Comment # 3 on bug 908167 from Dr. Werner Fink

IMHO it would help a lot if in the script xdg-su would be set the
XDG_RUNTIME_DIR to something more correct:


        if [ -z "$user" ] ; then
             XDG_RUNTIME_DIR=/run/user/0
        else
             XDG_RUNTIME_DIR=/run/user/$(id -u "$user") || exit 1
        fi
        export XDG_RUNTIME_DIR

short before the final

 case "$DE" in
   [...]
 esac

this may work with all the su like commands there out (generic, kde, gnome,
xfce, mate, lxde)