scheduleScrollByPixels method
Coalesced sub-cell pixel scroll (smooth wheel / trackpad / fling). Positive
deltaPx scrolls up into history. Aggregates with scheduleScrollBy into a
single per-frame flush. See scheduleScrollBy for the scheduling model.
Implementation
void scheduleScrollByPixels(double deltaPx) {
if (deltaPx == 0 || _disposed) return;
_pendingScrollPixels += deltaPx;
_ensureScrollScheduled();
}