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Whole-pixel motion was previously only available at multiples of the refresh rate -- 100, 200, 300 px/s on a 100Hz panel. 100 px/s crosses a 256px panel in 2.6s, which is brisk for reading, and everything slower had to blend (blur) or repeat frames unevenly (judder). There was no way to ask for 50 px/s and get clean motion. SwapOnVSync takes a framerate_fraction the display manager never passed. It holds each frame for N panel refreshes; the panel keeps refreshing at its full rate throughout, so holding costs nothing in flicker and only changes how often a NEW image is presented. That turns 50 px/s into one whole pixel every second refresh instead of half a pixel every refresh. The crisp speeds are therefore refresh_hz / hold * pixels_per_frame, and that ladder depends on the panel: a Pi Zero on a long chain has a different set of good speeds from a Pi 4 on a short one. crisp_ladder() enumerates them and solve_crisp() picks the best match for a requested speed. solve_crisp weights motion quality rather than picking the numerically nearest entry, which matters more than it sounds. Asked for 30 px/s, nearest-by-value answers 28.6 -- 2px jumps at 14fps -- over 33.3, which is single-pixel motion at 33fps and obviously better on the panel. The target is also clamped into the ladder's range first, because relative error saturates near 1.0 for a target far outside it and the quality penalty would otherwise answer "10000 px/s" with the slowest entry. configure() snaps to the ladder and applies the hold when given a display manager. Without one the hold silently cannot happen and motion falls back to fractional pixels, so it warns rather than failing quietly. set_frame_hold() resets to 1 when scrolling stops, so one plugin's pacing cannot leak into whatever is on screen next. scripts/scroll_speeds.py is the user-facing part: it prints the ladder for the configured rate, measures what the panel ACTUALLY manages (--measure, for hardware that cannot reach its configured limit), highlights the nearest option to a wanted speed, and demos one live. It never starts or stops the display service itself -- doing that inside a script stranded the panel twice today. Speeds below ~20 px/s remain stepped regardless. That is the pixel pitch, not a software limit. Also fixes the dirty-tracking test spy, which stubbed SwapOnVSync with a single-argument function and would have masked the new call as a failed push, and rewrites a configure() test that had started passing for the wrong reason: it asserted a judder warning, which snapping now prevents, and was matching the unrelated "hold could not be applied" warning instead. 183 tests pass on the Pi. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>