perf(studio): centralize timeline viewport geometry

packages/studio/src/components/nle/NLEContext.tsx

@@ -48,6 +48,7 @@ export interface NLEContextValue {
   compositionLoading: boolean;
   setCompositionLoading: (loading: boolean) => void;
   timelineDisabled: boolean;
+  timelineSessionEpoch: number;
   hasLoadedOnceRef: React.MutableRefObject<boolean>;
   // preview composition size (for preview block drop)
   previewCompositionSize: { width: number; height: number } | null;
@@ -103,7 +104,7 @@ export function NLEProvider({
   // project would otherwise keep rendering (and re-fetching from) the old project
   // after switching.
   useEffect(() => {
-    usePlayerStore.getState().reset();
+    usePlayerStore.getState().beginTimelineSession(projectId);
     useAssetPreviewStore.getState().clearPreviewAsset();
   }, [projectId]);
 
@@ -289,6 +290,7 @@ export function NLEProvider({
     setCompositionLoadingRaw(loading);
   }, []);
   const timelineDisabled = shouldDisableTimelineWhileCompositionLoading(compositionLoading);
+  const timelineSessionEpoch = usePlayerStore((state) => state.timelineSessionEpoch);
 
   useEffect(() => {
     onCompositionLoadingChange?.(compositionLoading);
@@ -319,6 +321,7 @@ export function NLEProvider({
     compositionLoading,
     setCompositionLoading,
     timelineDisabled,
+    timelineSessionEpoch,
     hasLoadedOnceRef,
     previewCompositionSize,
     setPreviewCompositionSize,

packages/studio/src/components/nle/TimelinePane.tsx

@@ -127,6 +127,7 @@ export function TimelinePane({
     persistTimelineH,
     containerRef,
     timelineDisabled,
+    timelineSessionEpoch,
   } = useNLEContext();
 
   // Move/resize/split come from the timeline edit context, not props — the
@@ -271,6 +272,7 @@ export function TimelinePane({
         >
           <div className="flex-shrink-0">{timelineToolbar}</div>
           <Timeline
+            sessionEpoch={timelineSessionEpoch}
             onSeek={seek}
             onDrillDown={handleDrillDown}
             renderClipContent={renderClipContent}

packages/studio/src/hooks/useStudioTestHooks.test.tsx

@@ -5,6 +5,8 @@ import { afterEach, describe, expect, it, vi } from "vitest";
 import { usePlayerStore } from "../player/store/playerStore";
 import {
   createTimelinePerformanceFixture,
+  hasTimelinePerformanceFixtureLease,
+  setTimelinePerformanceFixtureLease,
   type TimelinePerformanceFixtureProfile,
 } from "../player/lib/timelinePerformanceFixture";
 import { useStudioTestHooks } from "./useStudioTestHooks";
@@ -30,6 +32,7 @@ function Probe(): null {
 
 describe("timeline performance fixture", () => {
   afterEach(() => {
+    setTimelinePerformanceFixtureLease(false);
     window.__studioTest = undefined;
     usePlayerStore.getState().reset();
   });
@@ -101,9 +104,11 @@ describe("timeline performance fixture", () => {
     });
     expect(usePlayerStore.getState().elements).toHaveLength(1_000);
     expect(usePlayerStore.getState().expandedClipIds.size).toBe(1_000);
+    expect(hasTimelinePerformanceFixtureLease()).toBe(true);
     unsubscribe();
     act(() => root.unmount());
     expect(window.__studioTest).toBeUndefined();
+    expect(hasTimelinePerformanceFixtureLease()).toBe(false);
   });
 
   it("does not mutate state when the fixture request is invalid", () => {

packages/studio/src/hooks/useStudioTestHooks.ts

@@ -7,6 +7,7 @@ import {
 } from "../player/lib/timelinePerformanceDiagnostics";
 import {
   createTimelinePerformanceFixture,
+  setTimelinePerformanceFixtureLease,
   type TimelinePerformanceFixtureSpec,
   type TimelinePerformanceFixtureSummary,
 } from "../player/lib/timelinePerformanceFixture";
@@ -70,6 +71,7 @@ export function useStudioTestHooks({
       },
       loadTimelinePerformanceFixture: (spec) => {
         const fixture = createTimelinePerformanceFixture(spec);
+        setTimelinePerformanceFixtureLease(true);
         usePlayerStore.setState({
           currentTime: 0,
           duration: fixture.summary.duration,
@@ -91,6 +93,7 @@ export function useStudioTestHooks({
     };
     window.__studioTest = api;
     return () => {
+      setTimelinePerformanceFixtureLease(false);
       window.__studioTest = undefined;
     };
   }, [applyDomSelection, buildDomSelectionFromTarget, previewIframeRef]);

