fix(studio): undoing an agent's build takes its sections off the timeline too

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

@@ -118,9 +118,9 @@ export function useShadowPreviewReload({
       const liveTime = live?.getTime();
       const playing = usePlayerStore.getState().isPlaying;
       live?.pause();
-      // The store takes the new document's timeline only now that it is the one on screen.
-      pending.commit();
+      // The store takes the new document's timeline only now that it is the one on screen, and reads it there.
       iframeRef.current = shadow;
+      pending.commit();
       shadowIframeRef.current = null;
       attachIframeShortcutListeners();
       applyPreviewAudioState();

packages/studio/src/player/hooks/useTimelinePlayer.shadowReload.test.ts

@@ -543,6 +543,75 @@ describe("shadow reload store ownership and readiness", () => {
     unmount(root);
   });
 
+  it("takes a section the edit removed off the timeline, though the old document still has it", () => {
+    const { getApi, gen, liveIframe, shadowIframe, root } = setup();
+    const host = (id: string) =>
+      `<div id="${id}" data-composition-id="${id}" data-composition-src="compositions/${id}.html" data-start="0" data-duration="3"></div>`;
+    liveIframe.contentDocument!.body.innerHTML = `<div data-composition-id="main">${host("benefit")}<p id="a"></p></div>`;
+    shadowIframe.contentDocument!.body.innerHTML = `<div data-composition-id="main"><p id="a"></p></div>`;
+    const a = { id: "a", domId: "a", tag: "p", start: 0, duration: 3, track: 0 };
+    const benefit = {
+      id: "benefit",
+      domId: "benefit",
+      tag: "div",
+      kind: "composition" as const,
+      start: 0,
+      duration: 3,
+      track: 1,
+      compositionSrc: "compositions/benefit.html",
+    };
+    act(() => usePlayerStore.getState().setElements([a, benefit]));
+    (shadowIframe.contentWindow as Record<string, unknown>).__clipManifest = {
+      source: "hf-preview",
+      type: "timeline",
+      durationInFrames: 1260,
+      clips: [
+        { id: "a", label: "A", start: 0, duration: 3, track: 0, kind: "element", tagName: "p" },
+      ],
+    };
+    act(() => getApi().onShadowIframeLoad(gen));
+    act(() => getApi().onShadowReadyChange(gen, true));
+    expect(usePlayerStore.getState().elements.map((element) => element.id)).toEqual(["a"]);
+    unmount(root);
+  });
+
+  it("keeps a section the new document still hosts but its manifest misses, without dropping it once", () => {
+    const { getApi, gen, liveIframe, shadowIframe, root } = setup();
+    const film = `<div data-composition-id="main"><div id="benefit" data-composition-id="benefit"
+      data-composition-src="compositions/benefit.html" data-start="0" data-duration="3"></div><p id="a"></p></div>`;
+    liveIframe.contentDocument!.body.innerHTML = film;
+    shadowIframe.contentDocument!.body.innerHTML = film;
+    const a = { id: "a", domId: "a", tag: "p", start: 0, duration: 3, track: 0 };
+    // As enrichment adds it: no kind.
+    const benefit = {
+      id: "benefit",
+      domId: "benefit",
+      tag: "div",
+      start: 0,
+      duration: 3,
+      track: 1,
+      compositionSrc: "compositions/benefit.html",
+    };
+    act(() => usePlayerStore.getState().setElements([a, benefit]));
+    const seen: string[][] = [];
+    const stop = usePlayerStore.subscribe((state) =>
+      seen.push(state.elements.map((element) => element.id)),
+    );
+    (shadowIframe.contentWindow as Record<string, unknown>).__clipManifest = {
+      source: "hf-preview",
+      type: "timeline",
+      durationInFrames: 1260,
+      clips: [
+        { id: "a", label: "A", start: 0, duration: 3, track: 0, kind: "element", tagName: "p" },
+      ],
+    };
+    act(() => getApi().onShadowIframeLoad(gen));
+    act(() => getApi().onShadowReadyChange(gen, true));
+    stop();
+    expect(seen.every((ids) => ids.includes("benefit"))).toBe(true);
+    unmount(root);
+  });
+
   it("never writes a failed shadow's timeline into the store", () => {
     vi.spyOn(console, "error").mockImplementation(() => {});
     const { getApi, gen, root } = setup();

