+    const noteA = createMockMidiNote({ id: 'note-a', startBeat: 0, endBeat: 1, pitch: 60 });
+    const noteB = createMockMidiNote({ id: 'note-b', startBeat: 1, endBeat: 2, pitch: 62 });
+    const noteC = createMockMidiNote({ id: 'note-c', startBeat: 2, endBeat: 3, pitch: 64 });
+    const activeRegion = createMockMidiRegion({
+      id: 'region-1',
+      trackId: '1',
+      notes: [noteA, noteB, noteC],
+    });
+    const track = createMockMidiTrack({ id: 1, regions: [activeRegion] });
+    const updateTrack = vi.fn();
+
+    noteA.select();
+    noteB.select();
+    KGCore.instance().addSelectedItems([noteA, noteB]);
+
+    const { result } = renderNoteOperations(activeRegion, track, updateTrack);
+
+    act(() => {
+      result.current.handleNoteResizeStart(noteC.getId(), 'end', 120);
+    });
+
+    expect(KGCore.instance().getSelectedItems().map(item => item.getId())).toEqual([noteC.getId()]);
+    expect(useProjectStore.getState().selectedNoteIds).toEqual([noteC.getId()]);
+    expect(noteA.isSelected()).toBe(false);
+    expect(noteB.isSelected()).toBe(false);
+    expect(noteC.isSelected()).toBe(true);
+
+    act(() => {
+      result.current.handleNoteResizeEnd(noteC.getId(), 'end');
+    });
+
+    expect(coreState.executeCommand).toHaveBeenCalledTimes(1);
+    const resizeCommand = coreState.executeCommand.mock.calls[0][0];
+    expect(resizeCommand).toBeInstanceOf(ResizeNotesCommand);
+    expect((resizeCommand as ResizeNotesCommand).getNoteIdsToResize()).toEqual([noteC.getId()]);
+  });
+
+  it('keeps the existing multi-selection when resizing a selected note', () => {
+    const noteA = createMockMidiNote({ id: 'note-a', startBeat: 0, endBeat: 1, pitch: 60 });
+    const noteB = createMockMidiNote({ id: 'note-b', startBeat: 1, endBeat: 2, pitch: 62 });
+    const noteC = createMockMidiNote({ id: 'note-c', startBeat: 2, endBeat: 3, pitch: 64 });
+    const activeRegion = createMockMidiRegion({
+      id: 'region-1',
+      trackId: '1',
+      notes: [noteA, noteB, noteC],
+    });
+    const track = createMockMidiTrack({ id: 1, regions: [activeRegion] });
+    const updateTrack = vi.fn();
+
+    noteA.select();
+    noteB.select();
+    KGCore.instance().addSelectedItems([noteA, noteB]);
+
+    const { result } = renderNoteOperations(activeRegion, track, updateTrack);
+
+    act(() => {
+      result.current.handleNoteResizeStart(noteA.getId(), 'end', 0);
+    });
+
+    expect(KGCore.instance().getSelectedItems().map(item => item.getId())).toEqual([noteA.getId(), noteB.getId()]);
+    expect(useProjectStore.getState().selectedNoteIds).toEqual([noteA.getId(), noteB.getId()]);
+    expect(noteA.isSelected()).toBe(true);
+    expect(noteB.isSelected()).toBe(true);
+    expect(noteC.isSelected()).toBe(false);
+
+    act(() => {
+      result.current.handleNoteResizeEnd(noteA.getId(), 'end');
+    });
+
+    expect(coreState.executeCommand).toHaveBeenCalledTimes(1);
+    const resizeCommand = coreState.executeCommand.mock.calls[0][0];
+    expect(resizeCommand).toBeInstanceOf(ResizeNotesCommand);
+    expect((resizeCommand as ResizeNotesCommand).getNoteIdsToResize()).toEqual([noteA.getId(), noteB.getId()]);
+  });
+
+  it('previews end resize for all notes in the active multi-selection', () => {
+    const noteA = createMockMidiNote({ id: 'note-a', startBeat: 0, endBeat: 1, pitch: 60 });
+    const noteB = createMockMidiNote({ id: 'note-b', startBeat: 2, endBeat: 3, pitch: 62 });
+    const activeRegion = createMockMidiRegion({
+      id: 'region-1',
+      trackId: '1',
+      startFromBeat: 4,
+      notes: [noteA, noteB],
+    });
+
+    noteA.select();
+    noteB.select();
+    KGCore.instance().addSelectedItems([noteA, noteB]);
+
+    const { result } = renderNoteOperations(activeRegion);
+
+    act(() => {
+      result.current.handleNoteResizeStart(noteA.getId(), 'end', 0);
+      result.current.handleNoteResize(noteA.getId(), 'end', 20);
+    });
+
+    expect(result.current.tempNoteStyles).toEqual({
+      'note-a': { left: '160px', width: '80px' },
+      'note-b': { left: '240px', width: '80px' },
+    });
+  });
+
+  it('previews start resize for all notes in the active multi-selection', () => {
+    const noteA = createMockMidiNote({ id: 'note-a', startBeat: 1, endBeat: 3, pitch: 60 });
+    const noteB = createMockMidiNote({ id: 'note-b', startBeat: 4, endBeat: 6, pitch: 62 });
+    const activeRegion = createMockMidiRegion({
+      id: 'region-1',
+      trackId: '1',
+      startFromBeat: 2,
+      notes: [noteA, noteB],
+    });
+
+    noteA.select();
+    noteB.select();
+    KGCore.instance().addSelectedItems([noteA, noteB]);
+
+    const { result } = renderNoteOperations(activeRegion);
+
+    act(() => {
+      result.current.handleNoteResizeStart(noteA.getId(), 'start', 0);
+      result.current.handleNoteResize(noteA.getId(), 'start', 20);
+    });
+
+    expect(result.current.tempNoteStyles).toEqual({
+      'note-a': { left: '120px', width: '80px' },
+      'note-b': { left: '240px', width: '80px' },
+    });
+  });
+
+  it('previews only the grabbed note when it was outside the current selection', () => {
+    const noteA = createMockMidiNote({ id: 'note-a', startBeat: 0, endBeat: 1, pitch: 60 });
+    const noteB = createMockMidiNote({ id: 'note-b', startBeat: 1, endBeat: 2, pitch: 62 });
+    const noteC = createMockMidiNote({ id: 'note-c', startBeat: 2, endBeat: 3, pitch: 64 });
+    const activeRegion = createMockMidiRegion({
+      id: 'region-1',
+      trackId: '1',
+      notes: [noteA, noteB, noteC],
+    });
+
+    noteA.select();
+    noteB.select();
+    KGCore.instance().addSelectedItems([noteA, noteB]);
+
+    const { result } = renderNoteOperations(activeRegion);
+
+    act(() => {
+      result.current.handleNoteResizeStart(noteC.getId(), 'end', 0);
+      result.current.handleNoteResize(noteC.getId(), 'end', 20);
+    });
+
+    expect(useProjectStore.getState().selectedNoteIds).toEqual([noteC.getId()]);
+    expect(result.current.tempNoteStyles).toEqual({
+      'note-c': { left: '80px', width: '80px' },
+    });
+  });
+
+  it('clears resize preview styles after committing a multi-note resize', () => {
+    const noteA = createMockMidiNote({ id: 'note-a', startBeat: 0, endBeat: 1, pitch: 60 });
+    const noteB = createMockMidiNote({ id: 'note-b', startBeat: 2, endBeat: 3, pitch: 62 });
+    const activeRegion = createMockMidiRegion({
+      id: 'region-1',
+      trackId: '1',
+      notes: [noteA, noteB],
+    });
+
+    noteA.select();
+    noteB.select();
+    KGCore.instance().addSelectedItems([noteA, noteB]);
+
+    const { result } = renderNoteOperations(activeRegion);
+
+    act(() => {
+      result.current.handleNoteResizeStart(noteA.getId(), 'end', 0);
+      result.current.handleNoteResize(noteA.getId(), 'end', 20);
+    });
+
+    expect(Object.keys(result.current.tempNoteStyles)).toEqual(['note-a', 'note-b']);
+
+    act(() => {
+      result.current.handleNoteResizeEnd(noteA.getId(), 'end');
+    });
+
+    expect(result.current.tempNoteStyles).toEqual({});
+  });
+
+  it('commits the same snapped resize delta that is shown in the preview', () => {
+    const noteA = createMockMidiNote({ id: 'note-a', startBeat: 0, endBeat: 1, pitch: 60 });
+    const noteB = createMockMidiNote({ id: 'note-b', startBeat: 2, endBeat: 3, pitch: 62 });
+    const activeRegion = createMockMidiRegion({
+      id: 'region-1',
+      trackId: '1',
+      notes: [noteA, noteB],
+    });
+
+    noteA.select();
+    noteB.select();
+    KGCore.instance().addSelectedItems([noteA, noteB]);
+
+    const { result } = renderNoteOperations(activeRegion);
+
+    act(() => {
+      result.current.handleNoteResizeStart(noteA.getId(), 'end', 0);
+      result.current.handleNoteResize(noteA.getId(), 'end', 20);
+      result.current.handleNoteResizeEnd(noteA.getId(), 'end');
+    });
+
+    expect(coreState.executeCommand).toHaveBeenCalledTimes(1);
+    const resizeCommand = coreState.executeCommand.mock.calls[0][0] as ResizeNotesCommand;
+    expect(resizeCommand.getNoteIdsToResize()).toEqual([noteA.getId(), noteB.getId()]);
+    expect((resizeCommand as unknown as { primaryEndBeatDelta: number }).primaryEndBeatDelta).toBe(1);
+  });
+
+  it('previews drag movement for all notes in the active multi-selection', () => {
+    const noteA = createMockMidiNote({ id: 'note-a', startBeat: 0, endBeat: 1, pitch: 60 });
+    const noteB = createMockMidiNote({ id: 'note-b', startBeat: 2, endBeat: 3, pitch: 64 });
+    const activeRegion = createMockMidiRegion({
+      id: 'region-1',
+      trackId: '1',
+      startFromBeat: 4,
+      notes: [noteA, noteB],
+    });
+
+    noteA.select();
+    noteB.select();
+    KGCore.instance().addSelectedItems([noteA, noteB]);
+
+    const { result } = renderNoteOperations(activeRegion);
+
+    act(() => {
+      result.current.handleNoteDragStart(noteA.getId(), 0, 0);
+      result.current.handleNoteDrag(noteA.getId(), 20, 15);
+    });
+
+    expect(result.current.tempNoteStyles).toEqual({
+      'note-a': { left: '200px', top: '955px', width: '40px', height: '20px', zIndex: 100 },
+      'note-b': { left: '280px', top: '875px', width: '40px', height: '20px', zIndex: 100 },
+    });
+  });
+
+  it('previews only the grabbed note during drag when it was outside the current selection', () => {
+    const noteA = createMockMidiNote({ id: 'note-a', startBeat: 0, endBeat: 1, pitch: 60 });
+    const noteB = createMockMidiNote({ id: 'note-b', startBeat: 1, endBeat: 2, pitch: 62 });
+    const noteC = createMockMidiNote({ id: 'note-c', startBeat: 2, endBeat: 3, pitch: 64 });
+    const activeRegion = createMockMidiRegion({
+      id: 'region-1',
+      trackId: '1',
+      notes: [noteA, noteB, noteC],
+    });
+
+    noteA.select();
+    noteB.select();
+    KGCore.instance().addSelectedItems([noteA, noteB]);
+
+    const { result } = renderNoteOperations(activeRegion);
+
+    act(() => {
+      result.current.handleNoteDragStart(noteC.getId(), 0, 0);
+      result.current.handleNoteDrag(noteC.getId(), 20, 15);
+    });
+
+    expect(result.current.tempNoteStyles).toEqual({
+      'note-c': { left: '120px', top: '875px', width: '40px', height: '20px', zIndex: 100 },
+    });
+  });
+
+  it('clears drag preview styles after committing a multi-note move', () => {
+    const noteA = createMockMidiNote({ id: 'note-a', startBeat: 0, endBeat: 1, pitch: 60 });
+    const noteB = createMockMidiNote({ id: 'note-b', startBeat: 2, endBeat: 3, pitch: 64 });
+    const activeRegion = createMockMidiRegion({
+      id: 'region-1',
+      trackId: '1',
+      notes: [noteA, noteB],
+    });
+
+    noteA.select();
+    noteB.select();
+    KGCore.instance().addSelectedItems([noteA, noteB]);
+
+    const { result } = renderNoteOperations(activeRegion);
+
+    act(() => {
+      result.current.handleNoteDragStart(noteA.getId(), 0, 0);
+      result.current.handleNoteDrag(noteA.getId(), 20, 15);
+    });
+
+    expect(Object.keys(result.current.tempNoteStyles)).toEqual(['note-a', 'note-b']);
+
+    act(() => {
+      result.current.handleNoteDragEnd(noteA.getId());
+    });
+
+    expect(result.current.tempNoteStyles).toEqual({});
+  });
+
+  it('commits the same drag cohort and deltas that are shown in the preview', () => {
+    const noteA = createMockMidiNote({ id: 'note-a', startBeat: 0, endBeat: 1, pitch: 60 });
+    const noteB = createMockMidiNote({ id: 'note-b', startBeat: 2, endBeat: 3, pitch: 64 });
+    const activeRegion = createMockMidiRegion({
+      id: 'region-1',
+      trackId: '1',
+      notes: [noteA, noteB],
+    });
+
+    noteA.select();
+    noteB.select();
+    KGCore.instance().addSelectedItems([noteA, noteB]);
+
+    const { result } = renderNoteOperations(activeRegion);
+
+    act(() => {
+      result.current.handleNoteDragStart(noteA.getId(), 0, 0);
+      result.current.handleNoteDrag(noteA.getId(), 20, 15);
+      result.current.handleNoteDragEnd(noteA.getId());
+    });
+
+    expect(coreState.executeCommand).toHaveBeenCalledTimes(1);
+    const moveCommand = coreState.executeCommand.mock.calls[0][0] as MoveNotesCommand;
+    expect(moveCommand).toBeInstanceOf(MoveNotesCommand);
+    expect(moveCommand.getNoteIdsToMove()).toEqual([noteA.getId(), noteB.getId()]);
+    expect(moveCommand.getStartBeatDelta()).toBe(1);
+    expect(moveCommand.getPitchDelta()).toBe(-1);
+  });
+});

