One frame behind is the normal pipelined state when the render thread is
the slower side; skipping on it dropped nearly every present at 4K
widescreen. rt_frames_ahead() counts the complete frames queued after
the one being replayed; the backend skips only at two, where the
emulation thread is at the in-flight bound waiting on the render thread.
The unit test checks the count never exceeds the bound and that both
cases occur under a slow consumer.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Backend-neutral core for the opt-in [video] render_thread: the emulation
thread appends records to a single-producer/single-consumer ring and the
render thread replays them in order with the graphics context current.
rt_frame_end bounds closed frames in flight (backpressure), the render
thread can tell a frame is stale, and rt_acquire is the sync point:
drain, then move the context to the emulation thread until rt_release.
No lock is held while a record executes.
render_thread_test drives it with a fake executor and context token:
ordering and payload integrity across ring wraps, oversize refusal, the
in-flight bound under a slow consumer, stale detection, randomized
acquire/release interleaving and stop while held (TSan clean).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Shokunin