[Sergilol88]

What this fixes

Some Matroska HEVC/H.264 recordings with reordered B-frames expose non-monotonic or missing DTS. A normal FFmpeg stream-copy remux can clamp those decode timestamps and collapse multiple frames onto the same PTS.

Reproduced on an OBS HEVC MKV:

• source: 0 duplicate PTS, 16/17 ms frame spacing
• current VFR FastCut export: 195,035 duplicate PTS, dominant 50 ms spacing
• packet-level setts repair: 0 duplicate PTS, 16/17 ms spacing

Implementation

• probe a short packet sample before video export
• leave streams with valid monotonic DTS unchanged
• for affected streams, preserve source PTS and synthesize only monotonic DTS with FFmpeg setts
• apply the repair to single-range, multi-range, audio-processing and final concat/remux passes
• bump app version to 0.5.1
• add regression coverage and release notes

Video remains stream-copy; no video re-encoding is introduced.

Manual validation still required

Before merge/release, test the PR build with the reproduced OBS source through a real multi-range export and verify packet spacing plus MPC playback.

[Sergilol88]

Final validation before merge:

• Source checks: green on Python 3.10 / 3.12 / 3.13.
• Portable Windows build/audit: green.
• Minimal FFmpeg runtime verifies required capabilities including setts.
• Portable launch smoke test: passed.
• Manual regression test on the reproduced OBS HEVC/MKV source:
  • 47,400 video packets
  • 0 duplicate PTS
  • average interval 16.668 ms
  • median / P95 / P99 = 17 ms
  • 99.998% of positive intervals are 16/17 ms
  • one 77 ms interval occurs exactly at a new keyframe on a lossless multi-range splice boundary
• Visual playback is smooth in MPC.
• Audio sync checked around edits and is correct.

Code review found no release-blocking cleanup issues. The repair remains packet-level stream copy; source PTS are preserved and only DTS are synthesized when required. Final Matroska temp-part remux paths are covered so the timestamp corruption is not reintroduced during concat.

Ready for v0.5.1 hotfix release.