src/hooks/useNoteOperations.ts

@@ -25,6 +25,25 @@ interface UseNoteOperationsProps {
   pianoGridRef: MutableRefObject<HTMLDivElement | null>;
 }
 
+interface ResizePreviewBaseline {
+  noteId: string;
+  originalStartBeat: number;
+  originalEndBeat: number;
+  originalLeft: number;
+  originalWidth: number;
+}
+
+interface DragPreviewBaseline {
+  noteId: string;
+  originalStartBeat: number;
+  originalEndBeat: number;
+  originalPitch: number;
+  originalLeft: number;
+  originalTop: number;
+  originalWidth: number;
+  originalHeight: number;
+}
+
 export const useNoteOperations = ({
   activeRegion,
   timeSignature,
@@ -44,6 +63,8 @@ export const useNoteOperations = ({
   const currentResizeLeft = useRef<number | null>(null);
   const initialStartBeatRef = useRef<number | null>(null);
   const initialEndBeatRef = useRef<number | null>(null);
+  const resizePreviewBaselinesRef = useRef<ResizePreviewBaseline[]>([]);
+  const resizePreviewNoteIdsRef = useRef<string[]>([]);
   
   // Refs for drag operations
   const initialDragLeft = useRef<number | null>(null);
@@ -51,12 +72,29 @@ export const useNoteOperations = ({
   const currentDragLeft = useRef<number | null>(null);
   const currentDragTop = useRef<number | null>(null);
   const initialPitchRef = useRef<number | null>(null);
+  const dragPreviewBaselinesRef = useRef<DragPreviewBaseline[]>([]);
+  const dragPreviewNoteIdsRef = useRef<string[]>([]);
   
   // Counter to trigger re-renders when notes are updated
   const [noteUpdateCounter, setNoteUpdateCounter] = useState(0);
 
   // Get KGCore instance for accessing selected items
   const core = KGCore.instance();
+
+  const clearTempNoteStyles = (noteIds?: string[]) => {
+    if (!noteIds || noteIds.length === 0) {
+      setTempNoteStyles({});
+      return;
+    }
+
+    setTempNoteStyles(prev => {
+      const updated = { ...prev };
+      noteIds.forEach(id => {
+        delete updated[id];
+      });
+      return updated;
+    });
+  };
   
   // Utility function to delete selected notes from the active region using commands
   const deleteSelectedNotes = () => {
@@ -279,6 +317,32 @@ export const useNoteOperations = ({
     // Find the note being resized
     const note = activeRegion.getNotes().find(n => n.getId() === noteId);
     if (!note) return;
+
+    const selectedNotesInRegion = core.getSelectedItems().filter(item =>
+      item instanceof KGMidiNote &&
+      activeRegion.getNotes().some(regionNote => regionNote.getId() === item.getId())
+    ) as KGMidiNote[];
+    const isResizedNoteSelected = selectedNotesInRegion.some(selectedNote => selectedNote.getId() === noteId);
+
+    if (!isResizedNoteSelected) {
+      selectedNotesInRegion.forEach(selectedNote => {
+        selectedNote.deselect();
+      });
+      useProjectStore.getState().clearAllSelections();
+
+      note.select();
+      core.addSelectedItem(note);
+      KGPianoRollState.instance().setLastEditedNoteLength(note.getEndBeat() - note.getStartBeat());
+
+      const track = tracks.find(t => t.getId().toString() === activeRegion.getTrackId());
+      if (track) {
+        updateTrack(track);
+      }
+    }
+
+    const resizeTargetNotes = isResizedNoteSelected
+      ? selectedNotesInRegion
+      : [note];
     
