Usually gnomesu is used for XFCE. And, indeed, if you run "su -c env | grep XDG_RUNTIME_DIR", it shows "/run/user/0". Interestingly, it's set so no matter whether the directory actually is present or not. So, I guess we can align the behavior of xdg-su with su. Or, maybe fix gnomesu (and kdesu if needed, too) instead...