Dirty tracking skipped SwapOnVSync for byte-identical frames. That is the
right call for static content, but SwapOnVSync is also what paces the render
loop, so skipping it skips the wait for the panel: a duplicate frame returns
in ~8ms instead of ~10ms on a 100Hz panel, advances the strip only 0.8px
instead of 1.0px, and so makes the next frame more likely to repeat as well.
The effect sustains itself once it starts.
Measured over 20 minutes on a 2x128x64 chain, both scrollers configured
identically at 100 px/s:
leaderboard 10ms x35, 11ms x3 (clean)
odds-ticker 10ms x26, 8ms x7, 15ms x5 (~20% duplicates mid-scroll)
The duplicates were not end-of-cycle idling -- 38% of fast frames fell within
90s of a scroll completion against 35% of normal frames, a null result. The
trigger is per-frame work: odds does more of it, and more variably, so it is
first to land a frame that advances less than a whole pixel.
Pushing an identical frame costs one canvas copy. Falling out of vsync lock
costs smooth motion. Static content is untouched, because
is_currently_scrolling() expires on its own inactivity threshold -- covered
by test_stale_scrolling_state_stops_forcing_pushes so a plugin that stops
scrolling without saying so cannot pin the panel into always-push.
Also de-flakes test_snapshot_still_written_on_skip, which asserted a strict
mtime increase between two writes that can land in the same filesystem tick;
it failed about two runs in three on Windows regardless of the code under
test. The file is now backdated before the check.
156 tests pass on the Pi. Not yet confirmed by eye on the panel.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