packages/studio/src/player/components/Timeline.test.ts

@@ -17,6 +17,8 @@ import {
   shouldShowTimelineShortcutHint,
   shouldHandleTimelineDeleteKey,
   shouldAutoScrollTimeline,
+  getTimelineVisibleTimeRange,
+  getTimelineScrollTopForGeometryChange,
 } from "./Timeline";
 import {
   CLIP_Y,
@@ -32,6 +34,7 @@ import {
   getTimelineDisplayContentWidth,
   getTimelineFitPps,
   getTimelineLaneTop,
+  createTimelineRowGeometry,
 } from "./timelineLayout";
 import { formatTime } from "../lib/time";
 import { usePlayerStore } from "../store/playerStore";
@@ -44,6 +47,25 @@ afterEach(() => {
   usePlayerStore.getState().reset();
 });
 
+describe("timeline viewport geometry", () => {
+  it("derives a clamped visible time range from the raw viewport", () => {
+    expect(
+      getTimelineVisibleTimeRange({ scrollLeft: 300, clientWidth: 500 }, 100, 200, 20),
+    ).toEqual({ start: 1, end: 6 });
+    expect(getTimelineVisibleTimeRange({ scrollLeft: 0, clientWidth: 100 }, 100, 200, 20)).toEqual({
+      start: 0,
+      end: 0,
+    });
+  });
+
+  it("keeps the same row anchored when a row above it expands", () => {
+    const previous = createTimelineRowGeometry([1, 2, 3], [48, 48, 48]);
+    const next = createTimelineRowGeometry([1, 2, 3], [104, 48, 48]);
+    const scrollTop = previous.getRowTop(2) - RULER_H + 6;
+    expect(getTimelineScrollTopForGeometryChange(previous, next, scrollTop)).toBe(scrollTop + 56);
+  });
+});
+
 function getHorizontalGeometry(host: HTMLElement, clipId: string, tickLabel: string) {
   const clip = host.querySelector<HTMLElement>(`[data-el-id="${clipId}"]`);
   if (!clip) throw new Error(`Missing timeline clip ${clipId}`);

packages/studio/src/player/components/Timeline.tsx

@@ -1,6 +1,5 @@
-import { useRef, useMemo, useCallback, useState, memo } from "react";
+import { useRef, useMemo, useCallback, useState, useLayoutEffect, memo } from "react";
 import { useMusicBeatAnalysis } from "../../hooks/useMusicBeatAnalysis";
-import { isMusicTrack } from "../../utils/timelineInspector";
 import { remapBeatAnalysisToComposition } from "../../utils/beatEditActions";
 import { usePlayerStore, type TimelineElement } from "../store/playerStore";
 import { useExpandedTimelineElements } from "../hooks/useExpandedTimelineElements";
@@ -42,6 +41,8 @@ import {
 import { useTimelineSelectionLifecycle } from "./useTimelineSelectionLifecycle";
 import { useTimelineShiftModifier } from "./useTimelineShiftModifier";
 import { useTimelineTicks } from "./useTimelineTicks";
+import { getTimelineElementIndexes } from "../lib/timelineElementIndexes";
+import { getTimelineScrollTopForGeometryChange } from "./timelineViewportGeometry";
 
 // Re-export pure utilities so existing imports from "./Timeline" still resolve.
 export {
@@ -58,6 +59,11 @@ export {
   getDefaultDroppedTrack,
 } from "./timelineLayout";
 
+export {
+  getTimelineScrollTopForGeometryChange,
+  getTimelineVisibleTimeRange,
+} from "./timelineViewportGeometry";
+
 export const Timeline = memo(function Timeline({
   onSeek,
   onDrillDown,
@@ -76,6 +82,7 @@ export const Timeline = memo(function Timeline({
   onSplitElement: onSplitElementOverride,
   onSelectElement,
   theme: themeOverrides,
+  sessionEpoch = 0,
 }: TimelineProps = {}) {
   const {
     onMoveElement,
@@ -108,7 +115,7 @@ export const Timeline = memo(function Timeline({
   const rawElements = usePlayerStore((s) => s.elements);
   const expandedElements = useExpandedTimelineElements();
   const beatAnalysis = usePlayerStore((s) => s.beatAnalysis);
-  const musicElement = usePlayerStore((s) => s.elements.find(isMusicTrack) ?? null);
+  const musicElement = usePlayerStore((s) => getTimelineElementIndexes(s.elements).musicElement);
   const beatEdits = usePlayerStore((s) => s.beatEdits);
   const adjustedBeatAnalysis = useMemo(
     () => remapBeatAnalysisToComposition(beatAnalysis, musicElement, beatEdits),
@@ -163,8 +170,20 @@ export const Timeline = memo(function Timeline({
 
