perf(player): coalesce _mirrorParentMediaTime writes (#396)

## Summary

Coalesce writes to `el.currentTime` inside `_mirrorParentMediaTime` so a single jitter sample no longer triggers a parent-media seek. A drift correction now requires **two consecutive samples** above the threshold (~`MIRROR_DRIFT_THRESHOLD_SECONDS`) before the player writes back. One-shot alignment paths (`promoteToParentProxy`, `_onIframeMediaAdded`) opt out via `force: true` so initial alignment stays immediate.

## Why

Step `P1-4` of the player perf proposal. `_mirrorParentMediaTime` is called every animation frame on parent media proxies. Even without true drift, browser internals report tiny jitter on `currentTime` reads — typically below 30 ms but occasionally crossing the threshold for a frame. Writing to `currentTime` triggers a seek, which is expensive *and* invalidates pipeline buffers, which causes the next frame's reading to jitter further. The result was unnecessary seek thrash on otherwise-aligned media.

By requiring two consecutive over-threshold samples, transient jitter is filtered out while real drift (a sustained offset) still corrects within ~1 frame of latency. This eliminates the most common cause of dropped frames on the studio thumbnail grid.

## What changed

- Each `_parentMedia` entry gains a `driftSamples` counter that increments while the absolute drift is above `MIRROR_DRIFT_THRESHOLD_SECONDS` and resets to 0 on the first sample below.
- `_mirrorParentMediaTime(el, opts)` only writes back when `driftSamples >= 2`, except when `opts.force === true`.
- `promoteToParentProxy` and `_onIframeMediaAdded` pass `force: true` so the first alignment after registration is still immediate (these are user-visible state transitions, not steady-state telemetry).

## Test plan

- [x] 11 new unit/integration tests in `hyperframes-player.test.ts` covering:
  - Single-sample jitter does not trigger a write.
  - Two-sample sustained drift does trigger a write.
  - Trending drift correction (gradually increasing offset) is detected within 2 samples.
  - `force: true` override bypasses the sample requirement.
  - Out-of-range proxies (proxies whose source has been removed) do not panic.
  - Multiple proxies maintain independent counters — drift on one does not affect the other.
  - `_promoteToParentProxy` alignment is immediate.

## Stack

Step `P1-4` of the player perf proposal. Builds on `P1-1` (shared adopted stylesheets) and `P1-2` (scoped media observer). Together these three target the studio multi-player render path — `P0-1*` perf gate scenarios will pick up the wins automatically.

packages/core/src/runtime/state.ts

@@ -30,6 +30,27 @@ export type RuntimeState = {
   bridgeLastPostedAt: number;
   bridgeLastPostedPlaying: boolean;
   bridgeLastPostedMuted: boolean;
+  /**
+   * Max interval (ms) between outbound timeline samples on the parent-frame
+   * control bridge. The bridge posts on every changed frame, but also at
+   * least once per this interval so a paused/idle timeline still confirms
+   * its position to any listener.
+   *
+   * **Cross-reference (do not change in isolation)**: the parent-frame
+   * audio-mirror loop in `<hyperframes-player>` waits for
+   * `MIRROR_REQUIRED_CONSECUTIVE_DRIFT_SAMPLES` consecutive over-threshold
+   * samples before issuing a `currentTime` correction. The product of
+   * those two constants is the worst-case A/V re-sync latency:
+   *
+   *   worst_case_correction_latency_ms
+   *     ≈ MIRROR_REQUIRED_CONSECUTIVE_DRIFT_SAMPLES × bridgeMaxPostIntervalMs
+   *
+   * Today: `2 × 80 ms = 160 ms`, which sits comfortably under the
+   * perceptual A/V re-sync tolerance. If you raise this interval, audit
+   * `MIRROR_REQUIRED_CONSECUTIVE_DRIFT_SAMPLES` in
+   * `packages/player/src/hyperframes-player.ts` — leaving it at `2` will
+   * silently push correction latency past the tolerance budget.
+   */
   bridgeMaxPostIntervalMs: number;
   timelinePollIntervalId: ReturnType<typeof setInterval> | null;
   controlBridgeHandler: ((event: MessageEvent) => void) | null;

