handleTapDown method
If ignorePointer is false (the default) then this method is called by the internal gesture recognizer's TapGestureRecognizer.onTapDown callback.
When ignorePointer is true, an ancestor widget must respond to tap down events by calling this method.
Implementation
void handleTapDown(TapDownDetails details) {
_lastTapDownPosition = details.globalPosition + -_paintOffset;
}