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

@@ -28,6 +28,7 @@ export {
 import type { PlaybackAdapter, IframeWindow } from "../lib/playbackTypes";
 import { releaseStaticSeekCache, type StaticSeekCacheEntry } from "../lib/playbackAdapter";
 import { mergeTimelineElementsPreservingDowngrades } from "../lib/timelineDOM";
+import { findTimelineElementInIframe } from "../../hooks/timelineEditingHelpers";
 import { normalizeToZones } from "../components/timelineZones";
 import { applyPreviewAudioFlags, setPreviewPlaybackRate } from "../lib/timelineIframeHelpers";
 import { scrubMusicAtSeek, stopScrubPreviewAudio } from "../lib/playbackScrub";
@@ -92,6 +93,11 @@ export function useTimelinePlayer({
             elements,
             state.duration,
             resolvedDuration,
+            (element) =>
+              findTimelineElementInIframe(iframeRef.current, {
+                ...element,
+                kind: "composition",
+              }) !== null,
           ),
           state.timelineProjectId,
         ),

packages/studio/src/player/lib/timelineDOM.test.ts

@@ -534,6 +534,17 @@ describe("mergeTimelineElementsPreservingDowngrades — genuine removal vs trans
     expect(merged.map((e) => e.id).sort()).toEqual(["a", "sub-child"]);
   });
 
+  it("drops a section whose host left the preview (undo of an agent's build)", () => {
+    const current = [
+      el("a"),
+      el("benefit-fresh", { compositionSrc: "compositions/benefit-fresh.html" }),
+    ];
+    const next = [el("a")]; // the reverted film's manifest: the built section is gone
+    const inPreview = (element: { id: string }) => element.id !== "benefit-fresh";
+    const merged = mergeTimelineElementsPreservingDowngrades(current, next, 30, 30, inPreview);
+    expect(merged.map((e) => e.id)).toEqual(["a"]);
+  });
+
   it("trusts the fresh scan fully when it is not shorter", () => {
     const current = [el("a"), el("b", { compositionSrc: "sub.html" })];
     const next = [el("a"), el("c")];

packages/studio/src/player/lib/timelineDOM.ts

@@ -382,6 +382,7 @@ export function mergeTimelineElementsPreservingDowngrades(
   nextElements: TimelineElement[],
   currentDuration: number,
   nextDuration: number,
+  stillInPreview: (element: TimelineElement) => boolean = () => true,
 ): TimelineElement[] {
   const safeCurrentDuration = Number.isFinite(currentDuration) ? currentDuration : 0;
   const safeNextDuration = Number.isFinite(nextDuration) ? nextDuration : 0;
@@ -403,7 +404,8 @@ export function mergeTimelineElementsPreservingDowngrades(
       // re-adds. A TOP-LEVEL element missing from the fresh scan was genuinely
       // removed (undo of a split, a delete), so let it go — otherwise undoing a
       // split leaves a ghost clip in the timeline even though the file is reverted.
-      element.compositionSrc != null,
+      element.compositionSrc != null &&
+      stillInPreview(element),
   );
   if (preserved.length === 0) return nextElements;
   return [...nextElements, ...preserved];

packages/studio/src/player/lib/timelineIframeHelpers.test.ts

@@ -9,6 +9,7 @@ import {
   stopScrubPreviewAudio,
 } from "./timelineIframeHelpers";
 import type { IframeWindow } from "./playbackTypes";
+import { findTimelineElementInIframe } from "../../hooks/timelineEditingHelpers";
 
 function makeDoc(html: string): Document {
   const d = document.implementation.createHTMLDocument();
@@ -42,6 +43,20 @@ describe("buildMissingCompositionElements — hfId (R7)", () => {
     expect([patched, updatedEls[0]?.compositionSrc]).toEqual([true, "compositions/scene.html"]);
   });
 
+  it("adds a host the lookup finds in its preview, as a composition", () => {
+    const doc = makeDoc(`
+      <div data-composition-id="root">
+        <div id="benefit-fresh" data-composition-id="benefit-fresh" data-composition-src="compositions/benefit-fresh.html"
+          data-start="0" data-duration="3"></div>
+      </div>
+    `);
+    const { missing } = buildMissingCompositionElements(doc, window as IframeWindow, [], 10);
+    const iframe = { contentDocument: doc } as unknown as HTMLIFrameElement;
+    expect(findTimelineElementInIframe(iframe, { ...missing[0]!, kind: "composition" })).toBe(
+      doc.getElementById("benefit-fresh"),
+    );
+  });
+
   it("harvests hfId from data-hf-id on composition host elements", () => {
     const doc = makeDoc(`
       <div data-composition-id="root">