     // Store the initial start and end beats
     initialStartBeatRef.current = note.getStartBeat();
@@ -302,15 +366,28 @@ export const useNoteOperations = ({
     currentResizeWidth.current = width;
     currentResizeLeft.current = left;
     
-    // Set initial style to current position/size
-    const initialStyle = {
-      left: `${left}px`,
-      width: `${width}px`,
-    };
-    
+    resizePreviewBaselinesRef.current = resizeTargetNotes.map(targetNote => {
+      const targetAbsStartBeat = targetNote.getStartBeat() + regionStartBeat;
+      const targetAbsEndBeat = targetNote.getEndBeat() + regionStartBeat;
+      return {
+        noteId: targetNote.getId(),
+        originalStartBeat: targetNote.getStartBeat(),
+        originalEndBeat: targetNote.getEndBeat(),
+        originalLeft: targetAbsStartBeat * beatWidth,
+        originalWidth: (targetAbsEndBeat - targetAbsStartBeat) * beatWidth,
+      };
+    });
+    resizePreviewNoteIdsRef.current = resizeTargetNotes.map(targetNote => targetNote.getId());
+
     setTempNoteStyles(prev => ({
       ...prev,
-      [noteId]: initialStyle
+      ...Object.fromEntries(resizePreviewBaselinesRef.current.map(baseline => [
+        baseline.noteId,
+        {
+          left: `${baseline.originalLeft}px`,
+          width: `${baseline.originalWidth}px`,
+        },
+      ])),
     }));
   };
 
@@ -380,15 +457,41 @@ export const useNoteOperations = ({
     currentResizeWidth.current = snappedWidth;
     currentResizeLeft.current = newLeft;
     
-    // Update the temporary style for this note
-    const newStyle = {
-      left: `${newLeft}px`,
-      width: `${snappedWidth}px`,
-    };
-    
+    const startBeatDelta = resizeEdge === 'start'
+      ? (newLeft - originalLeft) / beatWidth
+      : 0;
+    const endBeatDelta = resizeEdge === 'end'
+      ? (snappedWidth - originalWidth) / beatWidth
+      : 0;
+
+    const previewBaselines = resizePreviewBaselinesRef.current.length > 0
+      ? resizePreviewBaselinesRef.current
+      : [{
+        noteId,
+        originalStartBeat: note.getStartBeat(),
+        originalEndBeat: note.getEndBeat(),
+        originalLeft: originalLeft,
+        originalWidth: originalWidth,
+      }];
+
     setTempNoteStyles(prev => ({
       ...prev,
-      [noteId]: newStyle
+      ...Object.fromEntries(previewBaselines.map(baseline => {
+        const previewLeft = resizeEdge === 'start'
+          ? baseline.originalLeft + (startBeatDelta * beatWidth)
+          : baseline.originalLeft;
+        const previewWidth = resizeEdge === 'end'
+          ? baseline.originalWidth + (endBeatDelta * beatWidth)
+          : baseline.originalWidth - (startBeatDelta * beatWidth);
+
+        return [
+          baseline.noteId,
+          {
+            left: `${previewLeft}px`,
+            width: `${previewWidth}px`,
+          },
+        ];
+      })),
     }));
     
     if (DEBUG_MODE.PIANO_ROLL) {
@@ -418,11 +521,9 @@ export const useNoteOperations = ({
         initialStartBeatRef.current === null || initialEndBeatRef.current === null) {
       // Reset resizing state
       setResizingNoteId(null);
-      setTempNoteStyles(prev => {
-        const updated = { ...prev };
-        delete updated[noteId];
-        return updated;
-      });
+      clearTempNoteStyles(resizePreviewNoteIdsRef.current);
+      resizePreviewBaselinesRef.current = [];
+      resizePreviewNoteIdsRef.current = [];
       return;
     }
     
@@ -513,15 +614,13 @@ export const useNoteOperations = ({
       console.error('Error resizing notes:', error);
       // Reset resizing state and return early on error
       setResizingNoteId(null);
-      setTempNoteStyles(prev => {
-        const updated = { ...prev };
-        delete updated[noteId];
-        return updated;
-      });
+      clearTempNoteStyles(resizePreviewNoteIdsRef.current);
       currentResizeWidth.current = null;
       currentResizeLeft.current = null;
       initialStartBeatRef.current = null;
       initialEndBeatRef.current = null;
+      resizePreviewBaselinesRef.current = [];
+      resizePreviewNoteIdsRef.current = [];
       return;
     }
 
@@ -538,15 +637,13 @@ export const useNoteOperations = ({
     
     // Reset resizing state
     setResizingNoteId(null);
-    setTempNoteStyles(prev => {
-      const updated = { ...prev };
-      delete updated[noteId];
-      return updated;
-    });
+    clearTempNoteStyles(resizePreviewNoteIdsRef.current);
     currentResizeWidth.current = null;
     currentResizeLeft.current = null;
     initialStartBeatRef.current = null;
     initialEndBeatRef.current = null;
+    resizePreviewBaselinesRef.current = [];
+    resizePreviewNoteIdsRef.current = [];
     
     // Increment the note update counter to trigger a re-render
     setNoteUpdateCounter(prev => prev + 1);
@@ -576,6 +673,15 @@ export const useNoteOperations = ({
     // Find the note being dragged
     const note = activeRegion.getNotes().find(n => n.getId() === noteId);
     if (!note) return;
+
+    const selectedNotesInRegion = core.getSelectedItems().filter(item =>
+      item instanceof KGMidiNote &&
+      activeRegion.getNotes().some(regionNote => regionNote.getId() === item.getId())
+    ) as KGMidiNote[];
+    const isDraggedNoteSelected = selectedNotesInRegion.some(selectedNote => selectedNote.getId() === noteId);
+    const dragTargetNotes = isDraggedNoteSelected
+      ? selectedNotesInRegion
+      : [note];
     
     // Store the initial pitch
     initialPitchRef.current = note.getPitch();
@@ -603,19 +709,37 @@ export const useNoteOperations = ({
     initialDragTop.current = top;
     currentDragLeft.current = left;
     currentDragTop.current = top;
-    
-    // Set initial style
-    const initialStyle = {
-      left: `${left}px`,
-      top: `${top}px`,
-      width: `${width}px`,
-      height: `${noteHeight}px`,
-      zIndex: 100, // Bring to front during drag
-    };
-    
+
+    dragPreviewBaselinesRef.current = dragTargetNotes.map(targetNote => {
+      const targetAbsStartBeat = targetNote.getStartBeat() + regionStartBeat;
+      const targetWidth = (targetNote.getEndBeat() - targetNote.getStartBeat()) * beatWidth;
+      const targetTop = (107 - targetNote.getPitch()) * noteHeight;
+
+      return {
+        noteId: targetNote.getId(),
+        originalStartBeat: targetNote.getStartBeat(),
+        originalEndBeat: targetNote.getEndBeat(),
+        originalPitch: targetNote.getPitch(),
+        originalLeft: targetAbsStartBeat * beatWidth,
+        originalTop: targetTop,
+        originalWidth: targetWidth,
+        originalHeight: noteHeight,
+      };
+    });
+    dragPreviewNoteIdsRef.current = dragTargetNotes.map(targetNote => targetNote.getId());
+
     setTempNoteStyles(prev => ({
       ...prev,
-      [noteId]: initialStyle
+      ...Object.fromEntries(dragPreviewBaselinesRef.current.map(baseline => [
+        baseline.noteId,
+        {
+          left: `${baseline.originalLeft}px`,
+          top: `${baseline.originalTop}px`,
+          width: `${baseline.originalWidth}px`,
+          height: `${baseline.originalHeight}px`,
+          zIndex: 100,
+        },
+      ])),
     }));
   };
   
@@ -659,21 +783,33 @@ export const useNoteOperations = ({
     currentDragLeft.current = newLeft;
     currentDragTop.current = newTop;
     
