Recreate WSLg sockets after /tmp and $XDG_RUNTIME_DIR mounted as tmpfs.

sudo cp wslg-tmp-x11.service /usr/lib/systemd/system/
sudo cp wslg-runtime-dir.service /usr/lib/systemd/user/

sudo systemctl --global disable pulseaudio.socket

sudo systemctl enable wslg-tmp-x11
sudo systemctl --global enable wslg-runtime-dir

Systemd will mount /tmp and $XDG_RUNTIME_DIR(/run/user/1000) as tmpfs, make /tmp/.X11-unix/X0 and $XDG_RUNTIME_DIR/wayland-0 missing.
Debian and Ubuntu always mask tmp.mount by put it in /usr/share/systemd/tmp.mount.
Ubuntu-22.04 won't mount /run/user/1000 due to a bug in /usr/lib/systemd/systemd-user-runtime-dir.
Ubuntu-24.04 is using a new version of systemd which fix the bug, so it will mount /run/user/1000, and make $XDG_RUNTIME_DIR/wayland-0 missing.
See https://viruscamp.github.io/posts/2024/04/what-does-wslg-do-3/ for more details.

Simply, we must recreate links to make sure that:

/tmp/.X11-unix/X0          -> /mnt/wslg/.X11-unix/X0
$XDG_RUNTIME_DIR/wayland-0 -> /mnt/wslg/runtime-dir/wayland-0

For /tmp/.X11-unix, we can:

1. link the directory ln -s /mnt/wslg/tmp/.X11-unix /tmp/.X11-unix
Xephyr :1 cannot create /tmp/.X11-unix/X1 but @/tmp/.X11-unix/X1 as abstract namespace socket.
2. link the X socket only ln -s /mnt/wslg/tmp/.X11-unix/X0 /tmp/.X11-unix/X0
   If there are multiple monitors, there maybe /tmp/.X11-unix/X1 or more.
3. mount the directory readwrite mount -m -o bind,rw /mnt/wslg/.X11-unix /tmp/.X11-unix
Xephyr may overwrite /tmp/.X11-unix/X0, maybe we can chown and chgrp it.
4. mount the directory readonlymount -m -o bind,ro /mnt/wslg/.X11-unix /tmp/.X11-unix
Xephyr :1 cannot create any X socket.

$ cat /etc/tmpfiles.d/wslg.conf
# See tmpfiles.d(5) for details
# link WSLg display files after system started

# Type Path              Mode UID  GID  Age Argument
#L+     /tmp/.X11-unix    -    -    -    -   /mnt/wslg/.X11-unix
L+     /tmp/.X11-unix/X0 -    -    -    -   /mnt/wslg/.X11-unix/X0
 
$ sudo systemctl enable systemd-tmpfiles-setup.service systemd-tmpfiles-clean.timer

$ cat ~/.config/user-tmpfiles.d/wslg.conf
# Type Path              Mode UID  GID  Age Argument
L+     %t/wayland-0      -    -    -    -   /mnt/wslg/runtime-dir/wayland-0
L+     %t/wayland-0.lock -    -    -    -   /mnt/wslg/runtime-dir/wayland-0.lock
L+     %t/pulse/native   -    -    -    -   /mnt/wslg/runtime-dir/pulse/native
L+     %t/pulse/pid      -    -    -    -   /mnt/wslg/runtime-dir/pulse/pid
L+     %t/dbus-1         -    -    -    -   /mnt/wslg/runtime-dir/dbus-1

$ systemctl --user enable systemd-tmpfiles-setup.service systemd-tmpfiles-clean.timer