fix(studio): keep sub-compositions expanded at the end of the timeline

Auto-expansion picks the composition whose clip window contains the
playhead, and those windows are half-open. With the playhead parked on
the last clip's end - where playback stops - it sat inside nothing, so
every expanded sub-composition row and its keyframe lanes collapsed to a
single host row.

Accept the closing boundary, but only when the strict pass matched
nothing, so a playhead on the seam between two adjacent clips still
expands the one that is starting.

packages/studio/src/player/hooks/useExpandedTimelineElements.test.ts

@@ -278,6 +278,49 @@ describe("resolveTimelineExpansionRawId", () => {
     ).toBe("scene");
   });
 
+  it("THE BUG: keeps a composition expanded with the playhead parked on its end", () => {
+    // Clip windows are half-open, so at the very end of the timeline the
+    // playhead was inside nothing and every expanded row collapsed.
+    const manifest = [
+      clip({ id: "scene", start: 0, duration: 12 }),
+      clip({ id: "headline", start: 0, duration: 12 }),
+    ];
+    const parentMap = new Map([["headline", "scene"]]);
+
+    expect(
+      resolveTimelineExpansionRawId({
+        selectedElementId: null,
+        isPlaying: false,
+        currentTime: 12,
+        manifest,
+        parentMap,
+      }),
+    ).toBe("scene");
+  });
+
+  it("prefers the starting clip over the ending one on a shared seam", () => {
+    const manifest = [
+      clip({ id: "first", start: 0, duration: 5 }),
+      clip({ id: "first-child", start: 0, duration: 5 }),
+      clip({ id: "second", start: 5, duration: 5 }),
+      clip({ id: "second-child", start: 5, duration: 5 }),
+    ];
+    const parentMap = new Map([
+      ["first-child", "first"],
+      ["second-child", "second"],
+    ]);
+
+    expect(
+      resolveTimelineExpansionRawId({
+        selectedElementId: null,
+        isPlaying: false,
+        currentTime: 5,
+        manifest,
+        parentMap,
+      }),
+    ).toBe("second");
+  });
+
   it("auto-expands the innermost active nested composition when paused", () => {
     const manifest = [
       clip({ id: "outer", start: 0, duration: 10 }),

packages/studio/src/player/hooks/useExpandedTimelineElements.ts

@@ -45,6 +45,11 @@ function clipContainsTime(clip: ClipManifestClip, time: number): boolean {
   return Number.isFinite(time) && time >= clip.start && time < clip.start + clip.duration;
 }
 
+/** Half-open containment plus the closing boundary — see the fallback below. */
+function clipTouchesTime(clip: ClipManifestClip, time: number): boolean {
+  return Number.isFinite(time) && time >= clip.start && time <= clip.start + clip.duration;
+}
+
 function getActiveParentDepth(id: string, parentMap: Map<string, string>, activeIds: Set<string>) {
   let depth = 0;
   let parent = parentMap.get(id);
@@ -65,11 +70,20 @@ function findActiveExpandableCompositionId(
   parentMap: Map<string, string>,
 ): string | null {
   const parentIds = new Set(parentMap.values());
-  const activeIds = new Set<string>();
-  for (const clip of manifest) {
-    if (!clip.id || !parentIds.has(clip.id) || !clipContainsTime(clip, currentTime)) continue;
-    activeIds.add(clip.id);
-  }
+  const collect = (matches: (clip: ClipManifestClip, time: number) => boolean) => {
+    const ids = new Set<string>();
+    for (const clip of manifest) {
+      if (clip.id && parentIds.has(clip.id) && matches(clip, currentTime)) ids.add(clip.id);
+    }
+    return ids;
+  };
+  // Clip windows are half-open, so a playhead parked exactly on a clip's end is
+  // inside nothing — at the end of the timeline that collapsed every expanded
+  // sub-composition row and its keyframe lanes. Only when the strict pass finds
+  // nothing do we accept the closing boundary, so a playhead landing on the seam
+  // between two adjacent clips still expands the one that is starting.
+  const strict = collect(clipContainsTime);
+  const activeIds = strict.size > 0 ? strict : collect(clipTouchesTime);
   let bestId: string | null = null;
   let bestDepth = -1;
   for (const id of activeIds) {