-    // Calculate width based on note duration
-    const width = (note.getEndBeat() - note.getStartBeat()) * beatWidth;
-    
-    // Update the temporary style for this note
-    const newStyle = {
-      left: `${newLeft}px`,
-      top: `${newTop}px`,
-      width: `${width}px`,
-      height: `${noteHeight}px`,
-      zIndex: 100, // Keep on top during drag
-    };
-    
+    const previewBaselines = dragPreviewBaselinesRef.current.length > 0
+      ? dragPreviewBaselinesRef.current
+      : [{
+        noteId,
+        originalStartBeat: note.getStartBeat(),
+        originalEndBeat: note.getEndBeat(),
+        originalPitch: note.getPitch(),
+        originalLeft,
+        originalTop,
+        originalWidth: (note.getEndBeat() - note.getStartBeat()) * beatWidth,
+        originalHeight: noteHeight,
+      }];
+    const leftDelta = newLeft - originalLeft;
+    const topDelta = newTop - originalTop;
+
     setTempNoteStyles(prev => ({
       ...prev,
-      [noteId]: newStyle
+      ...Object.fromEntries(previewBaselines.map(baseline => [
+        baseline.noteId,
+        {
+          left: `${baseline.originalLeft + leftDelta}px`,
+          top: `${baseline.originalTop + topDelta}px`,
+          width: `${baseline.originalWidth}px`,
+          height: `${baseline.originalHeight}px`,
+          zIndex: 100,
+        },
+      ])),
     }));
     
     if (DEBUG_MODE.PIANO_ROLL) {
@@ -705,11 +841,9 @@ export const useNoteOperations = ({
         initialPitchRef.current === null) {
       // Reset dragging state
       setDraggingNoteId(null);
-      setTempNoteStyles(prev => {
-        const updated = { ...prev };
-        delete updated[noteId];
-        return updated;
-      });
+      clearTempNoteStyles(dragPreviewNoteIdsRef.current);
+      dragPreviewBaselinesRef.current = [];
+      dragPreviewNoteIdsRef.current = [];
       return;
     }
 
@@ -771,16 +905,14 @@ export const useNoteOperations = ({
       console.error('Error moving notes:', error);
       // Reset dragging state and return early on error
       setDraggingNoteId(null);
-      setTempNoteStyles(prev => {
-        const updated = { ...prev };
-        delete updated[noteId];
-        return updated;
-      });
+      clearTempNoteStyles(dragPreviewNoteIdsRef.current);
       currentDragLeft.current = null;
       currentDragTop.current = null;
       initialDragLeft.current = null;
       initialDragTop.current = null;
       initialPitchRef.current = null;
+      dragPreviewBaselinesRef.current = [];
+      dragPreviewNoteIdsRef.current = [];
       return;
     }
     
@@ -798,16 +930,14 @@ export const useNoteOperations = ({
     
     // Reset dragging state
     setDraggingNoteId(null);
-    setTempNoteStyles(prev => {
-      const updated = { ...prev };
-      delete updated[noteId];
-      return updated;
-    });
+    clearTempNoteStyles(dragPreviewNoteIdsRef.current);
     currentDragLeft.current = null;
     currentDragTop.current = null;
     initialDragLeft.current = null;
     initialDragTop.current = null;
     initialPitchRef.current = null;
+    dragPreviewBaselinesRef.current = [];
+    dragPreviewNoteIdsRef.current = [];
     
     // Increment the note update counter to trigger a re-render
     setNoteUpdateCounter(prev => prev + 1);

src/stores/projectStore.test.ts

@@ -3,6 +3,11 @@ import { act } from '@testing-library/react';
 import { KGTrack } from '../core/track/KGTrack';
 import { KGMidiTrack } from '../core/track/KGMidiTrack';
 
+const pianoRollStateMocks = vi.hoisted(() => ({
+  setSheetMusicViewEnabled: vi.fn(),
+  setPianoRollZoom: vi.fn(),
+}));
+
 let mockTracks: KGTrack[] = [new KGMidiTrack('Track 1', 0, 'acoustic_grand_piano')];
 const mockProject = {
   getTimeSignature: () => ({ numerator: 4, denominator: 4 }),
@@ -15,13 +20,23 @@ const mockProject = {
   getSelectedMode: () => 'major',
   getIsLooping: () => false,
   getLoopingRange: () => [0, 0] as [number, number],
+  getPianoRollZoom: () => 1,
 };
+let currentProject = mockProject;
 
 const mockAudioInterface = {
   getTransportPosition: vi.fn().mockReturnValue(8),
   startAudioRecording: vi.fn().mockResolvedValue({ usedDeviceId: 'default', fellBackToDefault: false }),
   stopAudioRecording: vi.fn().mockResolvedValue(null),
   cancelAudioRecording: vi.fn().mockResolvedValue(undefined),
+  removeTrackSynth: vi.fn(),
+  removeTrackAudioPlayerBus: vi.fn(),
+  createTrackAudioPlayerBus: vi.fn().mockResolvedValue(undefined),
+  loadAudioBufferForTrack: vi.fn(),
+  createTrackSynth: vi.fn(),
+  setTrackVolume: vi.fn(),
+  setTrackMute: vi.fn(),
+  setTrackSolo: vi.fn(),
 };
 
 const configValues = new Map<string, unknown>([
@@ -29,7 +44,10 @@ const configValues = new Map<string, unknown>([
 ]);
 
 const mockCore = {
-  getCurrentProject: () => mockProject,
+  getCurrentProject: () => currentProject,
+  setCurrentProject: vi.fn((project: typeof mockProject) => {
+    currentProject = project;
+  }),
   setPlayheadUpdateCallback: vi.fn(),
   setPlaybackStateChangeCallback: vi.fn(),
   setLoopBoundaryReachedCallback: vi.fn(),
@@ -45,6 +63,7 @@ const mockCore = {
   redo: vi.fn(() => true),
   clearSelectedItems: vi.fn(),
   getStatus: () => 'Ready',
+  setStatus: vi.fn(),
   getPlayheadPosition: () => 0,
   setPlayheadPosition: vi.fn(),
   getIsPlaying: () => false,
@@ -76,11 +95,20 @@ vi.mock('../core/config/ConfigManager', () => ({
   },
 }));
 
+vi.mock('../core/state/KGPianoRollState', () => ({
+  KGPianoRollState: {
+    instance: () => pianoRollStateMocks,
+  },
+}));
+
 describe('projectStore piano roll state', () => {
   beforeEach(() => {
     vi.useFakeTimers();
     vi.resetModules();
+    pianoRollStateMocks.setSheetMusicViewEnabled.mockReset();
+    pianoRollStateMocks.setPianoRollZoom.mockReset();
     mockTracks = [new KGMidiTrack('Track 1', 0, 'acoustic_grand_piano')];
+    currentProject = mockProject;
     mockCore.startPlaying.mockReset();
     mockCore.startPlaying.mockResolvedValue(undefined);
     mockCore.stopPlaying.mockReset();
@@ -118,10 +146,34 @@ describe('projectStore piano roll state', () => {
     });
 
     state = useProjectStore.getState();
+    expect(pianoRollStateMocks.setSheetMusicViewEnabled).toHaveBeenCalledWith(false);
     expect(state.showPianoRoll).toBe(true);
     expect(state.pianoRollMode).toBe('midi-edit');
     expect(state.activeRegionId).toBe('midi-b');
     expect(state.hybridAudioRegionId).toBeNull();
+    expect(state.requestedSheetMusicViewEnabled).toBe(false);
+    expect(state.pianoRollViewRequestVersion).toBe(1);
+  });
+
+  it('opens a MIDI region in sheet music view when requested', async () => {
+    const { useProjectStore } = await import('./projectStore');
+
+    act(() => {
+      useProjectStore.getState().openHybridMode('midi-a', 'audio-a');
+    });
+
+    act(() => {
+      useProjectStore.getState().openMidiPianoRollWithSheetMusicView('midi-b', true);
+    });
+
+    const state = useProjectStore.getState();
+    expect(pianoRollStateMocks.setSheetMusicViewEnabled).toHaveBeenCalledWith(true);
+    expect(state.showPianoRoll).toBe(true);
+    expect(state.pianoRollMode).toBe('midi-edit');
+    expect(state.activeRegionId).toBe('midi-b');
+    expect(state.hybridAudioRegionId).toBeNull();
+    expect(state.requestedSheetMusicViewEnabled).toBe(true);
+    expect(state.pianoRollViewRequestVersion).toBe(1);
   });
 
   it('tracks playback preparation around startPlaying success', async () => {

src/stores/projectStore.ts

@@ -98,6 +98,8 @@ interface ProjectState {
   activeRegionId: string | null;
   pianoRollMode: 'midi-edit' | 'spectrogram' | 'hybrid';
   hybridAudioRegionId: string | null;
+  requestedSheetMusicViewEnabled: boolean;
+  pianoRollViewRequestVersion: number;
   automationRedrawVersion: number;
   activeTrackAutomationTrackId: string | null;
   activeTrackAutomationType: TrackAutomationType | null;
@@ -194,6 +196,7 @@ interface ProjectState {
   setShowPianoRoll: (show: boolean) => void;
   setActiveRegionId: (regionId: string | null) => void;
   openMidiPianoRoll: (regionId: string) => void;
+  openMidiPianoRollWithSheetMusicView: (regionId: string, sheetMusicViewEnabled: boolean) => void;
   openSpectrogramViewer: (regionId: string) => void;
   openHybridMode: (midiRegionId: string, audioRegionId: string) => void;
   bumpAutomationRedrawVersion: () => void;
@@ -305,6 +308,7 @@ export const useProjectStore = create<ProjectState>((set, get) => {
 