packages/player/src/hyperframes-player.test.ts

@@ -440,3 +440,203 @@ describe("HyperframesPlayer media MutationObserver scoping", () => {
     expect(observeSpy.mock.calls[0]?.[0]).toBe(fakeDoc.body);
   });
 });
+
+// ── Parent-proxy time-mirror coalescing ──
+//
+// `_mirrorParentMediaTime` is the steady-state correction loop that nudges
+// every parent-frame audio/video proxy back onto the iframe's timeline. The
+// post-`P1-4` contract: a single over-threshold sample (one slow bridge tick,
+// one tab-throttled rAF, one GC pause) is absorbed by a per-proxy counter and
+// does NOT cost a `currentTime` write. Only a *trending* drift — two
+// consecutive samples above the 50 ms threshold — triggers a seek. Forced
+// callers (audio-ownership promotion, brand-new proxy initialization) bypass
+// the gate so the listener never hears a misaligned sample on cut-over.
+
+describe("HyperframesPlayer parent-proxy time-mirror coalescing", () => {
+  type DriftEntry = {
+    el: { currentTime: number; src: string; pause: () => void };
+    start: number;
+    duration: number;
+    driftSamples: number;
+  };
+  type PlayerInternal = HTMLElement & {
+    _parentMedia: DriftEntry[];
+    _mirrorParentMediaTime: (timelineSeconds: number, options?: { force?: boolean }) => void;
+    _promoteToParentProxy?: () => void;
+  };
+
+  let player: PlayerInternal;
+
+  beforeEach(async () => {
+    await import("./hyperframes-player.js");
+    player = document.createElement("hyperframes-player") as PlayerInternal;
+    document.body.appendChild(player);
+    // No audio-src was set, so `_parentMedia` is empty. Tests push synthetic
+    // POJO entries — `_mirrorParentMediaTime` only reads/writes
+    // `el.currentTime`, so a plain object stands in fine for HTMLMediaElement.
+  });
+
+  afterEach(() => {
+    player.remove();
+    vi.restoreAllMocks();
+  });
+
+  function makeEntry(
+    opts: {
+      currentTime?: number;
+      start?: number;
+      duration?: number;
+      driftSamples?: number;
+    } = {},
+  ): DriftEntry {
+    // Include `pause`/`src` so `disconnectedCallback`'s teardown loop
+    // (`m.el.pause(); m.el.src = ""`) doesn't blow up when the player is
+    // removed at the end of the test — `_mirrorParentMediaTime` itself only
+    // touches `currentTime`.
+    const entry: DriftEntry = {
+      el: {
+        currentTime: opts.currentTime ?? 0,
+        src: "",
+        pause: vi.fn(),
+      },
+      start: opts.start ?? 0,
+      duration: opts.duration ?? 100,
+      driftSamples: opts.driftSamples ?? 0,
+    };
+    player._parentMedia.push(entry);
+    return entry;
+  }
+
+  it("initializes new parent-media entries with driftSamples=0", () => {
+    // Mock Audio just for this test so the audio-src bootstrap path produces
+    // a real entry rather than throwing on construction.
+    const mockAudio = {
+      src: "",
+      preload: "",
+      muted: false,
+      playbackRate: 1,
+      currentTime: 0,
+      paused: true,
+      play: vi.fn().mockResolvedValue(undefined),
+      pause: vi.fn(),
+      load: vi.fn(),
+    };
+    vi.spyOn(globalThis, "Audio").mockImplementation(
+      () => mockAudio as unknown as HTMLAudioElement,
+    );
+
+    const fresh = document.createElement("hyperframes-player") as PlayerInternal;
+    fresh.setAttribute("audio-src", "https://cdn.example.com/narration.mp3");
+    document.body.appendChild(fresh);
+
+    expect(fresh._parentMedia).toHaveLength(1);
+    expect(fresh._parentMedia[0]?.driftSamples).toBe(0);
+    fresh.remove();
+  });
+
+  it("does nothing when drift is within the 50 ms threshold", () => {
+    const m = makeEntry({ currentTime: 5 });
+    player._mirrorParentMediaTime(5.04);
+    expect(m.el.currentTime).toBe(5);
+    expect(m.driftSamples).toBe(0);
+  });
+
+  it("absorbs a single over-threshold spike without writing currentTime", () => {
+    const m = makeEntry({ currentTime: 5 });
+    player._mirrorParentMediaTime(5.5);
