fix: hold external sub-compositions through host duration (#917)

packages/core/src/runtime/init.test.ts

@@ -165,6 +165,52 @@ describe("initSandboxRuntimeModular", () => {
     expect(child.style.visibility).toBe("hidden");
   });
 
+  it("keeps external composition hosts visible through their authored duration", async () => {
+    const root = document.createElement("div");
+    root.setAttribute("data-composition-id", "main");
+    root.setAttribute("data-root", "true");
+    root.setAttribute("data-start", "0");
+    root.setAttribute("data-width", "1920");
+    root.setAttribute("data-height", "1080");
+    document.body.appendChild(root);
+
+    const child = document.createElement("div");
+    child.setAttribute("data-composition-id", "sub");
+    child.setAttribute("data-composition-src", "compositions/sub.html");
+    child.setAttribute("data-start", "0");
+    child.setAttribute("data-duration", "3");
+    root.appendChild(child);
+
+    const template = document.createElement("template");
+    template.id = "sub-template";
+    template.innerHTML = `
+      <div data-composition-id="sub" data-width="1920" data-height="1080">
+        <div id="hold-marker">HOLD ME</div>
+      </div>
+    `;
+    document.body.appendChild(template);
+
+    (window as Window & { __timelines?: Record<string, RuntimeTimelineLike> }).__timelines = {
+      main: createMockTimeline(3),
+      sub: createMockTimeline(1),
+    };
+
+    initSandboxRuntimeModular();
+    await new Promise<void>((resolve) => window.setTimeout(resolve, 0));
+
+    const player = (
+      window as Window & {
+        __player?: { renderSeek: (timeSeconds: number) => void };
+      }
+    ).__player;
+    expect(player).toBeDefined();
+    expect(child.querySelector("#hold-marker")?.textContent).toBe("HOLD ME");
+
+    player?.renderSeek(2);
+
+    expect(child.style.visibility).toBe("visible");
+  });
+
   it("pads the root timeline to the authored composition schedule before seeking visibility", () => {
     const root = document.createElement("div");
     root.setAttribute("data-composition-id", "main");

packages/core/src/runtime/init.ts

@@ -255,9 +255,9 @@ export function initSandboxRuntimeModular(): void {
       if (authoredEnd != null && !node.hasAttribute(AUTHORED_END_ATTR)) {
         node.setAttribute(AUTHORED_END_ATTR, authoredEnd);
       }
-      // Non-root compositions derive visible duration from timeline.
-      // Strip both data-duration AND data-end so the visibility system
-      // falls back to the GSAP timeline duration (parity with preview).
+      // Strip public timing attrs on non-root compositions after preserving
+      // authored values privately. Runtime timing can still distinguish
+      // authored host windows from live child timeline durations.
       node.removeAttribute("data-duration");
       node.removeAttribute("data-end");
     }
@@ -1335,9 +1335,18 @@ export function initSandboxRuntimeModular(): void {
           }
         }
 
-        // Composition hosts must respect both the authored parent clip window
-        // and the child composition's own live timeline duration.
-        if (duration != null && duration > 0 && liveDuration != null) {
+        const usesExternalCompositionSlot = rawNode.hasAttribute("data-composition-src");
+
+        // Generic child compositions retain legacy behavior and respect both
+        // the authored parent clip window and the live child timeline duration.
+        // External composition hosts render into an authored slot, so a shorter
+        // child timeline should hold its final state through that slot.
+        if (
+          duration != null &&
+          duration > 0 &&
+          liveDuration != null &&
+          !usesExternalCompositionSlot
+        ) {
           duration = Math.min(duration, liveDuration);
         } else if ((duration == null || duration <= 0) && liveDuration != null) {
           duration = liveDuration;