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fix(export): block browser fast export when video layers overlap on Windows - #227
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…indows Several video layers that overlap in time share Remotion's OffthreadVideo compositor and frame cache. On Windows the WebCodecs decoder stalls when more than one of them requests frames: the fast export neither advances nor reports an error, so the user can only cancel it. browserTimelineBlocker() was a stub that always returned null, so nothing stopped the browser path for those timelines. It now sweeps the clip intervals of the timeline (nested sequences included) and, when a Windows browser would run two or more video sources on the same frame, returns an explicit reason so the export falls back to the local renderer instead of hanging. The platform is a parameter with a browser-derived default, so the verify script drives the Windows, other-platform and empty-timeline cases without a DOM. Verification: npx tsc -b, npm run lint, src/export/browserExport.verify.ts
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Problem
On Windows, a timeline whose video layers overlap in time hangs the browser fast export (WebCodecs): the progress stops at some percentage and never reports an error, so the only way out is to cancel it manually.
Reproduction
Cause
browserTimelineBlocker()is a stub that always returnsnull, so nothing rejects the browser path for a timeline with several video sources. Multiple OffthreadVideo layers then request frames from the WebCodecs decoder at the same time, and on Windows that decoder stalls.Change
hasOverlappingVideoSources(): sweeps the clip intervals of the timeline, nested sequences included, and reports whether two or more video sources are live on the same frame.browserTimelineBlocker()now returns an explicit reason when the browser is Windows and such an overlap exists, so the export falls back to the local renderer. The server render path is untouched.Verification
src/export/browserExport.verify.ts: single track is not blocked, overlapping tracks are blocked, nested sequence overlap is blocked, non-Windows is not blockednpx tsc -bandnpm run lintpass locallyThis only rejects the fast path on the configurations that hang; the render itself is unchanged.