fix(studio): expand sub-comp children against their resolved parent host

buildExpandedElements synthesized DOM-only sub-composition children against
the top-level element rather than the parentHost it resolves immediately
after. Under two-level nesting every child row therefore inherited the
top-level window instead of its own host's, so the rows drew at the wrong
offset and duration.

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

@@ -243,6 +243,45 @@ describe("buildExpandedElements", () => {
     // The host row is replaced by its children.
     expect(out.some((e) => e.domId === "scene-host")).toBe(false);
   });
+
+  // Regression: DOM-only children were synthesized against the TOP-LEVEL element
+  // instead of the sub-comp host they actually live in, so every child row read
+  // the whole top-level window rather than its host's.
+  it("spans DOM-only children over their nested host's window, not the top-level one", () => {
+    const elements = [
+      el({ id: "scene-host", start: 0, duration: 20, compositionSrc: "scene.html" }),
+    ];
+    const manifest = [
+      clip({ id: "scene-host", start: 0, duration: 20, compositionSrc: "scene.html" }),
+      clip({ id: "sub-host", start: 5, duration: 6, compositionSrc: "sub.html" }),
+    ];
+    const parentMap = new Map([
+      ["sub-host", "scene-host"],
+      ["pill-1", "sub-host"],
+    ]);
+    const domClipChildren = [
+      {
+        id: "pill-1",
+        parentId: "sub-host",
+        hostId: "sub-host",
+        label: "pill-1",
+        stackingContextId: "css:0.0",
+      },
+    ];
+
+    const out = buildExpandedElements(
+      elements,
+      manifest,
+      parentMap,
+      "scene-host",
+      "sub-host",
+      domClipChildren,
+    );
+    const pill = out.find((e) => e.domId === "pill-1")!;
+    expect(pill.start).toBe(5);
+    expect(pill.duration).toBe(6);
+    expect(pill.sourceFile).toBe("sub.html");
+  });
 });
 
 describe("resolveTimelineExpansionRawId", () => {

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

@@ -209,7 +209,13 @@ function buildChildElements(
 function domSiblingClips(
   domClipChildren: DomClipChild[],
   siblingParentId: string,
-  host: TimelineElement,
+  host: {
+    id: string | null;
+    start: number;
+    duration: number;
+    track: number;
+    compositionSrc?: string | null;
+  },
 ): ClipManifestClip[] {
   return domClipChildren
     .filter((c) => c.parentId === siblingParentId)
@@ -243,20 +249,23 @@ export function buildExpandedElements(
   const topLevelElement = elements.find((el) => el.id === topLevelId || el.domId === topLevelId);
   if (!topLevelElement) return filterToTopLevel(elements, parentMap);
 
+  // The sub-comp host the children actually live in: top-level host for 1-level
+  // nesting, a nested host for deeper nesting. Its start/file anchor edits.
+  const parentHost = manifest.find((c) => c.id === siblingParentId);
+
   // Prefer real manifest children; fall back to DOM-only sub-comp children
   // (groups/pills) that have no data-start and thus never enter the manifest.
+  // Those are synthesized against the host they actually live in, not the
+  // top-level element, or every child row reads the whole top-level window.
   const siblings = (() => {
     const fromManifest = manifest.filter(
       (c) => c.id != null && parentMap.get(c.id) === siblingParentId,
     );
     if (fromManifest.length > 0) return fromManifest;
-    return domSiblingClips(domClipChildren, siblingParentId, topLevelElement);
+    return domSiblingClips(domClipChildren, siblingParentId, parentHost ?? topLevelElement);
   })();
   if (siblings.length === 0) return filterToTopLevel(elements, parentMap);
 
-  // The sub-comp host the children actually live in: top-level host for 1-level
-  // nesting, a nested host for deeper nesting. Its start/file anchor edits.
-  const parentHost = manifest.find((c) => c.id === siblingParentId);
   const editBasis = {
     start: parentHost?.start ?? topLevelElement.start,
     sourceFile: parentHost?.compositionSrc ?? topLevelElement.compositionSrc ?? undefined,