How to debug multiseat? Second seat turns off after login.

[OliW]

I had been using multiseat on 22.04 with great success. Two nvidia cards and loginctrl to assign the PCI IDs of those, a USB controller, sound, etc, etc.

Then I upgraded to 23.10 and it stopped working. You get a login screen on both displays. Seat0 works as expected but the secondary seat monitor turns off when I log in, as any user.

The session seems to start though, I just can't see anything.

systemd─┬─(sd-pam)
        ├─dbus-daemon
        ├─gnome-keyring-d───4*[{gnome-keyring-d}]
        ├─goa-daemon───4*[{goa-daemon}]
        ├─goa-identity-se───3*[{goa-identity-se}]
        ├─gvfs-afc-volume───4*[{gvfs-afc-volume}]
        ├─gvfs-goa-volume───3*[{gvfs-goa-volume}]
        ├─gvfs-gphoto2-vo───3*[{gvfs-gphoto2-vo}]
        ├─gvfs-mtp-volume───3*[{gvfs-mtp-volume}]
        ├─gvfs-udisks2-vo───4*[{gvfs-udisks2-vo}]
        ├─gvfsd───3*[{gvfsd}]
        ├─gvfsd-fuse───6*[{gvfsd-fuse}]
        ├─2*[pipewire───2*[{pipewire}]]
        ├─pipewire-pulse───2*[{pipewire-pulse}]
        ├─tracker-miner-f───6*[{tracker-miner-f}]
        └─wireplumber───5*[{wireplumber}]

I've tried:

• Several versions of the Nvidia driver
• Forcing Wayland through /etc/gdm3/custom.conf
• Forcing Xorg through /etc/gdm3/custom.conf

One oddity is that the login session remains running after login. It's c14 here. If I kill it, a new one is spawned and the screen switches to it.

SESSION  UID USER  SEAT       TTY 
    261 1000 oli   seat0      tty2
    262 1002 katie seat-katie 
    c14  120 gdm   seat-katie

I'm usually pretty resourceful when it comes to fixing my own things but I can't even find logs for the second display server. I'm getting pretty desperate. Any help appreciated.