feat: preserve the viewport position when zooming in and out

src/components/MainContent.tsx

@@ -1,4 +1,4 @@
-import React, { useState, useEffect, useRef, useCallback } from 'react';
+import React, { useState, useEffect, useRef, useCallback, useLayoutEffect } from 'react';
 import './MainContent.css';
 import { createPortal } from 'react-dom';
 import { useProjectStore } from '../stores/projectStore';
@@ -10,7 +10,7 @@ import TrackInfoPanel from './track/TrackInfoPanel';
 import TrackGridPanel from './track/TrackGridPanel';
 import PianoRoll from './piano-roll/PianoRoll';
 import type { RegionUI } from './interfaces';
-import { DEBUG_MODE, BAR_NUMBERS_CONSTANTS } from '../constants';
+import { DEBUG_MODE, BAR_NUMBERS_CONSTANTS, TOOLBAR_CONSTANTS } from '../constants';
 import { useRegionOperations } from '../hooks/useRegionOperations';
 import { regionDeleteManager } from '../util/regionDeleteUtil';
 import { KGMainContentState } from '../core/state/KGMainContentState';
@@ -26,6 +26,7 @@ const MainContent: React.FC<MainContentProps> = ({
   const {
     tracks,
     maxBars,
+    barWidthMultiplier,
     reorderTracks,
     updateTrack,
     updateTrackProperties,
@@ -96,6 +97,7 @@ const MainContent: React.FC<MainContentProps> = ({
   const mainContentRef = useRef<HTMLDivElement | null>(null);
   const expectedScrollLeftRef = useRef<number>(-1);
   const isPlayingRef = useRef(false);
+  const previousBarWidthMultiplierRef = useRef(barWidthMultiplier);
 
   // Refs for bar numbers and loop range drag functionality
   const barNumbersRef = useRef<HTMLDivElement | null>(null);
@@ -109,6 +111,37 @@ const MainContent: React.FC<MainContentProps> = ({
     isPlayingRef.current = isPlaying;
   }, [isPlaying]);
 
+  useLayoutEffect(() => {
+    const previousMultiplier = previousBarWidthMultiplierRef.current;
+    if (previousMultiplier === barWidthMultiplier) return;
+
+    previousBarWidthMultiplierRef.current = barWidthMultiplier;
+
+    const container = mainContentRef.current;
+    if (!container) return;
+
+    const infoWidth = parseInt(
+      getComputedStyle(document.documentElement).getPropertyValue('--track-info-panel-width')
+    ) || 200;
+    const visibleMusicWidth = Math.max(0, container.clientWidth - infoWidth);
+    const previousBarWidth = TOOLBAR_CONSTANTS.BASE_BAR_WIDTH * previousMultiplier;
+    const nextBarWidth = TOOLBAR_CONSTANTS.BASE_BAR_WIDTH * barWidthMultiplier;
+
+    if (visibleMusicWidth === 0 || previousBarWidth === 0 || nextBarWidth === 0) return;
+
+    const centerPixelBeforeZoom = container.scrollLeft + visibleMusicWidth / 2;
+    const anchorBeat = (centerPixelBeforeZoom / previousBarWidth) * timeSignature.numerator;
+    const targetPixel = (anchorBeat / timeSignature.numerator) * nextBarWidth;
+    const targetScrollLeft = targetPixel - visibleMusicWidth / 2;
+    const clampedScrollLeft = Math.max(
+      0,
+      Math.min(targetScrollLeft, container.scrollWidth - container.clientWidth)
+    );
+
+    expectedScrollLeftRef.current = clampedScrollLeft;
+    container.scrollLeft = clampedScrollLeft;
+  }, [barWidthMultiplier, timeSignature]);
+
   // Detect manual horizontal scroll during playback
   useEffect(() => {
     const container = mainContentRef.current;

src/components/piano-roll/PianoRoll.tsx

@@ -1,11 +1,11 @@
-import React, { useRef, useEffect, useState, useCallback } from 'react';
+import React, { useRef, useEffect, useState, useCallback, useLayoutEffect } from 'react';
 import './PianoRoll.css';
 import type { MouseEvent } from 'react';
 import { useProjectStore } from '../../stores/projectStore';
 import { FaGripLines } from 'react-icons/fa';
 import { KGMidiRegion } from '../../core/region/KGMidiRegion';
 import type { KGAudioRegion } from '../../core/region/KGAudioRegion';
-import { DEBUG_MODE, PIANO_ROLL_CONSTANTS } from '../../constants';
+import { DEBUG_MODE, PIANO_ROLL_CONSTANTS, TOOLBAR_CONSTANTS } from '../../constants';
 import PianoRollHeader from './PianoRollHeader';
 import PianoRollToolbar from './PianoRollToolbar';
 import PianoRollContent from './PianoRollContent';
@@ -81,6 +81,7 @@ const PianoRoll: React.FC<PianoRollProps> = ({
   // Refs for auto-scroll during playback
   const pianoRollExpectedScrollLeftRef = useRef<number>(-1);
   const pianoRollIsPlayingRef = useRef(false);
+  const pendingZoomAnchorBeatRef = useRef<number | null>(null);
 
