- The playback backup timer was using timebase_dbl() in seconds as its
QTimer interval, which truncated to 0 ms for normal frame rates. This
caused the timer to fire continuously and starve the event loop when
the video display was hidden, e.g. in Show Waveform Only mode, freezing
the UI and stopping the playhead. Multiply by 1000 to use milliseconds.
- If PlayInternal has neither video nor audio to prequeue, call
FinishPlayPreprocess immediately so the backup timer starts and the
playhead advances even in waveform-only mode with no audio.