Comment # 22 on bug 1089287 from Michal Srb

AFAIK the systemd-logind integration in X server is only meaningful when the X
server runs with the regular user's permissions inside the user's session. This
is currently only the case with GDM.

SDDM starts the X server as root. In this case you can consider the X server as
a system process. It is not tied to the user session that it displays - the
same X server may display the greeter and later the user session as well.

That's why X shows the error:

> [   840.157] (EE) systemd-logind: failed to get session: PID 2837 does not belong to any known session

It tries to find session that belongs to its own PID and that fails.

If -keeptty is not set, X server doesn't even try to use the systemd-logind
integration. If it is set, it tries and fails.

What is actually the motivation for adding the -keeptty parameter?