defer constant Null safety
A special class that indicates that the region with this cursor defers the choice of cursor to the next region behind it.
When an event occurs, MouseTracker will update each pointer's cursor by finding the list of regions that contain the pointer's location, from front to back in hit-test order. The pointer's cursor will be the first cursor in the list that is not a MouseCursor.defer.
static const MouseCursor defer = _DeferringMouseCursor._();