fix(core): reschedule FX automation when the transport changes rate

`attachElementFxChain` gets a frozen `{scheduledAt, elapsed, rate}` and commits
every lane to absolute context times from it. `setRate` only assigned
`playbackRate.value`, so the audio changed speed and the envelopes did not: a
lowpass sweeping over 10 clip-seconds, switched to 2x mid-play, eats 20 s of
material in 10 s of wall clock while the sweep keeps its original schedule.

The runtime's recovery — `stopAll()` plus a reschedule — only runs when
`hasBoundedActiveSources()` is true, and a project-level music bed with no
`data-duration` is unbounded, so that case never recovered at all.

The timing a chain measures from is now a mutable reference frame that
`setRate` rebases: `elapsed` is advanced to the playhead the OLD rate carried
it to, then the lanes are replayed from there at the new one. That also fixes
the second consequence — `timingNow()` was advancing `elapsed` with the stale
rate, so every later chain edit re-aimed the envelope at the wrong clip
position for as long as the track played.

`attachElementFxChain`'s return type is now the named `ElementFxHandle`, so the
transport's `ScheduledSource.fx` carries `setRate` rather than a structural
`{ dispose(): void }`.

Falsified against four mutations: `setRate` a no-op (envelope stays 8 s long),
rescheduled without adopting the new rate (6 s instead of 3), rate swapped
without rebasing the frame (booked from t=0 instead of t=2), and the guard
dropped (rate 0 books a NaN span).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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

@@ -315,6 +315,124 @@ describe("attachElementFxChain", () => {
     });
   });
 
+  /**
+   * Lanes are committed to absolute context times, so the schedule is only
+   * right for the rate it was booked at. Bumping `playbackRate` alone left a
+   * lowpass sweeping over its original 10 wall-clock seconds while the audio
+   * underneath ran through 20 clip-seconds of material — and the runtime's
+   * stopAll()+reschedule recovery never fired for an unbounded source.
+   */
+  describe("a rate change mid-playback", () => {
+    /** Records what was booked and when, without the browser's overlap rules. */
+    class TimedParam {
+      curves: { time: number; duration: number }[] = [];
+      ramps: number[] = [];
+      value = 0;
+      setValueAtTime(v: number): void {
+        this.value = v;
+      }
+      linearRampToValueAtTime(v: number, t: number): void {
+        this.ramps.push(t);
+        this.value = v;
+      }
+      setValueCurveAtTime(_v: Float32Array, time: number, duration: number): void {
+        this.curves.push({ time, duration });
+      }
+      cancelScheduledValues(): void {}
+      cancelAndHoldAtTime(): void {}
+      /** The last span booked, however the scheduler chose to express it. */
+      last(): { time: number; duration: number } | undefined {
+        return this.curves.at(-1);
+      }
+    }
+
+    const sweep = {
+      version: 1,
+      nodes: [{ type: "lowpass", id: "n1", params: { frequency: 300, q: 0.707 } }],
+    };
+    const lane = JSON.stringify({
+      version: 1,
+      lanes: [
+        {
+          target: "fx.n1.frequency",
+          points: [
+            { t: 0, v: 300 },
+            { t: 8, v: 3000 },
+          ],
+        },
+      ],
+    });
+
+    const build = () => {
+      const clock = { currentTime: 0 };
+      const made: { frequency: TimedParam }[] = [];
+      class TimedNode extends Node {
+        override frequency = new TimedParam() as unknown as { value: number };
+      }
+      class TimedCtx extends Ctx {
+        get currentTime(): number {
+          return clock.currentTime;
+        }
+        override createBiquadFilter(): Node {
+          const n = new TimedNode();
+          made.push(n as unknown as { frequency: TimedParam });
+          return n;
+        }
+      }
+      const node = document.createElement("audio");
+      node.setAttribute("data-fx-chain", JSON.stringify(sweep));
+      node.setAttribute("data-automation", lane);
+      document.body.append(node);
+      const handle = attachElementFxChain(
+        new TimedCtx() as unknown as BaseAudioContext,
+        node,
+        new Node() as never,
+        new Node() as never,
+        { scheduledAt: 0, elapsed: 0, rate: 1 },
+      );
+      return { clock, node, handle, param: () => made[0]?.frequency as unknown as TimedParam };
+    };
+
+    it("re-aims the envelope so the sweep still ends with the material", () => {
+      const { clock, handle, param } = build();
+      // Booked at 1x: the whole 8 s lane spans 8 s of context time.
+      expect(param().last()).toEqual({ time: 0, duration: 8 });
+
+      clock.currentTime = 2;
+      handle?.setRate(2);
+
+      // 6 clip-seconds are left, and at 2x they take 3 wall-clock seconds.
+      // Without this the sweep kept its original plan to t=8 while the audio
+      // ran out at t=5.
+      expect(param().last()).toEqual({ time: 2, duration: 3 });
+    });
+
+    it("measures later edits from the new rate, not the one it started at", async () => {
+      // `elapsed` advances at whatever rate the reference frame holds, so a
+      // frame left at 1x re-aims every subsequent edit at the wrong clip
+      // position for as long as the track plays.
+      const { clock, node, handle, param } = build();
+      clock.currentTime = 2;
+      handle?.setRate(2);
+
+      clock.currentTime = 4;
+      // 2 wall-clock seconds at 2x is 4 clip-seconds, so the playhead is at 6
+      // and 2 clip-seconds remain: 1 second of wall clock.
+      node.setAttribute("data-automation", lane);
+      await new Promise((r) => setTimeout(r, 0));
+      expect(param().last()).toEqual({ time: 4, duration: 1 });
+    });
+
+    it("ignores a rate that is not a rate", () => {
+      const { clock, handle, param } = build();
+      clock.currentTime = 2;
+      handle?.setRate(0);
+      handle?.setRate(Number.NaN);
+      handle?.setRate(1);
+      expect(param().last()).toEqual({ time: 0, duration: 8 });
+    });
+  });
+
   it("tears the chain down on dispose", () => {
     const src = new Node();
     const dst = new Node();

