How a stale audio batch caused 7 seconds of A/V desync after backward seeks

I have been building FastPlay, a minimal native Windows video player written in Rust.

FastPlay repo: github.com/CalvinSturm/FastPlay

FastPlay uses FFmpeg for demuxing and decoding, D3D11/DXGI for video presentation, and WASAPI for audio output. The product goal is intentionally narrow: fast local playback on Windows, smooth seeking and scrubbing, recent files, resume playback, and a UI that gets out of the way.

During the v0.3.0 release cycle, one of the most important bugs I fixed was not a UI issue. It was a playback correctness issue.

After certain backward seeks, audio could jump several seconds ahead of video.