The enabling of GUI snaps was handled with the following:
- this file copied to
/etc/systemd/system/user-runtime-dir@.service.d/override.conf - this file and this file copied to
/etc/systemd/system/
The first mounts /mnt/wslg/runtime-dir at /run/user/$UID to ensure that the pulseaudio and wayland sockets are accessible by snaps. The second re-enables the /tmp/.X11-unix socket when systemd mounts /tmp as a new tmpfs mount in the systemd mount namespace.