From 4cf474be7ee54a147366eacf40d085fe976d78e3 Mon Sep 17 00:00:00 2001
From: Xiaohan-Tian <157918347+Xiaohan-Tian@users.noreply.github.com>
Date: Mon, 18 May 2026 19:00:21 -0700
Subject: [PATCH] feat: added split notes function

---
 src/components/Toolbar.test.tsx               |  37 ++-
 src/components/Toolbar.tsx                    |   1 +
 src/core/commands/index.ts                    |   1 +
 .../note/SplitSelectedNotesCommand.test.ts    | 131 +++++++++++
 .../note/SplitSelectedNotesCommand.ts         | 164 ++++++++++++++
 src/core/config/ConfigManager.ts              |   5 +-
 src/hooks/useGlobalKeyboardHandler.test.tsx   |   6 +-
 src/util/regionEditUtil.test.ts               | 214 ++++++++++++++++++
 src/util/regionEditUtil.ts                    |  92 ++++++++
 9 files changed, 645 insertions(+), 6 deletions(-)
 create mode 100644 src/core/commands/note/SplitSelectedNotesCommand.test.ts
 create mode 100644 src/core/commands/note/SplitSelectedNotesCommand.ts
 create mode 100644 src/util/regionEditUtil.test.ts

diff --git a/src/components/Toolbar.test.tsx b/src/components/Toolbar.test.tsx
index fdbdbdd..871d847 100644
--- a/src/components/Toolbar.test.tsx
+++ b/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');
+    });
+  });
 });
diff --git a/src/components/Toolbar.tsx b/src/components/Toolbar.tsx
index be83bbb..19f4c66 100644
--- a/src/components/Toolbar.tsx
+++ b/src/components/Toolbar.tsx
@@ -16,6 +16,7 @@ import {
 } from 'react-icons/fa';
 import { KGProject, type KeySignature } from '../core/KGProject';
 import { KGMidiInput } from '../core/midi-input/KGMidiInput';
+import { KGMidiRegion } from '../core/region/KGMidiRegion';
 import { KGAudioTrack } from '../core/track/KGAudioTrack';
 import { plainToInstance } from 'class-transformer';
 import { FaPencil, FaCopy, FaPaste, FaTrash, FaWandMagicSparkles, FaListUl } from 'react-icons/fa6';
diff --git a/src/core/commands/index.ts b/src/core/commands/index.ts
index cc321f9..cbd015d 100644
--- a/src/core/commands/index.ts
+++ b/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';
diff --git a/src/core/commands/note/SplitSelectedNotesCommand.test.ts b/src/core/commands/note/SplitSelectedNotesCommand.test.ts
new file mode 100644
index 0000000..624cd00
--- /dev/null
+++ b/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.'
+    );
+  });
+});
diff --git a/src/core/commands/note/SplitSelectedNotesCommand.ts b/src/core/commands/note/SplitSelectedNotesCommand.ts
new file mode 100644
index 0000000..3de7260
--- /dev/null
+++ b/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`);
+  }
+}
diff --git a/src/core/config/ConfigManager.ts b/src/core/config/ConfigManager.ts
index 8a0027b..06e0464 100644
--- a/src/core/config/ConfigManager.ts
+++ b/src/core/config/ConfigManager.ts
@@ -231,13 +231,16 @@ 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', 
diff --git a/src/hooks/useGlobalKeyboardHandler.test.tsx b/src/hooks/useGlobalKeyboardHandler.test.tsx
index 305c7a8..795b12a 100644
--- a/src/hooks/useGlobalKeyboardHandler.test.tsx
+++ b/src/hooks/useGlobalKeyboardHandler.test.tsx
@@ -25,6 +25,7 @@ const storeState = {
   stopRecording: vi.fn(),
   activeRegionId: null,
   selectedRegionIds: ['region-a', 'region-b'],
+  selectedNoteIds: [],
   setActiveRegionId: vi.fn(),
   setShowPianoRoll: vi.fn(),
   showPianoRoll: false,
@@ -32,6 +33,7 @@ const storeState = {
   openSpectrogramViewer: vi.fn(),
   playheadPosition: 12,
   refreshProjectState: vi.fn(),
+  pianoRollMode: 'midi-edit' as const,
 };
 
 type StoreState = typeof storeState;
@@ -143,7 +145,7 @@ describe('useGlobalKeyboardHandler region shortcuts', () => {
   });
 
   it('triggers split on Ctrl+T', async () => {
-    regionEditUtilMocks.splitSelectedRegionAtPlayhead.mockResolvedValue('Split region at beat 12.00');
+    regionEditUtilMocks.splitSelectedRegionAtPlayhead.mockResolvedValue('Split 1 note at beat 12.00');
 
     render(<HookHarness />);
     fireEvent.keyDown(document.body, { key: 't', ctrlKey: true });
@@ -157,7 +159,7 @@ describe('useGlobalKeyboardHandler region shortcuts', () => {
     });
 
     await waitFor(() => {
-      expect(storeState.setStatus).toHaveBeenCalledWith('Split region at beat 12.00');
+      expect(storeState.setStatus).toHaveBeenCalledWith('Split 1 note at beat 12.00');
     });
   });
 
diff --git a/src/util/regionEditUtil.test.ts b/src/util/regionEditUtil.test.ts
new file mode 100644
index 0000000..ae596bf
--- /dev/null
+++ b/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();
+  });
+});
diff --git a/src/util/regionEditUtil.ts b/src/util/regionEditUtil.ts
index 669464f..5aa945e 100644
--- a/src/util/regionEditUtil.ts
+++ b/src/util/regionEditUtil.ts
@@ -1,7 +1,10 @@
 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 {
@@ -19,6 +22,95 @@ 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.');