greetd service specifies an alias (display-manager.service). That alias
should instead be managed via the alternatives system that other
display manager packages are using to make it possible to switch
between installed display managers.

Quack,

That would be nice indeed.

I looked at lightdm and gdm3 for example and I don't get how that's
supposed to work. lightdm brings a service file with the same Alias.
Both manage the /etc/systemd/system/display-manager.service symlink in
their postinst, similarly, but lightdm does not handle all possible
errors. None manages it using the alternative system
(debian/gdm3.alternatives is about PAM options).

So I think before doing any change we need to have all these packages
agree on some configuration and snippet to do that properly, and
documented.

Regards.
\_o<

I looked at /etc/alternatives/x-window-manager which points in my case to
/usr/bin/kwin_x11 and looked at the source on Salsa:
https://salsa.debian.org/qt-kde-team/kde/kwin/-/tree/master/debian has f.e.
kwin-x11.postinst which could provide some clues.

If you contact the the Debian Qt/KDE Team, they could probably help further.
I think that in any case contacting the relevant teams would be needed and/or
useful.