[Bug]: Linux preview 'aucune frame reçue' when seeking at/past the last frame of a recording

[Beetix]

Describe the bug

On Linux, the editor preview goes blank with "preview unavailable on this machine — decode_at(frame_idx=…) : aucune frame reçue" when playback or scrubbing needs a frame at or past the last frame of a recording.

The cause is decoder-side, in the Linux software decoder's seek path (crates/compositor/src/linux_decode.rs, SwDecoder::decode_at, reached from the preview player via live.rs → pipeline_linux::Decoder::seek_to). For a target timestamp at/after the last frame, av_seek_frame(…, AVSEEK_FLAG_BACKWARD) succeeds but lands on a single undecodable packet just before EOF; after avcodec_flush_buffers that reference-less packet yields no frame even when drained, so decode_at bails with "aucune frame reçue" instead of returning the last available frame.

It shows up most on frame-dropped recordings whose real avg_frame_rate is below the nominal 60 fps: a frame index computed on the 60 fps grid then points past the file's true frame count. Measured on a real recording: 631 frames / 11.2 s (≈56.34 fps) — sweeping decode_at over indices decodes 0..=630 fine and fails for every index ≥ 631. Those sub-60fps files are themselves produced by the capture-side bug #511.

Expected behavior

Seeking to or past the end of a clip returns the last available frame and the preview never errors — a requested frame index beyond the last frame should clamp to the last frame.

To Reproduce

1. On Linux, open a project containing a recording whose real average frame rate is below 60 fps (e.g. one produced under load per [Bug]: Linux screen capture time-compresses under frame drops, desyncing webcam/audio/cursor #511 — verify with ffprobe -select_streams v:0 -show_entries stream=avg_frame_rate,nb_frames,duration <file>.mp4).
2. Play or scrub the timeline through to the last clip backed by that recording.
3. The preview shows "preview unavailable on this machine — decode_at(frame_idx=…) : aucune frame reçue".

Decoder-level repro (no UI): open the file with SwDecoder and call decode_at(idx) for idx in 0..=nb_frames + 30; every index >= nb_frames fails.

OS

Linux

OS Version

Arch Linux (kernel 7.1.9)

Additional context

• Editor-side robustness bug. The malformed sub-60fps files that trigger it come from the Linux PipeWire capture frame-drop bug [Bug]: Linux screen capture time-compresses under frame drops, desyncing webcam/audio/cursor #511 — this issue is the decoder failing to tolerate them, and is fixable/mergeable independently.
• Affected component: crates/compositor/src/linux_decode.rs (SwDecoder::decode_at).
• Fix approach (in progress): when the backward-seek decode yields no frame, rewind to 0 and forward-scan, returning the last frame ≤ target (the same linear fallback already used for unindexed WebM). The accompanying PR also folds in two related latent decode_at seek-path hardening changes — draining the decoder at EOF (needed for B-frame streams; inert on these H.264-baseline has_b_frames=0 captures) and fixing an AVERROR_INVALIDDATA guard that compared against the wrong constant (-0x2A2A2A2A).
• App: Openscreen (Electron desktop), Linux/PipeWire capture path.