Arranging displays in LightDM / GDM Login screen

LightDM

For lightdm you would need to build yourself a display setup script using xrandr. You can do that the following way:

xrandr

• To get your display informations run xrandr
• You will see the different outputs for example DVI-0 or HDMI-0 including the possible resolutions and refresh rates. The currently used mode is marked with a *
• With that information you can now build your xrandr script.
• If your HDMI-0 output is the left screen, DVI-0 is in the middle and DP-0 is the right screen you would do the following:

xrandr --output HDMI-0 --mode 1920x1080 --left-of DVI-0 --output DVI-0 --mode 1920x1080 --rate 144 --output HDMI-1 --mode 1920x1080 --right-of DVI-0

man xrandr

display setup script

• If the file /etc/lightdm/lightdm.conf does not exist simply create it yourself.
• Using the above example your lightdm.conf would look like this:

[Seat:*]
display-setup-script=xrandr --output HDMI-0 --mode 1920x1080 --left-of DVI-0 --output DVI-0 --mode 1920x1080 --rate 144 --output HDMI-1 --mode 1920x1080 --right-of DVI-0

More information can be found here: askubuntu.com

GDM

In Gnome Display Manager this task is a little bit easier. You just need to setup your monitors the way you want them and then

cp ~/.config/monitors.xml /var/lib/gdm3/.config/

• /var/lib/gdm3 can also be /var/lib/gdm. You need to find out yourself.