Show video frame preview when hovering/dragging the progress slider

[Him188]

What

长按拖动进度条(触摸)/ 鼠标悬浮进度条(PC)时,原来只显示 "15:23" 的浮窗扩展为一张固定尺寸的卡片,在时间上方显示该位置的视频预览帧。

Built on the new mediamp FramePreview feature (open-ani/mediamp#39) — implemented for mpv, VLC and ExoPlayer; backends without it (AVKit/iOS) gracefully fall back to the time-only popup.

Screenshots

Hovering at ~48% while playing at 02:04 — the popup shows the frame at 11:05 (mpv backend, LocalTorrent media):

Moving to ~25% updates the frame (05:24):

While the frame is loading, the popup keeps its final fixed size with a placeholder area — no size jumping:

How

• MediaProgressFramePreviewState (video-player commonMain):
  • requests aligned to a 2s grid — extraction is keyframe-snapped anyway, finer granularity buys nothing;
  • LRU cache of 8 frames (~80 KB each at 192×108 ARGB, ~650 KB total) so scrubbing back over the recent trail hits instantly;
  • 50ms debounce, collectLatest semantics (newest position wins);
  • prewarm: the preview decoder is warmed at the current playback position when media loads, so the first hover doesn't pay the ~1.5s decoder startup.
• MediaProgressSlider renders the frame in both the hover popup and the drag (thumb) popup; the frame area is a fixed 160×90dp box with a placeholder background.
• BT sources: only positions whose pieces are DONE (from cacheProgressInfoFlow) are requested, so preview seeks never steal piece-deadline priority from the playback position. Non-BT sources are unrestricted.
• Platform pixel conversion via expect/actual (androidMain Bitmap / skikoMain installPixels).

Verification

• 7 new desktop Compose tests: hover shows frame, drag requests the dragged position, uncached-position gating, no-feature fallback, pixel conversion, and a screenshot-based regression test asserting the frame stays inside the popup.
• End-to-end in the packaged desktop app on both mpv and VLC backends with cached LocalTorrent media (frames match the requested positions across both backends). Measured: first hover < 0.8s, new position < 0.6s, scrub-back (cache hit) < 0.35s.

Notes

• Depends on FramePreview: seek-bar thumbnail extraction for mpv, VLC and ExoPlayer mediamp#39 — draft until a mediamp version containing FramePreview is published and the catalog is bumped (CI will fail against 0.0.30).

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