Comment # 11 on bug 1133876 from Maurizio Galli

(In reply to Takashi Iwai from comment #10)
> Right, basically it's a job of session manager.  The xfce4 session manages
> seats via systemd logind, and the processes should have been terminated
> along with it.
> So something wrong with seat management in xfce4-session side, I suppose.
> 
> The systemd KillUserProcess=yes is no solution.  If XFCE relies on it, it's
> a clear design mistake.

We could make a feature request. Does GNOME kill the processes at logout?