feat(linux)!: decouple video rendering from UI thread

[trin94]

Last updated at 13:25 Saturday, 15 November 2025 Coordinated Universal Time (UTC)

Replace the direct renderer with a threaded offscreen renderer that
decouples video rendering from UI rendering to improve UI
responsiveness.

Updated Video Rendering:
- Windows: unchanged - mpvQC uses winId/HWND embedding as before
- Linux: threaded offscreen renderer (replaces direct rendering)

Technical Implementation:
The new renderer uses double-buffered framebuffers with rendering in a
dedicated background thread. The UI thread blits the latest completed
frame during its render operations:

  ┌─────┐     ┌────────┐     ┌─────────┐     ┌────────┐     ┌────────┐
  │ mpv │  →  │ render │  →  │ display │  →  │   Qt   │  →  │ screen │
  │     │     │  fbo   │     │   fbo   │     │  fbo   │     │        │
  └─────┘     └────────┘     └─────────┘     └────────┘     └────────┘
                   └─ pointer swap ─┘             │
  └────────── Background thread ──────────┘       │
                                                  │
                                    └── UI thread ┘
                                          blit

This allows the UI to remain responsive at the display's refresh rate
while video frames render independently at the content's native
framerate.

Audio Delay Compensation:
The double-buffered design introduces variable latency as frames wait
in display fbo for the next Qt render cycle. Assuming Qt processes
the render request on the next vsync cycle, frames wait on average
half a display frame period before being blitted to screen.

  ┌─────────────────────────────────────────────────────────────────┐
  │ Consider a 60Hz display (16.67ms frame time):                   │
  │                                                                 │
  │   Display vsync:  |                                 |           │
  │   Time (ms):      0                                16.67        │
  │                                                                 │
  │   Frame arrives at t=5ms:                                       │
  │                   |         F                       |           │
  │                             ←------11.67ms wait-----→           │
  │                                                                 │
  │   Frame arrives at t=12ms:                                      │
  │                   |                       F         |           │
  │                                           ←-4.67ms--→           │
  │                                                                 │
  │   Average wait = 16.67ms / 2 ≈ 8.33ms                           │
  └─────────────────────────────────────────────────────────────────┘

To compensate, mpvQC automatically adjusts mpv's audio delay to half
the current monitor's frame time: (1 / refresh_rate) / 2. This value
updates dynamically when the window moves to a different monitor or
when the refresh rate changes.

  60Hz:  8.33ms
  120Hz: 4.17ms
  144Hz: 3.47ms
  240Hz: 2.08ms
  360Hz: 1.39ms

BREAKING CHANGE: Linux users must update their mpv.conf via Options ->
Edit mpv.conf by either restoring default values or removing the
video-timing-offset option. The new renderer handles timing
automatically through audio delay compensation.