In the case mutter is a x11 compositor, it doesn't matter much
since the stack tracker will go away soon. In the case this is a
wayland compositor with mandatory Xwayland, it matters even less
since the session would be shutting down in those paths.
But if this a wayland compositor that can start Xwayland on demand,
this is even harmful, as the MetaStackTracker should be cleared of
x11 windows at this moment, and we actually did right before dispose
on ::x11-display-closing.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/709
d3d1ff1aae