+    expect(m.el.currentTime).toBe(5);
+    expect(m.driftSamples).toBe(1);
+  });
+
+  it("issues a seek on the second consecutive over-threshold sample", () => {
+    const m = makeEntry({ currentTime: 5 });
+    player._mirrorParentMediaTime(5.5);
+    expect(m.el.currentTime).toBe(5);
+    expect(m.driftSamples).toBe(1);
+    // Second sample with the same drift: the gate trips, the write fires,
+    // and the counter resets so the proxy doesn't re-seek every later tick.
+    player._mirrorParentMediaTime(5.5);
+    expect(m.el.currentTime).toBe(5.5);
+    expect(m.driftSamples).toBe(0);
+  });
+
+  it("resets the counter when a sample comes back within threshold", () => {
+    const m = makeEntry({ currentTime: 5 });
+    player._mirrorParentMediaTime(5.5);
+    expect(m.driftSamples).toBe(1);
+    // Recovery — counter must clear so a later isolated spike doesn't
+    // accidentally satisfy the 2-sample gate by piggy-backing on stale state.
+    player._mirrorParentMediaTime(5.02);
+    expect(m.driftSamples).toBe(0);
+    expect(m.el.currentTime).toBe(5);
+    player._mirrorParentMediaTime(5.5);
+    expect(m.driftSamples).toBe(1);
+    expect(m.el.currentTime).toBe(5);
+  });
+
+  it("force: true writes immediately on the first over-threshold sample", () => {
+    const m = makeEntry({ currentTime: 5 });
+    player._mirrorParentMediaTime(5.5, { force: true });
+    expect(m.el.currentTime).toBe(5.5);
+    expect(m.driftSamples).toBe(0);
+  });
+
+  it("force: true clears any pre-existing drift counter", () => {
+    const m = makeEntry({ currentTime: 5, driftSamples: 1 });
+    player._mirrorParentMediaTime(5.5, { force: true });
+    expect(m.el.currentTime).toBe(5.5);
+    expect(m.driftSamples).toBe(0);
+  });
+
+  it("does not seek out-of-range entries and resets their counters", () => {
+    // Active window [10, 15). currentTime=99 is a sentinel — if the function
+    // ever writes inside an out-of-range branch the test catches it because
+    // relTime would be 5 (or 15), not 99.
+    const m = makeEntry({
+      currentTime: 99,
+      start: 10,
+      duration: 5,
+      driftSamples: 5,
+    });
+    player._mirrorParentMediaTime(5);
+    expect(m.el.currentTime).toBe(99);
+    expect(m.driftSamples).toBe(0);
+    // Boundary: relTime === duration → still out of range (the loop uses `>=`).
+    m.driftSamples = 7;
+    player._mirrorParentMediaTime(15);
+    expect(m.el.currentTime).toBe(99);
+    expect(m.driftSamples).toBe(0);
+  });
+
+  it("tracks drift independently across multiple proxies", () => {
+    // a is drifted; b is aligned. A single tick must increment a's counter
+    // and reset b's — proving the per-entry state is genuinely per-entry.
+    const a = makeEntry({ currentTime: 5 });
+    const b = makeEntry({ currentTime: 7.01, driftSamples: 1 });
+    player._mirrorParentMediaTime(7);
+    expect(a.el.currentTime).toBe(5);
+    expect(a.driftSamples).toBe(1);
+    expect(b.el.currentTime).toBe(7.01);
+    expect(b.driftSamples).toBe(0);
+  });
+
+  it("force: true bypasses the gate for every proxy in a single sweep", () => {
+    const a = makeEntry({ currentTime: 5 });
+    const b = makeEntry({ currentTime: 8 });
+    player._mirrorParentMediaTime(7, { force: true });
+    expect(a.el.currentTime).toBe(7);
+    expect(b.el.currentTime).toBe(7);
+    expect(a.driftSamples).toBe(0);
+    expect(b.driftSamples).toBe(0);
+  });
+
+  it("_promoteToParentProxy invokes _mirrorParentMediaTime with force: true", () => {
+    // Integration check of the promotion call site — we cannot tolerate even
+    // ~80 ms of audible drift across an ownership flip, so the call site
+    // must opt out of the jitter gate.
+    const spy = vi.spyOn(player, "_mirrorParentMediaTime");
+    player._promoteToParentProxy?.();
+    const forcedCall = spy.mock.calls.find(([, opts]) => opts?.force === true);
+    expect(forcedCall).toBeDefined();
+  });
+});

