diff --git a/packages/engine/src/services/frameCapture-subTimelinePoll.test.ts b/packages/engine/src/services/frameCapture-subTimelinePoll.test.ts
index 343f4acd41..5848a485d0 100644
--- a/packages/engine/src/services/frameCapture-subTimelinePoll.test.ts
+++ b/packages/engine/src/services/frameCapture-subTimelinePoll.test.ts
@@ -174,8 +174,15 @@ describe("pollSubCompositionTimelines when a script failed on a page nothing can
 describe("authorScriptsMayRegisterTimelines", () => {
   const RUNTIME_ATTR = "data-hyperframes-preview-runtime";
 
-  async function scan(html: string, files: Record<string, string> = {}): Promise<boolean> {
+  async function scan(
+    html: string,
+    files: Record<string, string> = {},
+    registered: string[] = [],
+  ): Promise<boolean> {
     const { document } = parseHTML(html);
+    vi.stubGlobal("window", {
+      __timelines: Object.fromEntries(registered.map((id) => [id, {}])),
+    });
     Object.defineProperty(document, "readyState", { value: "complete" });
     vi.stubGlobal("document", document);
     vi.stubGlobal("location", new URL("http://localhost:3000/index.html"));
@@ -193,7 +200,7 @@ describe("authorScriptsMayRegisterTimelines", () => {
         AUTHORED_SOURCE_END,
       );
     } finally {
-      delete (globalThis as { __hfAuthorScan?: boolean }).__hfAuthorScan;
+      delete (globalThis as { __hfAuthorScan?: unknown }).__hfAuthorScan;
       vi.unstubAllGlobals();
     }
   }
@@ -210,7 +217,8 @@ describe("authorScriptsMayRegisterTimelines", () => {
     const wrapped = (source: string) =>
       `<body><script>(function(){ window.__timelines = window.__timelines || {};\n${AUTHORED_SOURCE_BEGIN}\n${source}\n${AUTHORED_SOURCE_END}\n})();</script></body>`;
     expect(await scan(wrapped("draw()"))).toBe(false);
-    expect(await scan(wrapped("window.__timelines['a'] = tl"))).toBe(true);
+    const host = `<div data-composition-id="a"></div>`;
+    expect(await scan(host + wrapped("window.__timelines['a'] = tl"))).toBe(true);
   });
 
   it("ignores the runtime, which mentions __timelines itself", async () => {
@@ -220,9 +228,14 @@ describe("authorScriptsMayRegisterTimelines", () => {
   });
 
   it("is true when an inline script or a template script registers one", async () => {
-    expect(await scan(`<body><script>window.__timelines["a"] = tl</script></body>`)).toBe(true);
+    const host = (id: string) => `<div data-composition-id="${id}"></div>`;
     expect(
-      await scan(`<body><template><script>window.__timelines.b = tl</script></template></body>`),
+      await scan(`<body>${host("a")}<script>window.__timelines["a"] = tl</script></body>`),
+    ).toBe(true);
+    expect(
+      await scan(
+        `<body>${host("b")}<template><script>window.__timelines.b = tl</script></template></body>`,
+      ),
     ).toBe(true);
   });
 
@@ -244,7 +257,7 @@ describe("authorScriptsMayRegisterTimelines", () => {
       false,
     );
     expect(
-      await scan(`<body><script src="/a.js"></script></body>`, {
+      await scan(`<body><div data-composition-id="x"></div><script src="/a.js"></script></body>`, {
         "/a.js": "window.__timelines.x = 1",
       }),
     ).toBe(true);
@@ -253,4 +266,47 @@ describe("authorScriptsMayRegisterTimelines", () => {
       true,
     );
   });
+
+  it("is false when no script registers an id that is still waited on", async () => {
+    const page = (scripts: string, registered: string[] = []) =>
+      scan(
+        `<body><div data-composition-id="main"><div data-composition-id="still"></div></div>${scripts}</body>`,
+        {},
+        registered,
+      );
+    const main = `<script>window.__timelines = window.__timelines || {}; window.__timelines["main"] = tl</script>`;
+    expect(await page(main, ["main"])).toBe(false);
+    expect(await page(main)).toBe(true);
+    expect(await page(`<script>window.__timelines["still"] = tl</script>${main}`, ["main"])).toBe(
+      true,
+    );
+  });
+
+  it("answers true for a registration key it cannot read, so a late id is still waited on", async () => {
+    const hosts = `<div data-composition-id="still"></div>`;
+    expect(
+      await scan(`<body>${hosts}<script>window.__timelines[compId] = tl</script></body>`, {}, []),
+    ).toBe(true);
+    expect(
+      await scan(
+        `<body>${hosts}<script>Object.assign(window.__timelines, { still: tl })</script></body>`,
+      ),
+    ).toBe(true);
+  });
+
+  it("waits on a sub-composition mounted twice, whose hosts carry the authored id", async () => {
+    const html = `<body><div data-composition-id="main"><div data-composition-id="lt__hf1" data-hf-original-composition-id="lt"></div><div data-composition-id="lt__hf2" data-hf-original-composition-id="lt"></div></div><script>window.__timelines["main"] = tl</script><script>fetch("d.json").then(function(){ window.__timelines["lt"] = tl; })</script></body>`;
+    expect(await scan(html, {}, ["main"])).toBe(true);
+  });
+
+  it("answers true for a template-literal registration key", async () => {
+    const html =
+      '<body><div data-composition-id="still"></div><script>var id = "still"; setTimeout(function(){ window.__timelines[`${id}`] = tl; })</script></body>';
+    expect(await scan(html)).toBe(true);
+  });
+
+  it("does not wait on a host that opted out or is in the skipped list", async () => {
+    const html = `<body><div data-composition-id="x" data-no-timeline></div><script>window.__timelines["x"] = tl</script></body>`;
+    expect(await scan(html)).toBe(false);
+  });
 });
diff --git a/packages/engine/src/services/frameCapture.ts b/packages/engine/src/services/frameCapture.ts
index 9b367979f1..8e91a69069 100644
--- a/packages/engine/src/services/frameCapture.ts
+++ b/packages/engine/src/services/frameCapture.ts
@@ -2069,15 +2069,18 @@ export interface IsolatedComposition {
   elementId: string | null;
 }
 