packages/core/src/runtime/audioFx.ts

@@ -81,15 +81,29 @@ function readChain(el: { getAttribute?(name: string): string | null }): {
  * first effect is then heard without rescheduling the source.
  *
  * With `timing`, the element's automation lanes are scheduled onto the built
- * effects as AudioParam ramps, and rescheduled when the attribute is edited.
+ * effects as AudioParam ramps, and rescheduled when the attribute is edited or
+ * `setRate` reports the transport changed speed.
  */
+export interface ElementFxHandle {
+  dispose(): void;
+  /**
+   * Re-aim every booked envelope at a new playback rate.
+   *
+   * Lanes are committed to absolute context times, so a param scheduled at 1×
+   * keeps its original wall-clock plan while the audio underneath runs at the
+   * new speed: a lowpass sweeping over 10 clip-seconds, switched to 2×, eats
+   * 20 s of material in 10 s of wall clock with the sweep unchanged.
+   */
+  setRate(rate: number): void;
+}
+
 export function attachElementFxChain(
   ctx: BaseAudioContext,
   el: { getAttribute?(name: string): string | null },
   source: AudioNode,
   destination: AudioNode,
   timing?: AutomationTiming,
-): { dispose(): void } | null {
+): ElementFxHandle | null {
   const { chain } = readChain(el);
 
   // Null means the source runs straight into its gain: an empty chain, or one
@@ -166,20 +180,26 @@ export function attachElementFxChain(
       at && handle ? scheduleChainAutomation(readAutomation(el, next), next, handle.nodes, at) : [];
   };
 
+  // The reference frame every later reschedule measures from. Mutable because a
+  // rate change rebases it: `elapsed` has to stop advancing at the old rate the
+  // instant the new one takes effect, or every subsequent edit re-aims the
+  // envelope at the wrong clip position.
+  let frame: AutomationTiming | null = timing ? { ...timing } : null;
+
   attach(chain);
-  scheduleFor(chain, timing ?? null);
+  scheduleFor(chain, frame);
 
