To emulate X11 grabs, mutter as a Wayland compositor would disable its
own keyboard shortcuts and when the X11 window is an override redirect
window (which never receives focus), it also forces keyboard focus onto
that X11 O-R window so that all keyboard events are routed to the
window, just like an X11 server would.

But that's a bit of a “all-or-nothing” approach which prevents
applications that would legitimately grab the keyboard under X11 (like
virtual machine viewers) to work by default.

Change “xwayland-allow-grabs” to control whether the keyboard focus
should be locked onto override redirect windows in case of an X11 grab.

For stringent needs, careful users can still use the blacklisting
feature (i.e. a list containing “!*”) to prevent grabs from any X11
applications to affect other Wayland native applications.

#597