   const keyframeCache = usePlayerStore((s) => s.keyframeCache);
   useAutoExpandKeyframedClips(gsapAnimations);
-  const { tracks, trackStyles, trackOrder, trackOrderRef, laneCounts, rowHeights, rowHeightsRef } =
-    useTimelineTrackLayout(expandedElements, gsapAnimations, selectedElementId, selectedElementIds);
+  const {
+    tracks,
+    trackStyles,
+    trackOrder,
+    trackOrderRef,
+    laneCounts,
+    rowGeometry,
+    rowGeometryRef,
+  } = useTimelineTrackLayout(
+    expandedElements,
+    gsapAnimations,
+    selectedElementId,
+    selectedElementIds,
+  );
   const expandedElementsRef = useRef(expandedElements);
   expandedElementsRef.current = expandedElements;
 
@@ -231,7 +250,7 @@ export const Timeline = memo(function Timeline({
     ppsRef,
     durationRef,
     trackOrderRef,
-    rowHeightsRef,
+    rowGeometryRef,
     onMoveElement: pinnedOnMoveElement,
     onMoveElements: pinnedOnMoveElements,
     onResizeElement: pinnedOnResizeElement,
@@ -250,20 +269,47 @@ export const Timeline = memo(function Timeline({
       ppsRef,
       durationRef,
       trackOrderRef,
-      rowHeightsRef,
+      rowGeometryRef,
       contentOrigin,
       onFileDrop: pinnedOnFileDrop,
       onAssetDrop: pinnedOnAssetDrop,
       onBlockDrop: pinnedOnBlockDrop,
       onCompositionDrop: pinnedOnCompositionDrop,
     });
 
-  const displayLayout = useTimelineDisplayLayout(draggedClip, trackOrder, rowHeights);
-  const { viewportWidth, showShortcutHint, setScrollRef } = useTimelineScrollViewport(scrollRef, [
-    timelineReady,
-    expandedElements.length,
-    displayLayout.totalH,
-  ]);
+  const displayLayout = useTimelineDisplayLayout(draggedClip, trackOrder, rowGeometry);
+  const { viewport, showShortcutHint, setScrollRef, syncScrollViewport } =
+    useTimelineScrollViewport(scrollRef, [
+      timelineReady,
+      expandedElements.length,
+      displayLayout.totalH,
+    ]);
+  const previousLayoutRef = useRef(displayLayout.rowGeometry);
+  const previousSessionEpochRef = useRef(sessionEpoch);
+  useLayoutEffect(() => {
+    const scroll = scrollRef.current;
+    const previousGeometry = previousLayoutRef.current;
+    if (previousSessionEpochRef.current !== sessionEpoch) {
+      previousSessionEpochRef.current = sessionEpoch;
+      lastScrollLeftRef.current = 0;
+      if (scroll) {
+        scroll.scrollLeft = 0;
+        scroll.scrollTop = 0;
+        syncScrollViewport(scroll);
+      }
+    } else if (scroll && previousGeometry !== displayLayout.rowGeometry) {
+      const nextScrollTop = getTimelineScrollTopForGeometryChange(
+        previousGeometry,
+        displayLayout.rowGeometry,
+        scroll.scrollTop,
+      );
+      if (nextScrollTop !== scroll.scrollTop) {
+        scroll.scrollTop = nextScrollTop;
+        syncScrollViewport(scroll);
+      }
+    }
+    previousLayoutRef.current = displayLayout.rowGeometry;
+  }, [displayLayout.rowGeometry, sessionEpoch, syncScrollViewport]);
   const selectedKeyframes = usePlayerStore((s) => s.selectedKeyframes);
   const toggleSelectedKeyframe = usePlayerStore((s) => s.toggleSelectedKeyframe);
   const { onClickKeyframe, onSelectSegment, onShiftClickKeyframe, onContextMenuKeyframe } =
@@ -286,7 +332,7 @@ export const Timeline = memo(function Timeline({
     zoomModeRef,
     manualZoomPercentRef,
   } = useTimelineGeometry({
-    viewportWidth,
+    viewportWidth: viewport.clientWidth,
     effectiveDuration,
     zoomMode,
     manualZoomPercent,
@@ -369,7 +415,7 @@ export const Timeline = memo(function Timeline({
     setShowPopover,
     elementsRef: expandedElementsRef,
     trackOrderRef,
-    rowHeightsRef,
+    rowGeometryRef,
     onSelectElement,
     contentOrigin,
   });
@@ -403,6 +449,7 @@ export const Timeline = memo(function Timeline({
     <div
       ref={setContainerRef}
       aria-label="Timeline"
+      data-timeline-element-count={expandedElements.length}
       className={`relative border-t select-none h-full overflow-hidden ${isDragOver ? "ring-1 ring-inset ring-studio-accent/60" : ""} ${activeTool === "razor" ? "cursor-crosshair" : shiftHeld ? "cursor-crosshair" : "cursor-default"}`}
       onMouseMove={updateRazorGuide}
       onMouseLeave={clearRazorGuide}
@@ -414,10 +461,12 @@ export const Timeline = memo(function Timeline({
     >
       <div
         ref={setScrollRef}
+        data-timeline-scroll-viewport
         tabIndex={-1}
         className={`${zoomMode === "fit" ? "overflow-x-hidden" : "overflow-x-auto"} overflow-y-auto h-full outline-none`}
         onScroll={(e) => {
           lastScrollLeftRef.current = e.currentTarget.scrollLeft; // restored across post-edit reload
+          syncScrollViewport(e.currentTarget, true);
         }}
         onDragOver={handleAssetDragOver}
         onDragLeave={() => clearDropPreview()}

