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fix(core): cap the playable duration at the calculated document-level composition duration
The clock duration was max(gsap timeline, floors) with the declared duration only ever acting as a floor — so one ambient tween authored longer than the composition (a scene component's 20s background drift inside a 12s composition) stretched playback into a blank tail: 8s of empty frames after every clip had ended, the playhead pinned at the timeline edge, and play-at-end then restarting from 0. <html data-composition-duration> is machine-calculated from the content schedule by every emitter that stamps it, so it is now a CEILING on the playable duration. A composition HOST's data-duration deliberately stays floor-only: hand/agent-authored documents often declare a stale value there, and playing the full timeline remains the safer default (the 'keeps the timeline duration' test pins that).
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‎packages/core/src/runtime/init.test.ts‎

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@@ -516,6 +516,28 @@ describe("initSandboxRuntimeModular", () => {
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expect(warned).not.toContain("Root timeline not bound");
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});
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it("caps the playable duration at <html data-composition-duration>", () => {
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// The document-level duration is machine-calculated from the content
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// schedule, so it is a CEILING: one over-long ambient tween (a scene
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// component's 20s background drift) must not stretch playback into a
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// blank tail past the calculated end. (A composition HOST's declared
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// data-duration stays floor-only — see "keeps the timeline duration when
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// it exceeds the root's declared data-duration".)
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document.documentElement.setAttribute("data-composition-duration", "11.5");
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const root = document.createElement("div");
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root.className = "clip";
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root.setAttribute("data-root", "true");
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root.setAttribute("data-start", "0");
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document.body.appendChild(root);
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window.__timelines = { main: createMockTimeline(20) };
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initSandboxRuntimeModular();
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expect(window.__player?.getDuration()).toBe(11.5);
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document.documentElement.removeAttribute("data-composition-duration");
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});
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it("uses the shorter authored host window when the child timeline is longer", () => {
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const root = document.createElement("div");
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root.setAttribute("data-composition-id", "main");

‎packages/core/src/runtime/init.ts‎

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@@ -857,6 +857,26 @@ export function initSandboxRuntimeModular(): void {
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return maxSeconds > MIN_VALID_TIMELINE_DURATION_SECONDS ? maxSeconds : null;
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};
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/**
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* The MACHINE-CALCULATED composition duration from
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* `<html data-composition-duration>`, used as a playable-duration CEILING.
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* Emitters that stamp this attribute (the generator, external compilers)
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* compute it from the content schedule, so a GSAP master running longer —
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* one scene component's 20s ambient background drift inside a 12s
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* composition is enough — must not stretch playback into a blank tail past
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* the calculated end. A composition HOST's `data-duration` is deliberately
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* NOT a ceiling: hand/agent-authored documents often declare a stale value
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* there, and playing the full timeline is the safer default (see the
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* authored-duration floor above, and the "keeps the timeline duration"
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* test).
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*/
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const resolveCalculatedCompositionDurationCeilingSeconds = (): number | null => {
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const declared = Number.parseFloat(
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document.documentElement.getAttribute("data-composition-duration") ?? "",
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);
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return Number.isFinite(declared) && declared > 0 ? declared : null;
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};
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const getSafeTimelineDurationSeconds = (
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timeline: RuntimeTimelineLike | null,
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fallback = 0,
@@ -881,6 +901,16 @@ export function initSandboxRuntimeModular(): void {
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} else {
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safeDuration = fallbackDuration;
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}
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// The machine-calculated document-level duration is a CEILING. The floor
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// half already lives in resolveAuthoredCompositionDurationFloorSeconds (a
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// GSAP timeline ending slightly short must not shrink the playable
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// window); symmetrically, a timeline running LONGER than the calculated
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// schedule must not extend playback past it into frames where every clip
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// has ended.
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const declaredCeiling = resolveCalculatedCompositionDurationCeilingSeconds();
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if (isUsableTimelineDuration(declaredCeiling) && safeDuration > declaredCeiling) {
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safeDuration = declaredCeiling;
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}
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return safeDuration > 0 ? Math.max(0, safeDuration) : 0;
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};
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