[Bug] Playback jumps to next track if user seeks back within a window of the current track's ending

[PythonTryHard]

Affected version: 0.96.3 #b445db57 on Windows 10 2004 (build 19041.685, x64), not yet tested on Linux

Step to reproduce:

1. Ensure at least two (2) tracks in library/play queue.
2. Play the first track normally.
3. When first track has approximately 3-4 seconds left, seek back (both by mouse and keyboard can trigger the bug).

Expected behaviour: Playback seeks back on current track. Actual behaviour: Playback jumps to next track.

Note: Bug can triggered on both gapless and crossfade playback mode.

[clangen]

Confirmed. I believe this is because the playback engine has moved past the end of the file, and has already closed it, and is in the progress of draining its internal buffers. This may take considerable effort to fix.