Bug#1031671: resolved for me by mutter packages upgrade as suggested

[Rainer Schwarzbach]

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

As it seems, the mutter packages on my machine had been upgraded during
my after-login-upgrade routine, and I had not logged out and in again
yet. I guess that would have helped already.

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

Thank you – again!