packages/studio/src/player/components/TimelineTypes.ts

@@ -5,6 +5,8 @@ import type { TimelineTheme } from "./timelineTheme";
 import type { TimelineEditOverrides } from "./useResolvedTimelineEditCallbacks";
 
 export interface TimelineProps extends TimelineDropCallbacks, TimelineEditOverrides {
+  /** Project-scoped reset boundary; soft source refreshes retain the same epoch. */
+  sessionEpoch?: number;
   onSeek?: (time: number) => void;
   onDrillDown?: (element: TimelineElement) => void;
   renderClipContent?: (

packages/studio/src/player/components/timelineDragDrop.ts

@@ -5,15 +5,15 @@ import {
   TIMELINE_COMPOSITION_MIME,
 } from "../../utils/timelineCompositionDrop";
 import { usePlayerStore } from "../store/playerStore";
-import { resolveTimelineAssetDrop } from "./timelineLayout";
+import { resolveTimelineAssetDrop, type TimelineRowGeometry } from "./timelineLayout";
 import type { TimelineDropCallbacks } from "./timelineCallbacks";
 
 interface UseTimelineAssetDropOptions extends TimelineDropCallbacks {
   scrollRef: RefObject<HTMLDivElement | null>;
   ppsRef: RefObject<number>;
   durationRef: RefObject<number>;
   trackOrderRef: RefObject<number[]>;
-  rowHeightsRef: RefObject<readonly number[]>;
+  rowGeometryRef: RefObject<TimelineRowGeometry>;
   contentOrigin: number;
 }
 
@@ -56,7 +56,7 @@ export function useTimelineAssetDrop({
   ppsRef,
   durationRef,
   trackOrderRef,
-  rowHeightsRef,
+  rowGeometryRef,
   contentOrigin,
   onFileDrop,
   onAssetDrop,
@@ -93,7 +93,7 @@ export function useTimelineAssetDrop({
           pixelsPerSecond: ppsRef.current,
           duration: durationRef.current,
           clampStartToDuration: !usePointerStart,
-          rowHeights: rowHeightsRef.current,
+          rowHeights: rowGeometryRef.current.rowHeights,
           trackOrder: trackOrderRef.current,
         },
         clientX,
@@ -104,7 +104,7 @@ export function useTimelineAssetDrop({
         track: pointer.track,
       };
     },
-    [scrollRef, ppsRef, durationRef, trackOrderRef, rowHeightsRef, contentOrigin],
+    [scrollRef, ppsRef, durationRef, trackOrderRef, rowGeometryRef, contentOrigin],
   );
 
   const handleAssetDrop = useCallback(