Comment 6 for bug 1446760

I'm seeing this with my lab deployment. We install ubuntu-desktop, xubuntu-desktop, kubuntu-desktop, plasma-desktop, and gnome-shell, so users can select whatever desktop they want. What seems to be happening is that configure of gdm, lightdm, or sddm don't actually disable sddm if another dm is selected. Neither kubuntu-desktop nor plasma-desktop seem to depend on sddm, so removal is easy to do.

I have also discovered that removing the sddm package isn't necessary. I have been able to successfully disable the sddm service to prevent it from starting on boot, and that also allows lightdm to run just fine.

# systemctl disable sddm.service

I've added this to my management tool to make sure it stays disabled even after updates, rather than remove the package in case the dependencies change in the future.