Black gdm login screen ubuntu 18.04

 cf: https://askubuntu.com/questions/1036242/17-10-to-18-04-upgrade-freezes-during-boot/1037192#1037192

 Turns out that the problem isn't a gdm3 vs lightdm problem. It's a gdm3/wayland problem with older Intel GPU's. To fix...

1. Edit /etc/gdm3/custom.conf, uncomment
#WaylandEnable=false to WaylandEnable=false
   
2. sudo dpkg-reconfigure gdm3, select gdm3

3. reboot