dragStartBehavior property Null safety
Configure the behavior of offsets passed to onStart.
If set to DragStartBehavior.start, the onStart callback will be called with the position of the pointer at the time this gesture recognizer won the arena. If DragStartBehavior.down, onStart will be called with the position of the first detected down event for the pointer. When there are no other gestures competing with this gesture in the arena, there's no difference in behavior between the two settings.
For more information about the gesture arena: https://flutter.cn/docs/development/ui/advanced/gestures#gesture-disambiguation
By default, the drag start behavior is DragStartBehavior.start.
A HorizontalDragGestureRecognizer and a VerticalDragGestureRecognizer compete with each other. A finger presses down on the screen with offset (500.0, 500.0), and then moves to position (510.0, 500.0) before the HorizontalDragGestureRecognizer wins the arena. With dragStartBehavior set to DragStartBehavior.down, the onStart callback will be called with position (500.0, 500.0). If it is instead set to DragStartBehavior.start, onStart will be called with position (510.0, 500.0).