-// Runs in the page. False once every inline, template and same-origin script is read and none
-// mentions __timelines or imports code; an unreadable or module script, or a page still loading, answers true.
+// Runs in the page. False once every script is read and none can register an id still waited on;
+// an unreadable, module or importing script, a computed key, or a page still loading answers true.
 export async function authorScriptsMayRegisterTimelines(
   runtimeAttr: string,
   authoredBegin: string,
   authoredEnd: string,
+  skipIds: readonly string[] = [],
 ): Promise<boolean> {
   if (document.readyState !== "complete") return true;
-  const memo = globalThis as { __hfAuthorScan?: boolean };
+  const memo = globalThis as {
+    __hfAuthorScan?: { opaque: boolean; mentions: boolean; ids: string[] };
+  };
   memo.__hfAuthorScan ??= await (async () => {
     const scripts = [
       ...Array.from(document.querySelectorAll("script")),
@@ -2085,34 +2088,62 @@ export async function authorScriptsMayRegisterTimelines(
         Array.from(t.content.querySelectorAll("script")),
       ),
     ].filter((script) => !script.hasAttribute(runtimeAttr));
+    const scan = { opaque: false, mentions: false, ids: [] as string[] };
     // Code that imports another file may register through it, which is not read here.
-    const mayRegister = (code: string) =>
-      code.includes("__timelines") || /\bimport\s*\(|^\s*import\s*[\w{*"']/m.test(code);
+    const mayImport = (code: string) => /\bimport\s*\(|^\s*import\s*[\w{*"']/m.test(code);
+    const read = (code: string) => {
+      if (mayImport(code)) scan.opaque = true;
+      if (!code.includes("__timelines")) return;
+      scan.mentions = true;
+      const literalKey = /__timelines\s*(?:\[\s*(["'])((?:(?!\1).)+)\1\s*\]|\.([A-Za-z_$][\w$]*))/g;
+      const rest = code
+        .replace(/__timelines\s*=\s*(?:window\.__timelines\s*\|\|\s*)?\{\s*\}/g, "")
+        .replace(literalKey, (_m, _q, bracketId, dotId) => {
+          scan.ids.push(bracketId ?? dotId);
+          return "";
+        });
+      if (rest.includes("__timelines")) scan.opaque = true;
+    };
     for (const script of scripts) {
-      if (script.type === "module") return true;
+      if (script.type === "module") return { ...scan, opaque: true };
       if (!script.src) {
         const text = script.textContent ?? "";
-        const authored = text.includes(authoredBegin)
-          ? text
-              .split(authoredBegin)
-              .slice(1)
-              .map((region) => region.split(authoredEnd)[0])
-              .join("\n")
-          : text;
-        if (mayRegister(authored)) return true;
+        read(
+          text.includes(authoredBegin)
+            ? text
+                .split(authoredBegin)
+                .slice(1)
+                .map((region) => region.split(authoredEnd)[0])
+                .join("\n")
+            : text,
+        );
         continue;
       }
       const url = new URL(script.src, location.href);
-      if (url.origin !== location.origin) return true;
+      if (url.origin !== location.origin) return { ...scan, opaque: true };
       try {
-        if (mayRegister(await (await fetch(url)).text())) return true;
+        read(await (await fetch(url)).text());
       } catch {
-        return true;
+        return { ...scan, opaque: true };
       }
     }
-    return false;
+    return scan;
   })();
-  return memo.__hfAuthorScan;
+  const { opaque, mentions, ids } = memo.__hfAuthorScan;
+  if (opaque) return true;
+  if (!mentions) return false;
+  const timelines = (window as { __timelines?: Record<string, unknown> }).__timelines ?? {};
+  return Array.from(document.querySelectorAll("[data-composition-id]")).some((host) => {
+    const id = host.getAttribute("data-composition-id");
+    const authoredId = host.getAttribute("data-hf-original-composition-id") ?? id;
+    return (
+      id &&
+      !host.hasAttribute("data-no-timeline") &&
+      !timelines[id] &&
+      !skipIds.includes(id) &&
+      ids.includes(authoredId ?? id)
+    );
+  });
 }
 
 export interface SubTimelinePollOptions {
@@ -2182,6 +2213,7 @@ export async function pollSubCompositionTimelines(
           RUNTIME_BOOTSTRAP_ATTR,
           AUTHORED_SOURCE_BEGIN,
           AUTHORED_SOURCE_END,
+          knownUnregisteredIds,
         )));
     if (ready || shouldStop?.()) break;
     const now = Date.now();