fix: calculate EXT-X-START TIME-OFFSET from actual playlist end for live streams #1599
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Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
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gkatsev
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Jan 15, 2026
src/playlist-controller.js
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| const liveEdgeDelay = Vhs.Playlist.liveEdgeDelay(main, media); | ||
| const actualPlaylistEnd = seekableEnd + liveEdgeDelay; | ||
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| startPoint = Math.max(actualPlaylistEnd + offset, seekable.start(0)); |
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question(non-blocking): While it is only a "SHOULD NOT" in the spec, should we clamp to seekableEnd here? To disallow a negative offset that would end up within the live edge delay?
…ive streams For live streams with negative TIME-OFFSET in EXT-X-START, the offset was incorrectly calculated from seekableEnd, which already has liveEdgeDelay subtracted. This caused playback to start further behind the live edge than intended. Per HLS spec, negative TIME-OFFSET should be relative to the end of the last Media Segment. This fix adds liveEdgeDelay back to seekableEnd before applying the offset, making VHS behavior consistent with native HLS implementations (e.g., Safari). Example: With liveEdgeDelay=18 and TIME-OFFSET=-18: - Before: startPoint = seekableEnd - 18 (36s behind actual live edge) - After: startPoint = seekableEnd (18s behind, matching Safari)
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Summary
Fixes incorrect start position calculation for live streams with
EXT-X-STARTcontaining a negativeTIME-OFFSET.The offset was being calculated from
seekableEnd, which already hasliveEdgeDelaysubtracted. This caused playback to start further behind the live edge than intended, making VHS behavior inconsistent with native HLS implementations (e.g., Safari).Example with
liveEdgeDelay=18andTIME-OFFSET=-18:startPoint = seekableEnd - 18(36s behind actual live edge)startPoint = seekableEnd(18s behind, matching Safari)Changes
liveEdgeDelayback toseekableEndbefore applying negativeTIME-OFFSET, per HLS spec which defines negative offsets as relative to the end of the last Media SegmentseekableEndto prevent small negative offsets (e.g.,-5) from seeking into the unsafe live edge buffer zoneTIME-OFFSETTest plan
liveEdgeDelayreturns 0)