   // Get initial ChatBox state from config
   const configManager = ConfigManager.instance();
+  KGPianoRollState.instance().setPianoRollZoom(currentProject.getPianoRollZoom());
   const initialChatBoxState = configManager.getIsInitialized()
     ? (configManager.get('chatbox.default_open') as boolean) ?? false
     : false;
@@ -430,6 +434,8 @@ export const useProjectStore = create<ProjectState>((set, get) => {
     activeRegionId: null,
     pianoRollMode: 'midi-edit' as const,
     hybridAudioRegionId: null,
+    requestedSheetMusicViewEnabled: false,
+    pianoRollViewRequestVersion: 0,
     automationRedrawVersion: 0,
     activeTrackAutomationTrackId: null,
     activeTrackAutomationType: null,
@@ -873,6 +879,14 @@ export const useProjectStore = create<ProjectState>((set, get) => {
           }
         }
 
+        // Reapply restored mute/solo state after all buses exist so solo logic can be
+        // computed against the full track set.
+        for (const track of tracks) {
+          const trackId = track.getId().toString();
+          audioInterface.setTrackMute(trackId, track.getMuted());
+          audioInterface.setTrackSolo(trackId, track.getSolo());
+        }
+
         // Update CSS variables
         updateTimeSignatureCSS(timeSignature);
         updateMaxBarsCSS(maxBars);
@@ -929,6 +943,7 @@ export const useProjectStore = create<ProjectState>((set, get) => {
 
         // Reset piano roll state for new/loaded project
         KGPianoRollState.instance().setLastEditedNoteLength(1);
+        KGPianoRollState.instance().setPianoRollZoom(projectToLoad.getPianoRollZoom());
 
         // Add a status message
         KGCore.instance().setStatus(`Project "${projectToLoad.getName()}" loaded with audio setup`);
@@ -1552,15 +1567,47 @@ export const useProjectStore = create<ProjectState>((set, get) => {
     },
 
     openMidiPianoRoll: (regionId: string) => {
-      set({ showPianoRoll: true, activeRegionId: regionId, pianoRollMode: 'midi-edit', hybridAudioRegionId: null });
+      KGPianoRollState.instance().setSheetMusicViewEnabled(false);
+      set(state => ({
+        showPianoRoll: true,
+        activeRegionId: regionId,
+        pianoRollMode: 'midi-edit',
+        hybridAudioRegionId: null,
+        requestedSheetMusicViewEnabled: false,
+        pianoRollViewRequestVersion: state.pianoRollViewRequestVersion + 1,
+      }));
+    },
+
+    openMidiPianoRollWithSheetMusicView: (regionId: string, sheetMusicViewEnabled: boolean) => {
+      KGPianoRollState.instance().setSheetMusicViewEnabled(sheetMusicViewEnabled);
+      set(state => ({
+        showPianoRoll: true,
+        activeRegionId: regionId,
+        pianoRollMode: 'midi-edit',
+        hybridAudioRegionId: null,
+        requestedSheetMusicViewEnabled: sheetMusicViewEnabled,
+        pianoRollViewRequestVersion: state.pianoRollViewRequestVersion + 1,
+      }));
     },
 
     openSpectrogramViewer: (regionId: string) => {
-      set({ showPianoRoll: true, activeRegionId: regionId, pianoRollMode: 'spectrogram', hybridAudioRegionId: null });
+      set({
+        showPianoRoll: true,
+        activeRegionId: regionId,
+        pianoRollMode: 'spectrogram',
+        hybridAudioRegionId: null,
+        requestedSheetMusicViewEnabled: false,
+      });
     },
 
     openHybridMode: (midiRegionId: string, audioRegionId: string) => {
-      set({ showPianoRoll: true, activeRegionId: midiRegionId, hybridAudioRegionId: audioRegionId, pianoRollMode: 'hybrid' });
+      set({
+        showPianoRoll: true,
+        activeRegionId: midiRegionId,
+        hybridAudioRegionId: audioRegionId,
+        pianoRollMode: 'hybrid',
+        requestedSheetMusicViewEnabled: false,
+      });
     },
     bumpAutomationRedrawVersion: () => {
       set(state => ({ automationRedrawVersion: state.automationRedrawVersion + 1 }));
@@ -1579,6 +1626,8 @@ export const useProjectStore = create<ProjectState>((set, get) => {
         activeRegionId: null,
         hybridAudioRegionId: null,
         pianoRollMode: 'midi-edit',
+        requestedSheetMusicViewEnabled: false,
+        pianoRollViewRequestVersion: 0,
         activeTrackAutomationTrackId: null,
         activeTrackAutomationType: null,
         trackAutomationRedrawVersion: 0,

src/test/utils/mock-data.ts

@@ -165,7 +165,7 @@ export const createMockProject = (overrides: Partial<{
     [0, 0], // loopingRange
     1, // barWidthMultiplier
     defaults.tracks, // tracks
-    5 // projectStructureVersion
+    KGProject.CURRENT_PROJECT_STRUCTURE_VERSION // projectStructureVersion
   );
   
   return project;

src/util/localLLMConfig.ts

@@ -1,7 +1,8 @@
 export const LOCAL_LLM_PROVIDER_KEY = 'local_browser';
-export const LOCAL_LLM_MODEL_URL =
+export const LOCAL_LLM_DEFAULT_MODEL_URL =
   'https://huggingface.co/notabilia/gemma-4-E4B-it-litert-lm/resolve/main/gemma-4-E4B-it-web.task';
 export const LOCAL_LLM_MODEL_FILENAME = 'gemma-4-E4B-it-web.task';
+export const LOCAL_LLM_MODEL_EXPECTED_SIZE_BYTES = 2964324352;
 export const LOCAL_LLM_DISPLAY_NAME = 'Gemma 4 E4B';
 export const LOCAL_LLM_LEGACY_FILENAMES = [
   'gemma-3n-E4B-it-int4-Web.litertlm',

src/util/localLLMModelCache.ts

@@ -1,5 +1,5 @@
 import { OpfsModelCache, type ModelDownloadProgress } from './opfsModelCache';
-import { LOCAL_LLM_MODEL_FILENAME } from './localLLMConfig';
+import { LOCAL_LLM_MODEL_EXPECTED_SIZE_BYTES, LOCAL_LLM_MODEL_FILENAME } from './localLLMConfig';
 
 const cache = new OpfsModelCache({ directoryName: 'models' });
 let writingToCachePromise: Promise<void> | null = null;
@@ -51,7 +51,9 @@ const createProgressReader = (
 
 export class LocalLLMModelCache {
   public static async exists(filename: string = LOCAL_LLM_MODEL_FILENAME): Promise<boolean> {
-    return cache.exists(filename);
+    return cache.exists(filename, {
+      expectedSizeBytes: LOCAL_LLM_MODEL_EXPECTED_SIZE_BYTES,
+    });
   }
 
   public static async getFile(filename: string = LOCAL_LLM_MODEL_FILENAME): Promise<File> {
@@ -98,6 +100,9 @@ export class LocalLLMModelCache {
       streamForCache,
       filename,
       totalBytes > 0 ? totalBytes : null,
+      {
+        expectedSizeBytes: LOCAL_LLM_MODEL_EXPECTED_SIZE_BYTES,
+      },
       progress => onProgress?.({ ...progress, fromCache: false }),
     );
     writingToCachePromise = writingToCachePromise.finally(() => {
@@ -117,6 +122,13 @@ export class LocalLLMModelCache {
     filename: string = LOCAL_LLM_MODEL_FILENAME,
     onProgress?: (progress: ModelDownloadProgress) => void,
   ): Promise<void> {
-    await cache.download(sourceUrl, filename, onProgress);
+    await cache.download(
+      sourceUrl,
+      filename,
+      {
+        expectedSizeBytes: LOCAL_LLM_MODEL_EXPECTED_SIZE_BYTES,
+      },
+      onProgress,
+    );
   }
 }

src/util/localLLMModelManager.ts

@@ -2,7 +2,6 @@ import {
   detectLocalLLMRuntimeSupport,
   LOCAL_LLM_LEGACY_FILENAMES,
   LOCAL_LLM_MODEL_FILENAME,
-  LOCAL_LLM_MODEL_URL,
   type LocalLLMRuntimeSupport,
 } from './localLLMConfig';
 import { LocalLLMModelCache } from './localLLMModelCache';

src/util/localSeparatorConfig.ts

@@ -1,9 +1,10 @@
 import type { LocalSeparatorModelConfig } from './localSeparatorTypes';
 
