On Wayland, if a client issues a inhibit-shortcut request, the Wayland
compositor will disable its own shortcuts.
We should also disable the default handler for the button grab modifier
so that button events with the window grab modifiers pressed are not
caught by the compositor but are forwarded to the client surface.
That also fixes the same issue with Xwayland applications issuing grabs,
as those end up being emulated like shortcut inhibition.
Closes: https://gitlab.gnome.org/GNOME/mutter/issues/642
040de396b2