How to adjust display settings for LightDM greeter on 18.04?

Solution 1:

I was able to solve this problem by installing arandr:

$ sudo apt install arandr

and running it:

$ arandr (it is also available in the settings manager).

Then I set my monitors the way I like using its GUI. You can try out its configurations on your current desktop by clicking the green checkmark:

Once things were correct, I saved the configuration to my desktop, which generated a .sh file:

#!/bin/sh xrandr --output DP-0 --mode 1600x1200 --pos 0x0 --rotate normal --output DVI-I-1 --primary --mode 1600x1200 --pos 3200x0 --rotate normal --output DVI-I-0 --off --output DP-1 --off --output HDMI-0 --mode 1600x1200 --pos 1600x0 --rotate normal

I then copied that file to /etc/lightdm/monitor-config.sh as root.

Finally, with su I edited /etc/lightdm/lightdm.conf (which did not exist) to include the following lines:

[Seat:*]
display-setup-script=/etc/lightdm/monitor-config.sh

I did that, logged out, and my login screen had the right monitor positions.