-export const LOCAL_SEPARATOR_MODEL_URL =
+export const LOCAL_SEPARATOR_DEFAULT_MODEL_URL =
   'https://huggingface.co/notabilia/uvr5-models/resolve/main/UVR-MDX-NET-Inst_HQ_3.onnx';
 
 export const LOCAL_SEPARATOR_MODEL_FILENAME = 'UVR-MDX-NET-Inst_HQ_3.onnx';
+export const LOCAL_SEPARATOR_MODEL_EXPECTED_SIZE_BYTES = 66759214;
 
 export const LOCAL_SEPARATOR_MODEL_CONFIG: LocalSeparatorModelConfig = {
   filename: LOCAL_SEPARATOR_MODEL_FILENAME,

src/util/localSeparatorModelCache.ts

@@ -1,4 +1,7 @@
-import { LOCAL_SEPARATOR_MODEL_FILENAME } from './localSeparatorConfig';
+import {
+  LOCAL_SEPARATOR_MODEL_EXPECTED_SIZE_BYTES,
+  LOCAL_SEPARATOR_MODEL_FILENAME,
+} from './localSeparatorConfig';
 import { OpfsModelCache, type ModelDownloadProgress } from './opfsModelCache';
 
 const cache = new OpfsModelCache({ directoryName: 'models' });
@@ -7,7 +10,9 @@ export { type ModelDownloadProgress };
 
 export class LocalSeparatorModelCache {
   public static async exists(filename: string = LOCAL_SEPARATOR_MODEL_FILENAME): Promise<boolean> {
-    return cache.exists(filename);
+    return cache.exists(filename, {
+      expectedSizeBytes: LOCAL_SEPARATOR_MODEL_EXPECTED_SIZE_BYTES,
+    });
   }
 
   public static async getFile(filename: string = LOCAL_SEPARATOR_MODEL_FILENAME): Promise<File> {
@@ -27,6 +32,13 @@ export class LocalSeparatorModelCache {
     filename: string = LOCAL_SEPARATOR_MODEL_FILENAME,
     onProgress?: (progress: ModelDownloadProgress) => void,
   ): Promise<void> {
-    await cache.download(sourceUrl, filename, onProgress);
+    await cache.download(
+      sourceUrl,
+      filename,
+      {
+        expectedSizeBytes: LOCAL_SEPARATOR_MODEL_EXPECTED_SIZE_BYTES,
+      },
+      onProgress,
+    );
   }
 }

src/util/opfsModelCache.ts

@@ -9,6 +9,10 @@ interface OpfsModelCacheOptions {
   sizeSuffix?: string;
 }
 
+interface ModelCacheValidationOptions {
+  expectedSizeBytes?: number | null;
+}
+
 export class OpfsModelCache {
   private readonly directoryName: string;
   private readonly sizeSuffix: string;
@@ -18,7 +22,7 @@ export class OpfsModelCache {
     this.sizeSuffix = options.sizeSuffix ?? '.size';
   }
 
-  public async exists(filename: string): Promise<boolean> {
+  public async exists(filename: string, options: ModelCacheValidationOptions = {}): Promise<boolean> {
     try {
       const dir = await this.getDir();
       const fileHandle = await dir.getFileHandle(filename);
@@ -29,6 +33,10 @@ export class OpfsModelCache {
         await this.delete(filename);
         return false;
       }
+      if (options.expectedSizeBytes != null && expectedSize !== options.expectedSizeBytes) {
+        await this.delete(filename);
+        return false;
+      }
       if (file.size !== expectedSize) {
         await this.delete(filename);
         return false;
@@ -64,6 +72,7 @@ export class OpfsModelCache {
   public async download(
     sourceUrl: string,
     filename: string,
+    options: ModelCacheValidationOptions = {},
     onProgress?: (progress: ModelDownloadProgress) => void,
   ): Promise<void> {
     const response = await fetch(sourceUrl);
@@ -75,13 +84,14 @@ export class OpfsModelCache {
     if (!response.body) {
       throw new Error('Model download response did not include a readable body.');
     }
-    await this.downloadStream(response.body, filename, totalBytes, onProgress);
+    await this.downloadStream(response.body, filename, totalBytes, options, onProgress);
   }
 
   public async downloadStream(
     stream: ReadableStream<Uint8Array>,
     filename: string,
     totalBytes: number | null,
+    options: ModelCacheValidationOptions = {},
     onProgress?: (progress: ModelDownloadProgress) => void,
   ): Promise<void> {
     const dir = await this.getDir();
@@ -111,6 +121,9 @@ export class OpfsModelCache {
       if (!Number.isFinite(sizeValue) || sizeValue <= 0) {
         throw new Error('Model download did not provide a valid size.');
       }
+      if (options.expectedSizeBytes != null && receivedBytes !== options.expectedSizeBytes) {
+        throw new Error(`Model download size mismatch for ${filename}: expected ${options.expectedSizeBytes} bytes, got ${receivedBytes}.`);
+      }
 
       const sizeHandle = await dir.getFileHandle(this.getSizeFilename(filename), { create: true });
       const sizeWritable = await sizeHandle.createWritable();

