Scan-order compensation only ran at one frame per refresh, so a crisp scroll
like 60 px/s on a 120 Hz panel (1px every 2 refreshes) showed a half-pixel
step across the middle of the panel. A held frame is now presented as a
sequence of swaps (scan_order.refresh_plan): the lagging half shows the
previous frame for its first refresh and the new one for the rest, so it
steps one refresh after the rest. Skipped when a blit takes over half a
refresh, since the second blit has to land before the next vsync.
Soaked on ledpi (60 px/s, 120 Hz): 0.16% late frames, as before the change.
Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>
A 1:N-scan HUB75 panel lights the two rows either side of its middle at opposite ends of each refresh, so a scroll at one pixel per refresh shows a 1px step across the middle of every panel. While something scrolls at one frame per refresh, DisplayManager now shows the half whose seam row lights first one refresh behind the other (src/scan_order.py), which lines the two up again. Only for layouts whose row order is known; display.scan_order_compensation "off" disables it. Confirmed on hdpi before and after.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
