scheduleScrollBy method
Coalesced scroll. Multiple calls within one scheduling tick aggregate into a single FFI scrollLines + refreshView. Use this for input-driven scroll paths (wheel, trackpad pan, touch fling) where the per-event snapshot rate would otherwise saturate the UI thread.
Programmatic callers that need the future (PageUp/PageDown, ScrollTo*) should keep using scrollLines / scrollToBottom which await the underlying FFI.
Implementation
void scheduleScrollBy(int delta) {
if (delta == 0 || _disposed) return;
_pendingScrollDelta += delta;
_ensureScrollScheduled();
}