src/util/regionEditUtil.test.ts

@@ -0,0 +1,214 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import type { Selectable } from '../components/interfaces';
+import { KGMidiNote } from '../core/midi/KGMidiNote';
+import { createMockMidiRegion, createMockMidiTrack, createMockProject } from '../test/utils/mock-data';
+import { splitSelectedRegionAtPlayhead } from './regionEditUtil';
+
+const storeState = {
+  showPianoRoll: false,
+  pianoRollMode: 'midi-edit' as 'midi-edit' | 'spectrogram' | 'hybrid',
+  activeRegionId: null as string | null,
+  selectedNoteIds: [] as string[],
+  setShowPianoRoll: vi.fn(),
+  setActiveRegionId: vi.fn(),
+};
+
+const mockCore = {
+  getCurrentProject: vi.fn(),
+  getSelectedItems: vi.fn<() => Selectable[]>(() => []),
+  clearSelectedItems: vi.fn(),
+  addSelectedItems: vi.fn(),
+  executeCommand: vi.fn((command: { execute: () => void }) => {
+    command.execute();
+  }),
+};
+
+const dialogMocks = vi.hoisted(() => ({
+  showAlert: vi.fn(),
+  showConfirm: vi.fn(),
+}));
+
+const pianoRollStateMock = vi.hoisted(() => ({
+  getSheetMusicViewEnabled: vi.fn(() => false),
+}));
+
+vi.mock('../stores/projectStore', () => ({
+  useProjectStore: {
+    getState: vi.fn(() => storeState),
+  },
+}));
+
+vi.mock('../core/KGCore', () => ({
+  KGCore: {
+    instance: vi.fn(() => mockCore),
+  },
+}));
+
+vi.mock('../core/state/KGPianoRollState', () => ({
+  KGPianoRollState: {
+    instance: vi.fn(() => pianoRollStateMock),
+  },
+}));
+
+vi.mock('./dialogUtil', () => ({
+  showAlert: dialogMocks.showAlert,
+  showConfirm: dialogMocks.showConfirm,
+}));
+
+describe('splitSelectedRegionAtPlayhead', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    storeState.showPianoRoll = false;
+    storeState.pianoRollMode = 'midi-edit';
+    storeState.activeRegionId = null;
+    storeState.selectedNoteIds = [];
+    storeState.setShowPianoRoll.mockReset();
+    storeState.setActiveRegionId.mockReset();
+    pianoRollStateMock.getSheetMusicViewEnabled.mockReturnValue(false);
+    mockCore.getSelectedItems.mockReset();
+    mockCore.getSelectedItems.mockReturnValue([]);
+    mockCore.clearSelectedItems.mockReset();
+    mockCore.addSelectedItems.mockReset();
+  });
+
+  it('falls back to region split when the piano roll is closed', async () => {
+    const region = createMockMidiRegion({ id: 'region-1', trackId: '1', trackIndex: 0, startFromBeat: 0, length: 8 });
+    const track = createMockMidiTrack({ id: 1, regions: [region] });
+    mockCore.getCurrentProject.mockReturnValue(createMockProject({ tracks: [track] }));
+
+    const status = await splitSelectedRegionAtPlayhead({
+      selectedRegionIds: ['region-1'],
+      playheadPosition: 4,
+      refreshProjectState: vi.fn(),
+    });
+
+    expect(status).toBe('Split region at beat 4.00');
+    expect(mockCore.executeCommand).toHaveBeenCalledTimes(1);
+    expect(dialogMocks.showAlert).not.toHaveBeenCalled();
+  });
+
+  it('falls back to region split when not in piano-roll view', async () => {
+    const region = createMockMidiRegion({ id: 'region-1', trackId: '1', trackIndex: 0, startFromBeat: 0, length: 8 });
+    const track = createMockMidiTrack({ id: 1, regions: [region] });
+    mockCore.getCurrentProject.mockReturnValue(createMockProject({ tracks: [track] }));
+    storeState.showPianoRoll = true;
+    storeState.pianoRollMode = 'spectrogram';
+    storeState.selectedNoteIds = ['note-1'];
+
+    const status = await splitSelectedRegionAtPlayhead({
+      selectedRegionIds: ['region-1'],
+      playheadPosition: 4,
+      refreshProjectState: vi.fn(),
+    });
+
+    expect(status).toBe('Split region at beat 4.00');
+    expect(mockCore.executeCommand).toHaveBeenCalledTimes(1);
+  });
+
+  it('falls back to region split when sheet music view is enabled', async () => {
+    const region = createMockMidiRegion({ id: 'region-1', trackId: '1', trackIndex: 0, startFromBeat: 0, length: 8 });
+    const track = createMockMidiTrack({ id: 1, regions: [region] });
+    mockCore.getCurrentProject.mockReturnValue(createMockProject({ tracks: [track] }));
+    storeState.showPianoRoll = true;
+    storeState.activeRegionId = 'region-1';
+    storeState.selectedNoteIds = ['note-1'];
+    pianoRollStateMock.getSheetMusicViewEnabled.mockReturnValue(true);
+
+    const status = await splitSelectedRegionAtPlayhead({
+      selectedRegionIds: ['region-1'],
+      playheadPosition: 4,
+      refreshProjectState: vi.fn(),
+    });
+
+    expect(status).toBe('Split region at beat 4.00');
+    expect(mockCore.executeCommand).toHaveBeenCalledTimes(1);
+  });
+
+  it('splits selected notes when the piano roll is open in midi-edit view', async () => {
+    const note = new KGMidiNote('note-1', 1, 5, 60, 100);
+    const region = createMockMidiRegion({
+      id: 'region-1',
+      trackId: '1',
+      trackIndex: 0,
+      notes: [note],
+    });
+    const track = createMockMidiTrack({ id: 1, regions: [region] });
+    mockCore.getCurrentProject.mockReturnValue(createMockProject({ tracks: [track] }));
+    storeState.showPianoRoll = true;
+    storeState.activeRegionId = 'region-1';
+    storeState.selectedNoteIds = ['note-1'];
+    const refreshProjectState = vi.fn();
+    mockCore.getSelectedItems.mockReturnValue([note]);
+
+    const status = await splitSelectedRegionAtPlayhead({
+      selectedRegionIds: ['region-1'],
+      playheadPosition: 3,
+      refreshProjectState,
+    });
+
+    expect(status).toBe('Split 1 note at beat 3.00');
+    expect(mockCore.executeCommand).toHaveBeenCalledTimes(1);
+    expect(refreshProjectState).toHaveBeenCalledTimes(1);
+    expect(region.getNotes().map(candidate => [candidate.getStartBeat(), candidate.getEndBeat()])).toEqual([
+      [1, 3],
+      [3, 5],
+    ]);
+  });
+
+  it('splits selected notes using playhead position relative to the region start', async () => {
+    const note = new KGMidiNote('note-1', 1, 5, 60, 100);
+    const region = createMockMidiRegion({
+      id: 'region-1',
+      trackId: '1',
+      trackIndex: 0,
+      startFromBeat: 8,
+      length: 8,
+      notes: [note],
+    });
+    const track = createMockMidiTrack({ id: 1, regions: [region] });
+    mockCore.getCurrentProject.mockReturnValue(createMockProject({ tracks: [track] }));
+    storeState.showPianoRoll = true;
+    storeState.activeRegionId = 'region-1';
+    storeState.selectedNoteIds = ['note-1'];
+    mockCore.getSelectedItems.mockReturnValue([note]);
+
+    const status = await splitSelectedRegionAtPlayhead({
+      selectedRegionIds: ['region-1'],
+      playheadPosition: 11,
+      refreshProjectState: vi.fn(),
+    });
+
+    expect(status).toBe('Split 1 note at beat 11.00');
+    expect(region.getNotes().map(candidate => [candidate.getStartBeat(), candidate.getEndBeat()])).toEqual([
+      [1, 3],
+      [3, 5],
+    ]);
+  });
+
+  it('shows an alert when no selected note crosses the playhead', async () => {
+    const note = new KGMidiNote('note-1', 1, 2, 60, 100);
+    const region = createMockMidiRegion({
+      id: 'region-1',
+      trackId: '1',
+      trackIndex: 0,
+      notes: [note],
+    });
+    const track = createMockMidiTrack({ id: 1, regions: [region] });
+    mockCore.getCurrentProject.mockReturnValue(createMockProject({ tracks: [track] }));
+    storeState.showPianoRoll = true;
+    storeState.activeRegionId = 'region-1';
+    storeState.selectedNoteIds = ['note-1'];
+
+    const status = await splitSelectedRegionAtPlayhead({
+      selectedRegionIds: ['region-1'],
+      playheadPosition: 3,
+      refreshProjectState: vi.fn(),
+    });
+
+    expect(status).toBeNull();
+    expect(dialogMocks.showAlert).toHaveBeenCalledWith(
+      'The playhead is not inside any selected note. Move the playhead inside a selected note before splitting.'
+    );
+    expect(mockCore.executeCommand).not.toHaveBeenCalled();
+  });
+});

