I seem to be ailed with a problem most people consider a solution: my user is hidden from the login greeter. I've been frustrated by the login system for quite a while, there doesn't seem to be clear configuration or possible configuration of the login greeter user list settings.
My setup is pretty simple, Debian Stable (Stretch), add libpam-krb5 and configure it to point at the krb5-kdc server. gdm3 as the greeter, though I'm open to suggestions I am pretty unhappy with login greeters across the board. Regular old linux account.
su <user> works, entering the user name and password into the login greeter works, just not an option to select (which I wouldn't mind if the "other users" dialog was keyboard accessible)