CuePatch field guide
Fix subtitle timing drift on a Mac
Progressive drift usually means the subtitle timeline and video timeline run at slightly different speeds. A constant offset may fix the first scene while making the final scene worse; the correct repair is a scale adjustment anchored to two known points.
Recognize drift instead of delay
Check one line near the beginning and another near the end. If the timing error grows steadily, record the subtitle time and correct video time for both anchors. Avoid using fades, music cues or ambiguous speech.
Calculate the scale
The scale is the duration between correct anchors divided by the duration between subtitle anchors. Apply that factor to every timestamp, then add the offset needed to align the first anchor.
Validate the transformed timeline
Scaling changes cue durations and gaps as well as start times. Check for overlaps, zero-length cues and timestamps that were clamped at zero. Review the middle of the video even when both anchors match.
Keep the recipe reproducible
Record the scale, offset, source filename and output filename. CuePatch places those details in a JSON manifest so the same repair can be reviewed or repeated without guessing.