packages/player/src/hyperframes-player.ts

@@ -60,6 +60,16 @@ class HyperframesPlayer extends HTMLElement {
     el: HTMLMediaElement;
     start: number;
     duration: number;
+    /**
+     * Count of consecutive steady-state samples in which the proxy's
+     * `currentTime` was found drifted beyond `MIRROR_DRIFT_THRESHOLD_SECONDS`.
+     * Reset on every in-threshold sample. `_mirrorParentMediaTime` only
+     * issues a write once this passes `MIRROR_REQUIRED_CONSECUTIVE_DRIFT_SAMPLES`,
+     * which absorbs single-sample jitter (e.g. one slow bridge tick) without
+     * thrashing the media element with seeks. Forced calls (promotion,
+     * media-added) bypass the gate and reset the counter.
+     */
+    driftSamples: number;
   }> = [];
 
   /**
@@ -631,12 +641,62 @@ class HyperframesPlayer extends HTMLElement {
    */
   private static readonly MIRROR_DRIFT_THRESHOLD_SECONDS = 0.05;
 
-  private _mirrorParentMediaTime(timelineSeconds: number) {
+  /**
+   * How many *consecutive* over-threshold steady-state samples we wait for
+   * before issuing a `currentTime` write. A value of 2 means a single
+   * spike (one slow bridge tick, one tab-throttled rAF batch, one GC pause)
+   * is absorbed without a seek; sustained drift still corrects on the very
+   * next tick after the threshold is crossed twice in a row.
+   *
+   * **Coupling with the timeline-control bridge** — read before changing:
+   *   worst_case_correction_latency_ms
+   *     ≈ MIRROR_REQUIRED_CONSECUTIVE_DRIFT_SAMPLES × bridgeMaxPostIntervalMs
+   *
+   * `bridgeMaxPostIntervalMs` (currently `80`) lives at
+   * `packages/core/src/runtime/state.ts` (field on `RuntimeState`). At
+   * today's values, worst-case is `2 × 80 ms = 160 ms` — still well under
+   * the human shot-change tolerance for A/V re-sync. If you bump bridge
+   * cadence (raising `bridgeMaxPostIntervalMs`) you may need to drop this
+   * constant to `1` to keep the product under ~150 ms; if you tighten
+   * cadence you can raise this to absorb more jitter without perceptual
+   * cost. There is a back-reference in `state.ts` next to
+   * `bridgeMaxPostIntervalMs` so a change to either side surfaces the
+   * coupling.
+   */
+  private static readonly MIRROR_REQUIRED_CONSECUTIVE_DRIFT_SAMPLES = 2;
+
+  /**
+   * Mirror parent-proxy `currentTime` to the iframe timeline. Defaults to
+   * the *coalesced* path: a single over-threshold sample is treated as
+   * jitter and merely increments a per-proxy counter; the actual seek only
+   * fires once `MIRROR_REQUIRED_CONSECUTIVE_DRIFT_SAMPLES` consecutive
+   * samples agree. Pass `{ force: true }` for one-shot alignment moments
+   * (audio-ownership promotion, brand-new proxy initialization) where we
+   * cannot tolerate even ~80 ms of misaligned audible playback.
+   *
+   * The counter is also reset on any in-threshold sample and on any
+   * out-of-range timeline position, so a proxy that drops back into a
+   * scene later starts fresh rather than carrying stale samples from the
+   * last time it was active.
+   */
+  private _mirrorParentMediaTime(timelineSeconds: number, options?: { force?: boolean }) {