src/util/regionEditUtil.ts

@@ -0,0 +1,250 @@
+import { KGCore } from '../core/KGCore';
+import { SplitSelectedNotesCommand } from '../core/commands/note/SplitSelectedNotesCommand';
+import { SplitRegionCommand } from '../core/commands/region/SplitRegionCommand';
+import { MergeMidiRegionsCommand } from '../core/commands/region/MergeMidiRegionsCommand';
+import { KGMidiRegion } from '../core/region/KGMidiRegion';
+import { KGPianoRollState } from '../core/state/KGPianoRollState';
+import { useProjectStore } from '../stores/projectStore';
+import { showAlert, showConfirm } from './dialogUtil';
+
+interface SplitSelectedRegionParams {
+  selectedRegionIds: string[];
+  playheadPosition: number;
+  refreshProjectState: () => void;
+}
+
+interface MergeSelectedMidiRegionsParams {
+  selectedRegionIds: string[];
+  refreshProjectState: () => void;
+}
+
+export const splitSelectedRegionAtPlayhead = async ({
+  selectedRegionIds,
+  playheadPosition,
+  refreshProjectState,
+}: SplitSelectedRegionParams): Promise<string | null> => {
+  const {
+    activeRegionId,
+    pianoRollMode,
+    selectedNoteIds,
+    showPianoRoll,
+  } = useProjectStore.getState();
+  const sheetMusicViewEnabled = KGPianoRollState.instance().getSheetMusicViewEnabled();
+
+  if (
+    showPianoRoll &&
+    pianoRollMode === 'midi-edit' &&
+    !sheetMusicViewEnabled &&
+    activeRegionId &&
+    selectedNoteIds.length > 0
+  ) {
+    return splitSelectedNotesAtPlayhead({
+      activeRegionId,
+      selectedNoteIds,
+      playheadPosition,
+      refreshProjectState,
+    });
+  }
+
+  return splitSingleSelectedRegionAtPlayhead({
+    selectedRegionIds,
+    playheadPosition,
+    refreshProjectState,
+  });
+};
+
+const splitSelectedNotesAtPlayhead = async ({
+  activeRegionId,
+  selectedNoteIds,
+  playheadPosition,
+  refreshProjectState,
+}: {
+  activeRegionId: string;
+  selectedNoteIds: string[];
+  playheadPosition: number;
+  refreshProjectState: () => void;
+}): Promise<string | null> => {
+  const tracks = KGCore.instance().getCurrentProject().getTracks();
+  let activeRegion: KGMidiRegion | null = null;
+
+  for (const track of tracks) {
+    const region = track.getRegions().find(candidate => candidate.getId() === activeRegionId);
+    if (region instanceof KGMidiRegion) {
+      activeRegion = region;
+      break;
+    }
+  }
+
+  if (!activeRegion) {
+    await showAlert('The active MIDI region could not be found. Please reopen the piano roll and try again.');
+    return null;
+  }
+
+  const selectedNoteIdSet = new Set(selectedNoteIds);
+  const selectedNotes = activeRegion.getNotes().filter(note => selectedNoteIdSet.has(note.getId()));
+  if (selectedNotes.length === 0) {
+    await showAlert('The selected notes could not be found in the active MIDI region. Please reselect the notes and try again.');
+    return null;
+  }
+
+  const regionRelativePlayhead = playheadPosition - activeRegion.getStartFromBeat();
+  const splitCount = selectedNotes.filter(note => (
+    note.getStartBeat() < regionRelativePlayhead && regionRelativePlayhead < note.getEndBeat()
+  )).length;
+  if (splitCount === 0) {
+    await showAlert('The playhead is not inside any selected note. Move the playhead inside a selected note before splitting.');
+    return null;
+  }
+
+  try {
+    const command = new SplitSelectedNotesCommand(activeRegionId, selectedNoteIds, regionRelativePlayhead);
+    KGCore.instance().executeCommand(command, { rethrow: true });
+    refreshProjectState();
+    return `Split ${splitCount} note${splitCount === 1 ? '' : 's'} at beat ${playheadPosition.toFixed(2)}`;
+  } catch (error) {
+    await showAlert(error instanceof Error ? error.message : 'Unable to split the selected notes.');
+    return null;
+  }
+};
+
+const splitSingleSelectedRegionAtPlayhead = async ({
+  selectedRegionIds,
+  playheadPosition,
+  refreshProjectState,
+}: SplitSelectedRegionParams): Promise<string | null> => {
+  if (selectedRegionIds.length === 0) {
+    await showAlert('Please select a region to split.');
+    return null;
+  }
+
+  if (selectedRegionIds.length > 1) {
+    await showAlert('Please select exactly one region to split.');
+    return null;
+  }
+
+  const regionId = selectedRegionIds[selectedRegionIds.length - 1];
+  const tracks = KGCore.instance().getCurrentProject().getTracks();
+  let targetRegion = null;
+
+  for (const track of tracks) {
+    const foundRegion = track.getRegions().find(region => region.getId() === regionId);
+    if (foundRegion) {
+      targetRegion = foundRegion;
+      break;
+    }
+  }
+
+  if (!targetRegion) {
+    await showAlert('Selected region not found.');
+    return null;
+  }
+
+  const regionStart = targetRegion.getStartFromBeat();
+  const regionEnd = regionStart + targetRegion.getLength();
+
+  if (playheadPosition <= regionStart || playheadPosition >= regionEnd) {
+    await showAlert('The playhead is not inside the selected region. Move the playhead inside the region before splitting.');
+    return null;
+  }
+
+  const command = new SplitRegionCommand(regionId, playheadPosition);
+  KGCore.instance().executeCommand(command);
+  refreshProjectState();
+
+  return `Split region at beat ${playheadPosition.toFixed(2)}`;
+};
+
+export const mergeSelectedMidiRegions = async ({
+  selectedRegionIds,
+  refreshProjectState,
+}: MergeSelectedMidiRegionsParams): Promise<string | null> => {
+  if (selectedRegionIds.length < 2) {
+    await showAlert('Please select at least two MIDI regions on the same track to merge.');
+    return null;
+  }
+
+  const tracks = KGCore.instance().getCurrentProject().getTracks();
+  const selectedRegionIdSet = new Set(selectedRegionIds);
+  const selectedMidiRegions: KGMidiRegion[] = [];
+  let targetTrackId: string | null = null;
+
+  for (const track of tracks) {
+    for (const region of track.getRegions()) {
+      if (!selectedRegionIdSet.has(region.getId())) {
+        continue;
+      }
+
+      if (!(region instanceof KGMidiRegion)) {
+        await showAlert('Only MIDI regions can be merged. Please adjust your selection and try again.');
+        return null;
+      }
+
+      const regionTrackId = track.getId().toString();
+      if (targetTrackId && targetTrackId !== regionTrackId) {
+        await showAlert('Please select only MIDI regions from a single track before merging.');
+        return null;
+      }
+
+      targetTrackId = regionTrackId;
+      selectedMidiRegions.push(region);
+    }
+  }
+
+  if (selectedMidiRegions.length !== selectedRegionIds.length || !targetTrackId) {
+    await showAlert('Some selected regions could not be found. Please reselect the MIDI regions and try again.');
+    return null;
+  }
+
+  const sortedSelectedRegions = [...selectedMidiRegions].sort((a, b) => {
+    const startDelta = a.getStartFromBeat() - b.getStartFromBeat();
+    if (startDelta !== 0) {
+      return startDelta;
+    }
+    return a.getLength() - b.getLength();
+  });
+
+  let regionIdsToMerge = selectedRegionIds;
+  const firstSelectedRegion = sortedSelectedRegions[0];
+  const lastSelectedRegion = sortedSelectedRegions[sortedSelectedRegions.length - 1];
+  const spanStart = firstSelectedRegion.getStartFromBeat();
+  const spanEnd = lastSelectedRegion.getStartFromBeat() + lastSelectedRegion.getLength();
+
+  const targetTrack = tracks.find(track => track.getId().toString() === targetTrackId);
+  const inBetweenRegions = targetTrack
+    ?.getRegions()
+    .filter(region => (
+      region instanceof KGMidiRegion &&
+      !selectedRegionIdSet.has(region.getId()) &&
+      region.getStartFromBeat() >= spanStart &&
+      region.getStartFromBeat() <= spanEnd
+    )) ?? [];
+
+  if (inBetweenRegions.length > 0) {
+    const shouldIncludeInBetweenRegions = await showConfirm(
+      'There are additional MIDI regions between the first and last selected regions on this track. Would you like KGStudio to merge those as well?',
+      {
+        confirmLabel: 'Merge All In Between',
+        cancelLabel: 'Stop',
+      }
+    );
+
+    if (!shouldIncludeInBetweenRegions) {
+      return null;
+    }
+
+    regionIdsToMerge = Array.from(new Set([
+      ...selectedRegionIds,
+      ...inBetweenRegions.map(region => region.getId()),
+    ]));
+  }
+
+  try {
+    const command = new MergeMidiRegionsCommand(regionIdsToMerge);
+    KGCore.instance().executeCommand(command, { rethrow: true });
+    refreshProjectState();
+    return `Merged ${regionIdsToMerge.length} MIDI regions`;
+  } catch (error) {
+    await showAlert(error instanceof Error ? error.message : 'Unable to merge the selected MIDI regions.');
+    return null;
+  }
+};

src/util/scaleUtil.test.ts

@@ -387,6 +387,12 @@ describe('scaleUtil', () => {
       expect(typeof result).toBe('string');
       expect(result).toContain('#282828');
       expect(result).toContain('#303030');
+      expect(result).toContain('#404040 calc(var(--region-grid-bar-width) - 1px)');
+      expect(result).toContain('#343434 calc(var(--region-grid-beat-width) - 1px)');
+      expect(result).toContain('#303030 calc(var(--region-piano-key-height) * 0)');
+      expect(result).toContain('#404040 calc(var(--region-piano-key-height) * 12 - 1px)');
+      expect(result).toContain('#282828 calc(var(--region-piano-key-height) * 1)');
+      expect(result).toContain('#343434 calc(var(--region-piano-key-height) * 2 - 1px)');
     });
 
     it('should generate different backgrounds for different modes', () => {

src/util/scaleUtil.ts

@@ -304,6 +304,9 @@ export const generatePianoGridBackground = (
   selectedMode: string,
   keySignature: KeySignature
 ): string => {
+  const majorGridLineColor = '#404040';
+  const minorGridLineColor = '#343434';
+
   // Get root note and scale pitch classes
   const rootNote = getRootNoteFromKeySignature(keySignature);
   const modeSteps = getModeSteps(selectedMode);
@@ -314,37 +317,36 @@ export const generatePianoGridBackground = (
     const pitch = pianoRollIndexToPitch(index);
     const pitchClass = pitch % 12;
     const isInScale = scalePitchClasses.includes(pitchClass);
+    const isCRow = pitchClass === 0;
 
     // Calculate row positions using CSS calc() with --region-piano-key-height variable
     const rowTop = `calc(var(--region-piano-key-height) * ${index})`;
     const rowBottomMinusOne = `calc(var(--region-piano-key-height) * ${index + 1} - 1px)`;
     const rowBottom = `calc(var(--region-piano-key-height) * ${index + 1})`;
+    const rowFillColor = isInScale ? '#303030' : '#282828';
+    const horizontalLineColor = isCRow ? majorGridLineColor : minorGridLineColor;
 
     // Match the event list palette while preserving scale-aware row distinction.
-    if (isInScale) {
-      return `
-        #282828 ${rowTop},
-        #282828 ${rowBottomMinusOne},
-        #3a3a3a ${rowBottomMinusOne},
-        #3a3a3a ${rowBottom}
-      `.trim();
-    } else {
-      return `
-        #303030 ${rowTop},
-        #303030 ${rowBottomMinusOne},
-        #3a3a3a ${rowBottomMinusOne},
-        #3a3a3a ${rowBottom}
-      `.trim();
-    }
+    return `
+      ${rowFillColor} ${rowTop},
+      ${rowFillColor} ${rowBottomMinusOne},
+      ${horizontalLineColor} ${rowBottomMinusOne},
+      ${horizontalLineColor} ${rowBottom}
+    `.trim();
   }).join(',\n');
 
   // Return complete background-image with vertical and horizontal gradients
-  // Note: Vertical beat lines gradient should be preserved from existing CSS
+  // Major bar lines sit above minor beat lines so bar boundaries remain the primary anchors.
   return `
+    linear-gradient(to right,
+      transparent calc(var(--region-grid-bar-width) - 1px),
+      ${majorGridLineColor} calc(var(--region-grid-bar-width) - 1px),
+      ${majorGridLineColor} var(--region-grid-bar-width)
+    ),
     linear-gradient(to right,
       transparent calc(var(--region-grid-beat-width) - 1px),
-      #3a3a3a calc(var(--region-grid-beat-width) - 1px),
-      #3a3a3a var(--region-grid-beat-width)
+      ${minorGridLineColor} calc(var(--region-grid-beat-width) - 1px),
+      ${minorGridLineColor} var(--region-grid-beat-width)
     ),
     linear-gradient(to bottom, ${horizontalLines})
   `;