fix(runtime): don't restart non-loop media that has naturally ended (#1203)

* fix(runtime): don't restart non-loop media that has naturally ended

When a media element's authored data-duration exceeds the actual file
length, el.ended becomes true at the file's natural end while the clip
is still considered 'active' (timeSeconds < clip.end). The runtime was
calling el.play() on the ended element every rAF tick, resetting
currentTime to 0 and causing audible stutter for the overshoot duration.

Fix: treat el.ended as inactive for non-loop clips. The element sits
silently until the composition ends. el.ended resets to false on any
seek, so scrubbing backward correctly resumes playback.

Reproducer: bg-music WAV is 60s but data-duration='68.6' (composition
duration). Last 8.6s: rapid play->clamp->end->play cycle at 60fps.

* test(runtime): add seek-recovery contract test for el.ended guard

Adds a third test case pinning the seek-recovery property called out in
the PR body: a clip that went silent at t=62 (el.ended=true) should
resume playing after a backward seek resets el.ended to false.

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

@@ -320,6 +320,39 @@ describe("syncRuntimeMedia", () => {
     expect(clip.el.pause).toHaveBeenCalled();
   });
 
+  it("does not restart a non-loop clip that has naturally ended before the clip's end time", () => {
+    // Reproduces: bg-music WAV is 60s but data-duration="68.6" (composition duration).
+    // At t=62 the file has ended; without this guard the runtime calls el.play() every
+    // rAF tick, resetting currentTime to 0 and causing audible stutter for 8.6s.
+    const clip = createMockClip({ start: 0, end: 68.6, loop: false });
+    Object.defineProperty(clip.el, "paused", { value: true, writable: true });
+    Object.defineProperty(clip.el, "ended", { value: true, writable: true, configurable: true });
+    syncRuntimeMedia({ clips: [clip], timeSeconds: 62, playing: true, playbackRate: 1 });
+    expect(clip.el.play).not.toHaveBeenCalled();
+  });
+
+  it("does restart a loop clip that has naturally ended while still within its active window", () => {
+    const clip = createMockClip({ start: 0, end: 68.6, loop: true, sourceDuration: 60 });
+    Object.defineProperty(clip.el, "paused", { value: true, writable: true });
+    Object.defineProperty(clip.el, "ended", { value: true, writable: true, configurable: true });
+    syncRuntimeMedia({ clips: [clip], timeSeconds: 62, playing: true, playbackRate: 1 });
+    expect(clip.el.play).toHaveBeenCalled();
+  });
+
+  it("resumes a previously-ended non-loop clip after a backward seek resets el.ended", () => {
+    // el.ended resets to false when the browser processes a seek (per WHATWG spec).
+    // This test pins the contract: silent at t=62 (ended), playable again at t=30 (seeked back).
+    const clip = createMockClip({ start: 0, end: 68.6, loop: false });
+    Object.defineProperty(clip.el, "paused", { value: true, writable: true });
+    Object.defineProperty(clip.el, "ended", { value: true, writable: true, configurable: true });
+    syncRuntimeMedia({ clips: [clip], timeSeconds: 62, playing: true, playbackRate: 1 });
+    expect(clip.el.play).not.toHaveBeenCalled();
+    // Simulate backward seek: browser resets ended to false before committing new currentTime
+    Object.defineProperty(clip.el, "ended", { value: false, writable: true, configurable: true });
+    syncRuntimeMedia({ clips: [clip], timeSeconds: 30, playing: true, playbackRate: 1 });
+    expect(clip.el.play).toHaveBeenCalledTimes(1);
+  });
+
   it("sets volume when clip has volume", () => {
     const clip = createMockClip({ start: 0, end: 10, volume: 0.7 });
     syncRuntimeMedia({ clips: [clip], timeSeconds: 5, playing: false, playbackRate: 1 });

packages/core/src/runtime/media.ts

@@ -159,8 +159,15 @@ export function syncRuntimeMedia(params: {
     const { el } = clip;
     if (!el.isConnected) continue;
     let relTime = (params.timeSeconds - clip.start) * clip.playbackRate + clip.mediaStart;
+    // An ended non-loop element has played its file to natural completion.
+    // Don't restart it — if the authored duration extends past the file's
+    // actual length, the element sits silently until the composition ends.
+    // (el.ended resets to false when the user scrubs back, so seeks work.)
     const isActive =
-      params.timeSeconds >= clip.start && params.timeSeconds < clip.end && relTime >= 0;
+      params.timeSeconds >= clip.start &&
+      params.timeSeconds < clip.end &&
+      relTime >= 0 &&
+      (!el.ended || clip.loop);
     if (isActive) {
       // Loop wrapping: when media reaches end, restart from mediaStart
       if (clip.loop && clip.sourceDuration != null && clip.sourceDuration > 0) {