   /**
    * Re-aim the envelope at the live playhead. An edit lands mid-playback, so
    * the clip has advanced past the offset the source was scheduled with.
    */
   const timingNow = (): AutomationTiming | null => {
-    if (!timing) return null;
-    const now = typeof ctx.currentTime === "number" ? ctx.currentTime : timing.scheduledAt;
+    if (!frame) return null;
+    const now = typeof ctx.currentTime === "number" ? ctx.currentTime : frame.scheduledAt;
     return {
       scheduledAt: now,
-      elapsed: timing.elapsed + (now - timing.scheduledAt) * timing.rate,
-      rate: timing.rate,
+      elapsed: frame.elapsed + (now - frame.scheduledAt) * frame.rate,
+      rate: frame.rate,
     };
   };
 
@@ -239,6 +259,15 @@ export function attachElementFxChain(
   }
 
   return {
+    setRate: (rate: number) => {
+      const at = timingNow();
+      if (disposed || !at || !Number.isFinite(rate) || rate <= 0 || rate === at.rate) return;
+      // Rebased at the playhead the OLD rate carried us to, then replayed from
+      // there at the new one.
+      frame = { ...at, rate };
+      cancelParamLane(automated, at.scheduledAt);
+      scheduleFor(readChain(el).chain, frame);
+    },
     dispose: () => {
       disposed = true;
       observer?.disconnect();

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

@@ -289,6 +289,23 @@ describe("WebAudioTransport", () => {
       expect(mock.sourceNode.playbackRate.value).toBe(2);
     });
 
+    it("setRate re-aims each source's FX automation, not just its playback rate", async () => {
+      // The lanes are committed to absolute context times when the source is
+      // scheduled, so bumping playbackRate alone left every automated parameter
+      // running its original plan over audio moving at a different speed.
+      const { transport, mock, gen } = setupTransport(100);
+      await transport.schedulePlayback(mockEl, mockBuffer, 5, 0, 8, 1, gen, 1);
+      const active = (transport as unknown as { _activeSources: { fx?: unknown }[] })
+        ._activeSources;
+      const setRate = vi.fn();
+      active[0]!.fx = { dispose: vi.fn(), setRate };
+
+      transport.setRate(2);
+
+      expect(setRate).toHaveBeenCalledWith(2);
+      expect(mock.sourceNode.playbackRate.value).toBe(2);
+    });
+
     it("setRate before any sources are scheduled does not throw", () => {
       const transport = new WebAudioTransport();
       expect(() => transport.setRate(2)).not.toThrow();

packages/core/src/runtime/webAudioTransport.ts

@@ -1,4 +1,4 @@
-import { attachElementFxChain } from "./audioFx.js";
+import { attachElementFxChain, type ElementFxHandle } from "./audioFx.js";
 import type { AutomationTiming } from "../audio/audioFxAutomation.js";
 import { swallow } from "./diagnostics";
 import { getDebugSurface } from "./globals.js";
@@ -63,7 +63,7 @@ export type ScheduledSource = {
   sourceNode: AudioBufferSourceNode;
   gainNode: GainNode;
   /** FX chain spliced between source and gain, when the element carries one. */
-  fx?: { dispose(): void } | null;
+  fx?: ElementFxHandle | null;
   compositionStart: number;
   mediaStart: number;
   scheduledAt: number;
@@ -274,6 +274,14 @@ export class WebAudioTransport {
    * `getTime()` stays continuous across the change. Sources scheduled to
    * start in the future keep their original wallclock start time — callers
    * that need rate-correct future starts should `stopAll()` and reschedule.
+   *
+   * Each source's FX automation is re-aimed too. Lanes are committed to
+   * absolute context times when the source is scheduled, so bumping only
+   * `playbackRate` left every automated parameter running its original plan
+   * over audio moving at a different speed. The `stopAll()`+reschedule recovery
+   * in the runtime is no help here: it only fires for bounded sources, and a
+   * project-level music bed with no `data-duration` is unbounded, so it never
+   * recovered at all.
    */
   setRate(rate: number): boolean {
     const safeRate = normalizeRate(rate);
@@ -286,6 +294,7 @@ export class WebAudioTransport {
     for (const source of this._activeSources) {
       try {
         source.sourceNode.playbackRate.value = safeRate;
+        source.fx?.setRate(safeRate);
       } catch (err) {
         swallow("webAudioTransport.setRate", err);
       }