When shutting down or rebooting while atrium is running, the desktop environment shows a blocking confirmation dialog: "Other users are logged in: atriumdm." The cause is that atrium registers every logind session -- including the greeter session running as the atriumdm system account -- with session class "user". Any desktop or tool that queries logind session classes (GNOME, KDE Plasma, polkit, loginctl) treats "user" sessions as real user logins and refuses to shut down silently.

The greeter session should use class "greeter", which tells logind (and anything that queries it) that this is a transient display-manager session that should be ignored during shutdown decisions. GDM uses "greeter" for its greeter sessions and "user" only for the compositor sessions it hands to the user.