Focus follows mouse is meant to avoid focusing windows that happened
to pop up under the pointer, e.g. due to mapping, workspace changes,
etc... On X11, this has been done since ancient times through a
moderately complex synchronization mechanism, so mutter would know
to ignore crossing events caused on those situations.
This mechanism is much prior to XInput 2 though, where we may know
this in a more straightforward way: If the sourceid of the crossing
event is a logical pointer (i.e. equals deviceid), the crossing event
was triggered logically, and not through user input.
Perform this simpler check, and drop the existing mechanism to
ignore logically induced crossing events.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3267>
8b0da940cf