I installed Debian (somewhere around Dec 2016) using the Debian live "standard" iso flashed onto a USB-drive. It was a minimal install (without X) so I installed Xfce with LightDM (using the default GTK greeter) on it. I upgraded to Testing after a week of using this installation and have used that without any (major) problems so far. A few weeks back, when Ubuntu dropped Unity in favor of GNOME, I decided to try it on Debian. It worked perfectly fine with LightDM but there was no sleep button so I installed GDM. It installed and worked OK but I decided to switch back to LightDM because GDM was hardly configurable and theming it was a pain.
So I apt purge'd gdm3 and switched back to LightDM. But when I rebooted Debian, it was showing a blank screen with a blinking cursor at the top-left corner. I immediately opened tty1 (CTRL + ALT + F1) and typed startx which dropped me into Xfce. I tried searching the web for a solution but all hits I got were related to Arch (or an Arch-based distro) and it was all due to a bug in Arch (now resolved) which was not my case. Seeing no apparent solution, I reinstalled GDM and it again worked perfectly and booted me into GNOME.
But I noticed a strange problem with GDM; though I was able to boot into GNOME (both on X and Wayland) I wasn't able to boot into Xfce. It would just drop me back to the login prompt. So I tried opening a TTY to see if startx works, strangely CTRL+ALT+F1 didn't work so I had to use TTY2 instead. Predictably, startx on TTY2 logged me into Xfce so I logged out and when I tried to switch back to the GUI using CTRL+ALT+F7, I was dropped to a blank screen with a cursor blinking in the top-corner. I switched to TTY1 and I was taken to the GDM prompt. So yeah, GDM (and the display server) was, for some unknown reason, on TTY1!
At this point, logging into GNOME from GDM (on TTY1) has no effect whatsoever on GNOME (i.e. logout works, sleep works, etc) but it can't login to Xfce. I installed XDM and it works the same as startx and it properly starts from TTY7 but unfortunately I can't change session so it can only login to Xfce.
Summing up the problems what I can conclude is that:
- LightDM doesn't work, boots into a blank screen
- GDM does work but for some reason it's on TTY1. It also can't log into Xfce.
- startx at any TTY drops me into Xfce
- XDM works, is on TTY7 and logs into Xfce. There's no option to change session.
I'm using an Intel machine with a 3GHz x2 processor and 4GB of RAM. I've installed no dedicated GPU on this machine so I'm using the Intel-integrated graphics.
Right now I'm using no display-manager and I'm typing this on Xfce started through startx on TTY1. If anyone has any idea for a fix then it would be greatly appreciated.