fix(preview): restart the playback clock at the requested time on a held frame

[EtienneLescot]

Summary

On Linux, a seek into a still stretch of a held-frame take no longer sends the preview into a re-anchor storm. The seek itself is unchanged. Found in the Linux pass of v2.1.0-rc.3 (#1290).

Cause

• In live.rs, the playback clock is the screen frame's pts plus the accumulator acc.
• pipeline_linux's seek lands on the frame held at the target, which can be seconds earlier, and acc restarted at 0 after a seek. The view's clock then ran seconds behind the playhead.
• nativeSync's drift watch re-anchored the view every 0.5 s, and each re-anchor landed in the same place and paid a screen and camera seek. This lasted until the playhead left the still stretch, or until the end of the clip when the screen's last frame comes before it.

Fix: the clock part of #1291, without its seek (seek_held, decoder changes)

• anchor_acc(t, pts) = max(0, t - pts) after a seek, an app re-anchor and an automatic clip switch. advance_to_next_scene_clip returns the start it switched to.
• present_frame, and step on a frame just positioned, compose at the playhead (max(target, pts)) instead of the held frame's pts.
• A paused recompose (settle pass, param, scene or clip change) now redraws at the time the frame was last composed at, not at its pts. Otherwise a paused seek onto a held frame, or a pause during one, sent the cursor back to the pts on the next settle pass. Repositioning without composing (seek_active, a clip swap) resets that time, and an app re-anchor sets it to the playhead (anchor_at). It is one Cell read per paused recompose, with no cost per frame.
• Windows and macOS keep main's seek and clock: their seek lands at or after the target, so the anchor is 0 there. Etienne chose to keep main's seek on 16/10, for its cost. The paused-recompose fix applies everywhere: pausing during a held frame and changing a setting no longer moves the cursor back.

Related issue

Refs #1290, #1291 (closed).

Type of change

• Bug fix

Release impact

• Patch

Desktop impact

• Linux
• Windows
• macOS

Testing

Unit and GPU tests

• New the_clock_restarts_at_the_requested_time_on_a_held_frame. tests/held_frame_clock.rs also checks that a seek reports the playhead, and passes on the real D3D11 decoder with fix(preview): land a seek on the frame held at the playhead, and restart the clock there #1291's held-frame take.
• tests/held_frame_clock.rs's "recompose stays at the pts" assertion is reversed on purpose.
  • It froze the old behaviour, which is the paused-recompose bug above: after a hold, recompose must keep the clock.
  • It now fails on the old recompose (programme time 1.517 instead of 1.767 s) and passes with the fix.
  • A re-anchor check is added: seek_active + anchor_at, then recompose, lands at the anchored playhead.
  • held_frame_clock and no_camera_stand_in pass on the D3D11 decoder.
• cargo test -p openscreen-compositor --lib on Windows: 455 pass. One test fails, and it fails on main too: playback_off_the_end_of_the_programme_stays_on_the_last_clip, GPU-gated, already noted in fix(preview): land a seek on the frame held at the playhead, and restart the clock there #1291.

Linux, Vivobook (Ryzen 5 7520U, Radeon 610M, GNOME Wayland, 60 Hz)

• The rc.4 AppImage, with only compositor_view.node swapped between main and this branch.
• 6 interleaved pairs per case; the seek is a click on the ruler, then play.
• Synthetic takes built like a PipeWire take: no B-frames, a 30-frame GOP, fragmented MP4, with a WebM webcam like a Linux one.

main → fix	Re-anchors	Time re-seeking	Updates/s	Gaps > 100 ms	Longest gap	Seek
Seek past the screen's last frame, webcam, heavy project	18 → 1	1662 → 114 ms	25.6 → 29.8	6.5 → 0	128 → 49 ms	100 → 99 ms
Seek into a still stretch, webcam, heavy project	9 → 0	886 → 0 ms	28.3 → 29.8	4 → 0	143 → 54 ms	75 → 75 ms
Seek into a still stretch, light project	9 → 0	496 → 0 ms	same	0 → 0	83 → 51 ms	53 → 55 ms
Clip switch onto a held frame (trim)	8 → 1	423 → 66 ms	same	1 → 1	same	38 → 38 ms
Constant-rate take, same seek	0 → 0	0	same	0 → 0	same	54 → 55 ms

• Every difference listed is z = ±2.88, and the seek lands on the same frame in every case.
• The one re-anchor left happens in both builds: at play start in the first case, at the cut in the trim case.
• The pass's 1–2 updates/s did not reproduce as such on synthetic media. The mechanism is the same, and it is gone.