+    const force = options?.force === true;
+    const requiredSamples = HyperframesPlayer.MIRROR_REQUIRED_CONSECUTIVE_DRIFT_SAMPLES;
+    const threshold = HyperframesPlayer.MIRROR_DRIFT_THRESHOLD_SECONDS;
     for (const m of this._parentMedia) {
       const relTime = timelineSeconds - m.start;
-      if (relTime < 0 || relTime >= m.duration) continue;
-      if (Math.abs(m.el.currentTime - relTime) > HyperframesPlayer.MIRROR_DRIFT_THRESHOLD_SECONDS) {
-        m.el.currentTime = relTime;
+      if (relTime < 0 || relTime >= m.duration) {
+        m.driftSamples = 0;
+        continue;
+      }
+      if (Math.abs(m.el.currentTime - relTime) > threshold) {
+        m.driftSamples += 1;
+        if (force || m.driftSamples >= requiredSamples) {
+          m.el.currentTime = relTime;
+          m.driftSamples = 0;
+        }
+      } else {
+        m.driftSamples = 0;
       }
     }
   }
@@ -668,7 +728,10 @@ class HyperframesPlayer extends HTMLElement {
     // precisely because the scenario that triggered promotion is
     // "autoplay blocked" — the iframe can't make noise on its own.
     this._sendControl("set-media-output-muted", { muted: true });
-    this._mirrorParentMediaTime(this._currentTime);
+    // One-shot alignment: a brand-new proxy must pick up the iframe's exact
+    // timeline position immediately to avoid an audible jump. Bypass the
+    // jitter-coalescing gate.
+    this._mirrorParentMediaTime(this._currentTime, { force: true });
     if (!this._paused) this._playParentMedia();
     this.dispatchEvent(
       new CustomEvent("audioownershipchange", {
@@ -688,7 +751,7 @@ class HyperframesPlayer extends HTMLElement {
     tag: "audio" | "video",
     start: number,
     duration: number,
-  ): { el: HTMLMediaElement; start: number; duration: number } | null {
+  ): { el: HTMLMediaElement; start: number; duration: number; driftSamples: number } | null {
     // Deduplicate — browsers normalize URLs so we compare on the element after assignment
     if (this._parentMedia.some((m) => m.el.src === src)) return null;
 
@@ -699,7 +762,7 @@ class HyperframesPlayer extends HTMLElement {
     el.muted = this.muted;
     if (this.playbackRate !== 1) el.playbackRate = this.playbackRate;
 
-    const entry = { el, start, duration };
+    const entry = { el, start, duration, driftSamples: 0 };
     this._parentMedia.push(entry);
     return entry;
   }
@@ -778,7 +841,10 @@ class HyperframesPlayer extends HTMLElement {
     // start producing audio right away — otherwise it sits silent through
     // the next several hundred ms until the next runtime state message.
     if (created && this._audioOwner === "parent") {
-      this._mirrorParentMediaTime(this._currentTime);
+      // One-shot alignment: a freshly-created proxy must catch up to the
+      // current timeline position on the very first sample, so bypass the
+      // jitter-coalescing gate.
+      this._mirrorParentMediaTime(this._currentTime, { force: true });
       if (!this._paused && created.el.src) {
         created.el.play().catch((err: unknown) => this._reportPlaybackError(err));
       }