   // Ref for storing the setNoteUpdateCounter function
   const triggerNoteUpdateRef = useRef<React.Dispatch<React.SetStateAction<number>> | null>(null);
@@ -613,6 +614,30 @@ const PianoRoll: React.FC<PianoRollProps> = ({
     }
   }, [quantizeSelectedNotes, quantizeNoteLength]);
 
+  const handleZoomChange = useCallback((nextZoom: number) => {
+    if (nextZoom === pianoRollZoom) return;
+
+    const container = pianoRollContentRef.current;
+    pendingZoomAnchorBeatRef.current = null;
+    if (container) {
+      const keysWidth = parseInt(
+        getComputedStyle(document.documentElement).getPropertyValue('--region-piano-key-width')
+      ) || 60;
+      const visibleMusicWidth = Math.max(0, container.clientWidth - keysWidth);
+      const beatWidth = parseInt(
+        getComputedStyle(document.documentElement).getPropertyValue('--region-grid-beat-width')
+      ) || TOOLBAR_CONSTANTS.BASE_BAR_WIDTH;
+
+      if (visibleMusicWidth > 0 && beatWidth > 0) {
+        pendingZoomAnchorBeatRef.current = (container.scrollLeft + visibleMusicWidth / 2) / beatWidth;
+      } else {
+        pendingZoomAnchorBeatRef.current = null;
+      }
+    }
+
+    setPianoRollZoom(nextZoom);
+  }, [pianoRollZoom]);
+
   // Calculate C4 position and scroll to it when piano roll opens
   useEffect(() => {
     if (pianoRollContentRef.current) {
@@ -717,12 +742,33 @@ const PianoRoll: React.FC<PianoRollProps> = ({
     useProjectStore.setState({ pianoRollScrollRequest: null });
   }, [pianoRollScrollRequest]);
 
-  // Update --region-grid-beat-width when zoom changes; reset on unmount
+  // Update --region-grid-beat-width when zoom changes and preserve the centered beat position.
-  useEffect(() => {
+  useLayoutEffect(() => {
-    document.documentElement.style.setProperty('--region-grid-beat-width', `${40 * pianoRollZoom}px`);
+    const beatWidth = TOOLBAR_CONSTANTS.BASE_BAR_WIDTH * pianoRollZoom;
+    document.documentElement.style.setProperty('--region-grid-beat-width', `${beatWidth}px`);
     if (triggerNoteUpdateRef.current) {
       triggerNoteUpdateRef.current(prev => prev + 1);
     }
+
+    const anchorBeat = pendingZoomAnchorBeatRef.current;
+    const container = pianoRollContentRef.current;
+    if (anchorBeat !== null && container) {
+      const keysWidth = parseInt(
+        getComputedStyle(document.documentElement).getPropertyValue('--region-piano-key-width')
+      ) || 60;
+      const visibleMusicWidth = Math.max(0, container.clientWidth - keysWidth);
+      const targetPixel = anchorBeat * beatWidth;
+      const targetScrollLeft = targetPixel - visibleMusicWidth / 2;
+      const clampedScrollLeft = Math.max(
+        0,
+        Math.min(targetScrollLeft, container.scrollWidth - container.clientWidth)
+      );
+
+      pianoRollExpectedScrollLeftRef.current = clampedScrollLeft;
+      container.scrollLeft = clampedScrollLeft;
+      pendingZoomAnchorBeatRef.current = null;
+    }
+
     return () => {
       document.documentElement.style.setProperty('--region-grid-beat-width', '40px');
     };
@@ -970,7 +1016,7 @@ const PianoRoll: React.FC<PianoRollProps> = ({
         power={spectrogramPower}
         onPowerChange={setSpectrogramPower}
         zoom={pianoRollZoom}
-        onZoomChange={setPianoRollZoom}
+        onZoomChange={handleZoomChange}
       />
 
       <PianoRollContent