Windows (279 Hz, RTX 4070 Ti), perf-ab bench (scripts/perf-ab)

• fix(preview): land a seek on the frame held at the playhead, and restart the clock there #1291's bundles and takes, with only the addon swapped (main / this branch, both built here).
• 6 pairs per seek case, 3 light and 24 heavy playback pairs.

main → fix	Seek latency	Lands on	Re-anchors	Updates/s on screen	CPU	GPU
Held take, seek to 7.5 s	20.9 → 16.4 ms (z -0.82)	8.02 → 8.02 s	2 → 2	59.1 → 59.0	99.5 → 100.3 %	10.1 → 9.5 %
Held take, backward 12 → 7.5 s	16.8 → 12.1 ms (z -1.83)	8.02 → 8.02 s	2 → 2	59.3 → 59.5	96.5 → 98.4 %	8.3 → 7.5 %
Constant-rate take, seek to 7.5 s	245 → 182 ms (z -0.91)	7.50 → 7.50 s	0 → 0	59.9 → 59.8	96.8 → 103.0 % (z 1.76)	10.8 → 11.2 %

Heavy playback from 0, 24 pairs: no difference in behaviour (0 re-anchors and one setActiveClip at play start in every session), and nothing over the bench's 5 % bar.

Heavy, 24 pairs	main	fix
Paints per second	60.0	60.0	same
Paint interval p99	36.6 ms	36.6 ms	same
Paints later than 25 ms, per 30 s	124	124	same
CPU, all processes	128.3 %	130.0 %	+1.4 %, under 5 % (z 2.31)
CPU, main process	19.6 %	20.8 %	within noise (z 1.88)
GPU, all engines	48.9 %	49.2 %	+0.4 %, under 5 %

The main-process CPU first came out at +12.7 % on 12 pairs (z 2.48). It did not replicate on 12 more pairs (20.2 → 20.1 %, z 0.06). The change adds no work per frame: the playback loop only differs after a seek, a re-anchor or a clip switch.

• Light playback: every metric the same (paints 59.9 → 60.0/s, paint p99 27.8 → 27.8 ms, cursor 6.00 → 5.91 px RMS, CPU 123.5 → 121.5 %, GPU 21.2 → 21.1 %).

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Playback now preserves the requested position when seeking or switching clips, even if the displayed frame is from an earlier time.
  • Automatic clip transitions keep the playback clock aligned with the new clip’s start time, including when playback recovers from a decoder rewind.
  • Stepping and recomposing while paused now keep the published playback position aligned with the clock and the most recently composed programme time.

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📝 Walkthrough

Playback timing now preserves the requested playhead when a seek or clip switch positions a frame with an earlier PTS. Recomposition uses the last composed time, and automatic clip advancement returns a source-time anchor for the playback accumulator.

Changes: Playback clock anchoring

Layer / File(s)	Summary
Anchor seek and explicit positioning crates/compositor/src/live.rs, crates/compositor/tests/held_frame_clock.rs	Seek and active-clip positioning anchor the accumulator to the requested time relative to the positioned frame’s PTS. Composition and recomposition preserve the later of the requested time and frame PTS. Tests check the anchored clock and composed position.
Anchor automatic clip advancement crates/compositor/src/live.rs	Clip advancement returns the next clip’s source start time on success, or None otherwise. Boundary and EOF-triggered switches use that time to anchor the accumulator.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~15 minutes

Change: Bug fix

Suggested reviewers: marcgabe15

Merge Risk: ⚪ Minimal · up to 0a431

This change restarts the playback clock at the requested time after seeking into a held frame, which reduces repeated re-anchors on Linux. No merge-blocking risk is evident from the supplied context.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 78.57% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 14 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Title check		The title clearly and concisely describes the primary change: restarting playback timing at the requested position for held frames.
Description check		The description is complete and follows the repository template. It includes the change summary, related issues, change type, release and platform impact, and detailed testing results.

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Inline comments:
Review comments at @crates/compositor/src/live.rs:
- Line 732: Update the paused seek and explicit clip-change recomposition paths
in Player::recompose so they preserve the positioned playhead time instead of
resetting it to the held frame’s PTS; retain the time established by
present_frame or the clip-change acc anchor through settle passes.

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