onPanDelta method
Implementation
void onPanDelta(double deltaX, double canvasWidth) {
if (_disposed) return;
if (!constraints.enablePan) return;
if (canvasWidth <= 0 || !canvasWidth.isFinite || !deltaX.isFinite) return;
final deltaFrac = -(deltaX / canvasWidth) * value.windowSize;
value = value.panBy(deltaFrac);
}