scrollByPixels method
Fire-and-forget sub-cell pixel scroll for smooth wheel / trackpad / fling
input. Positive deltaPx scrolls up into history. Coalesces per frame like
scrollBy; the engine tracks the fractional offset and exposes it on the
grid as scrollFraction for the painter.
Implementation
void scrollByPixels(double deltaPx) => _client?.scheduleScrollByPixels(deltaPx);