Weird GDM issue when two monitors connected to eGPU
I just finished installing the latest PopOS on my Razer Blade Stealth 2019 and found a weird issue when connected to my egpu with two monitors plugged in. Basically, GDM seems to be making two separate X11 heads, one for the internal display (assuming it's the iGPU) and one for the eGPU displays.
The weird part is, if I detach one of the monitors from the GPU, I get the proper login screen and everything acts as if the internal and external displays are on the same session. Once I login to an account, I can plug the other monitor back in and it will kill the internal display, but I'll have dual eGPU displays working as expected.
I think GDM uses a Wayland session for the login screen by default though I'm not sure how this is pre-configured on Pop!OS. You could go into the file /etc/gdm3/custom.conf and either uncomment or comment the line "WaylandEnable=false" to go between X11 or Wayland login screens and see if that makes a difference. The Arch Wiki also has good info on how to change the default monitor settings.
Thanks! Turns out PopOS already has Wayland disabled for gdm3, so it was Xorg that was messing up. I think Ubuntu 18.04 has Wayland enabled and that's why I hadn't encountered it before. It's all working properly now