fix(core): the picture keeps playing while a just-started take's audio catches up

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

@@ -353,6 +353,32 @@ describe("TransportClock", () => {
       expect(clock.now()).toBeCloseTo(6.37, 5);
     });
 
+    it("runs the playhead past a just-started element frozen 300 ms behind it, as iOS leaves it", () => {
+      const { clock, advance } = createClock({ duration: 30 });
+      clock.play();
+      advancePolled(clock, advance, 6300);
+      const take = createMockAudioEl(0, false); // playing, its time frozen at its start
+      clock.attachAudioSource({ el: take, compositionStart: 6, mediaStart: 0 });
+      let previous = clock.now();
+      for (let i = 0; i < 20; i++) {
+        advance(16);
+        const now = clock.now();
+        expect(now).toBeGreaterThan(previous); // never held, though the element has not moved
+        previous = now;
+      }
+      expect(previous).toBeCloseTo(6.3 + 0.32, 1);
+    });
+
+    it("holds for an element no more than the steer limit behind, so a small lag is not seeked", () => {
+      const { clock, advance } = createClock({ duration: 30 });
+      clock.play();
+      advancePolled(clock, advance, 6300);
+      const take = createMockAudioEl(0, false); // 50 ms behind the playhead
+      clock.attachAudioSource({ el: take, compositionStart: 6.25, mediaStart: 0 });
+      advance(100);
+      expect(clock.now()).toBeCloseTo(6.3, 5);
+    });
+
     it("holds the playhead while its element seeks, even to a time ahead", () => {
       const { clock, advance } = createClock({ duration: 30 });
       clock.play();
@@ -400,7 +426,7 @@ describe("TransportClock", () => {
       clock.play();
       advancePolled(clock, advance, 2000);
       const bed = Object.assign(createMockAudioEl(0, false), { loop: true, duration: 8 });
-      clock.attachAudioSource({ el: bed, compositionStart: 1.9, mediaStart: 0 });
+      clock.attachAudioSource({ el: bed, compositionStart: 1.95, mediaStart: 0 });
       advance(16);
       expect(clock.now()).toBeCloseTo(2, 5);
     });

packages/core/src/runtime/clock.ts

@@ -1,5 +1,5 @@
 import { timeAtSourceTime, type RateSpec } from "../speedRamp.js";
-import { MEDIA_HARD_SYNC_SECONDS } from "./mediaSteer.js";
+import { AUDIO_STEER_LIMIT_SECONDS } from "./mediaSteer.js";
 import { playRangeHoldTime } from "./protocol.js";
 
 export type TransportClockSnapshot = {
@@ -99,7 +99,8 @@ export class TransportClock {
   private _heldTimeWhenBehind(el: HTMLMediaElement, behind: number, spec: RateSpec): number | null {
     if (el.loop && behind > timeAtSourceTime(spec, el.duration) / 2) return null;
     const buffering = el.readyState < HAVE_FUTURE_DATA;
-    return buffering || behind <= MEDIA_HARD_SYNC_SECONDS ? this._lastNow : null;
+    // Past the steer limit the sync seeks the audio onto the playhead, so the picture does not wait for it.
+    return buffering || behind <= AUDIO_STEER_LIMIT_SECONDS ? this._lastNow : null;
   }
 
   /** Folds a >500ms gap since the last read into `_playStartMs` so it's never reported as played time. */

packages/core/src/runtime/mediaSteer.ts

@@ -11,7 +11,7 @@ const MEDIA_STEER_STEPS: readonly (readonly [lag: number, steer: number])[] = [
 const MEDIA_STEER_RELEASE_SECONDS = 0.01;
 // Audio never steers past 10%, where speech changes tempo; a lag held past 100 ms is seeked instead.
 const AUDIO_MAX_STEER = 0.1;
-const AUDIO_STEER_LIMIT_SECONDS = 0.1;
+export const AUDIO_STEER_LIMIT_SECONDS = 0.1;
 
 /** Direction (+1 fast, -1 slow) an element is being steered in; absent when it plays at its authored rate. */
 const steering = new WeakMap<HTMLMediaElement, number>();