Bug#1031671: Re-updating gnome-shell packages solved the issue for me

[Rainer Schwarzbach]

The mutter packages had already been at version 43.3-3.

Upgrading all gnome-shell packages to 43.3-1 and rebooting seems to have 
solved the issue, no more focus stealing occurs anymore.

So it seems to have been the upstream bug indeed, maybe resolved (in my 
case) by upgrading gnome-shell _after_ having the fixed mutter version 
installed.

Probably now-irrelevant additional information: the focus-stealing 
applications were all installed from distribution packages; no 
snap/flatpak/etc involved.

Thank you!