I know that the xauth information of a logged in user is available via
"/run/user/UID/xauth_some-string"

Usecase: log into remote laptop via SSH and port-forwarding, start x11vnc on
that remote machine, run vncviewer on local machine --> shared remote
assistance for far-away relatives.

This used to work as long as .Xauthority was around. Now that it is missing,
x11vnc fails to start and it is very inconvenient to get access to it via
"/run/...." (especially knowing the string before having logged in). I just use
simple scripts!

Unfortunately running "xauth extract - $DISPLAY" in an SSH session will not
give me the correct cookie either. 

Current workaround: run a startup-script (KDE) that fetches the xauth data and
stores it in $HOME/.Xauthority

(xauth extract - $DISPLAY > ~/.Xauthority)

It would be nice to have a working .Xauthority out of the box (again).