Two compounding causes behind 'playhead advances but the picture stays
frozen' and 'pause freezes the app':
- The wall-anchored clock teleported the playhead past the pre-render
window during any long stall (the first render after pressing play
costs seconds while the worker pool spins up: measured +104 frames in
one 4.1s block). The window then started behind and, with stale
in-flight frames occupying the workers, never converged.
RealClock::tick now clamps the advance to 2 frames/tick and
re-anchors the dropped time (NLE drop-frames semantics).
- Window frames the playhead had already passed stayed pending/in
flight, burning worker time on frames that could never be displayed.
update_preview_window now cancels them per tick via the new
JobDispatch::cancel_preview_frame, keeping the workers on frames
around the playhead.
Includes a production-shaped regression test (real 1080p media on the
timeline, actual cpu_frame display path) that failed with the exact
production signature (playhead 240 / displayed 0 / 36 stale slots)
before the fix and passes after.
Mike-Solar
46e43b51d9