It tests that if we go from (x is the pointer cursor)
+--------+
| |
| X |
+--------+
to
+----------------+
| |
| |
+--------+ |
| | |
| X | |
+--------+----------------+
i.e. making sure that X ends up somewhere within the logical monitor
region.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2237>