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
@override
void handleTapDown(TapDownDetails details) {
  _lastTapDownPosition = details.globalPosition;
}