fix(subtitle): remove the DASH presentationTimeOffset from stpp and wvtt cues

STPP and WVTT cues sit on the media timeline, so they played late by the offset after the mux.
2026-11-14 13:05:09 +00:00 · 2026-10-30 23:22:47 -06:00
parent 97711e799c
commit 0c45195fe2
4 changed files with 126 additions and 13 deletions
@@ -35,7 +35,9 @@ shown in track listings. The **Extension** column is the file extension used on
    `fTTML` (STPP) and `fVTT` (WVTT) are the fragmented, MP4-box-encapsulated forms that
    DASH and HLS deliver. You never keep these as-is: right after download they are
    unwrapped into their plain counterparts (`fTTML` becomes TTML and `fVTT` becomes
-    WebVTT), so you always end up with an editable text subtitle. Note that this is a
+    WebVTT), so you always end up with an editable text subtitle. For a DASH track, the
+    unwrap also subtracts the manifest's `presentationTimeOffset` from the cue times, so the
+    subtitles stay in sync with the video. Note that this is a
    one-way, source-only relationship: you can convert *from* these boxed formats, but no
    backend can write them as an output target, so they can never be a conversion
    destination.