The above comment (https://bugs.launchpad.net/ubuntu/+source/mutter/+bug/2016308/comments/6) pertained to running Wayland.

I switched to Xorg and the behaviour was slightly different.

Under Xorg the libmutter assert failed leading to gnome-shell "Bail out!' same as before, except that instead of killing my X session and returning to login prompt the display flickered (apparent Xorg restart) and my session continued - this time with no programs closed, and the OWASP ZAP application continuing.

From this I conclude that the error behaviour in libmutter/gnome-shell is the same, but problem recovery under Xorg is better than under Wayland.

Log follows:

May 09 21:09:59 gnome-shell[339768]: Window manager warning: Overwriting existing binding of keysym 31 with keysym 31 (keycode a).
May 09 21:09:59 gnome-shell[339768]: Window manager warning: Overwriting existing binding of keysym 32 with keysym 32 (keycode b).
May 09 21:09:59 gnome-shell[339768]: Window manager warning: Overwriting existing binding of keysym 34 with keysym 34 (keycode d).
May 09 21:09:59 gnome-shell[339768]: Window manager warning: Overwriting existing binding of keysym 35 with keysym 35 (keycode e).
May 09 21:09:59 gnome-shell[339768]: Window manager warning: Overwriting existing binding of keysym 36 with keysym 36 (keycode f).
May 09 21:09:59 gnome-shell[339768]: Window manager warning: Overwriting existing binding of keysym 37 with keysym 37 (keycode 10).
May 09 21:09:59 gnome-shell[339768]: Window manager warning: Overwriting existing binding of keysym 39 with keysym 39 (keycode 12).
May 09 21:09:59 gnome-shell[339768]: Window manager warning: Overwriting existing binding of keysym 33 with keysym 33 (keycode c).
May 09 21:09:59 gnome-shell[339768]: Window manager warning: Overwriting existing binding of keysym 38 with keysym 38 (keycode 11).
May 09 21:10:09 gnome-shell[339768]: **
May 09 21:10:09 gnome-shell[339768]: libmutter:ERROR:../src/core/window.c:1533:meta_window_unmanage: assertion failed: (window->display->focus_window != window)
May 09 21:10:09 gnome-shell[339768]: Bail out! libmutter:ERROR:../src/core/window.c:1533:meta_window_unmanage: assertion failed: (window->display->focus_window != window)
May 09 21:10:09 gnome-shell[339768]: GNOME Shell crashed with signal 6
May 09 21:10:09 gnome-shell[339768]: == Stack trace for context 0x55cc24c2cf70 ==
May 09 21:10:10 snapd-desktop-i[340464]: atk-bridge: get_device_events_reply: unknown signature
May 09 21:10:10 systemd[339459]: <email address hidden>: Main process exited, code=dumped, status=6/ABRT
May 09 21:10:10 polkitd(authority=local)[8258]: Unregistered Authentication Agent for unix-session:15 (system bus name :1.318, object path /org/freedesktop/PolicyKit1/AuthenticationAgen>
May 09 21:10:10 systemd[339459]: <email address hidden>: Failed with result 'core-dump'.
May 09 21:10:10 systemd[339459]: <email address hidden>: Consumed 2.610s CPU time.
May 09 21:10:10 systemd[339459]: <email address hidden>: Scheduled restart job, restart counter is at 1.
May 09 21:10:10 systemd[339459]: Stopped <email address hidden> - GNOME Shell on X11.
May 09 21:10:10 systemd[339459]: <email address hidden>: Consumed 2.610s CPU time.
May 09 21:10:10 snap-store[340144]: atk-bridge: get_device_events_reply: unknown signature
May 09 21:10:10 systemd[339459]: Starting <email address hidden> - GNOME Shell on X11...
May 09 21:10:10 gnome-shell[341653]: Running GNOME Shell (using mutter 44.0) as a X11 window and compositing manager