During grabs, it is expected that the X11 focus does not correspond
to the display's focus window, as focus should be on the stage's
XWindow instead.
This still messes up the keyboard focus even after we stopped moving
the X11 focus, because we end up with a presumed X11 focus window
of None, and as a result the stage is considered unfocused.
https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5932
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2776>