Does two main changes:
* Handle regions in the order as visible on screen. Practically this
just means handling overlapping regions before non-overlapping ones.
* Don't handle any other regions after having found one containing the
mouse pointer.
Fixes: T94016, T91538, T91579, T71899 (and a whole bunch of duplicates)
Addresses: T92364
Differential Revision: https://developer.blender.org/D13539
Reviewed by: Campbell Barton
87c13ac68c