Comment # 3 on bug 1212932 from Fabian Vogt

(In reply to Andrei Borzenkov from comment #2)
> (In reply to Fabian Vogt from comment #1)
> > 
> > Is there any particular issue with having the file in /tmp instead?
> 
> Not for me (although $XDG_TUNTIME_DIR /feels/ more clean

Agreed.

). But there are
> users who  try to start graphical programs from outside of GUI sessions; in
> this case it complicates things. I guess you better ask OP in this thread
> 
> https://lists.opensuse.org/archives/list/support@lists.opensuse.org/thread/
> PDA4D72XJKAHTOUHWT7QMPMNPUVXADWN/

There's an easy way to run it in the proper environment: systemd-run --user.

e.g. systemd-run --user -P x11vnc.

This will pick up $DISPLAY, $XAUTHORITY and even $WAYLAND_DISPLAY.