Where is X display set on startup?

English Install/Boot/Login
1

The machine is a laptop in a dock with a large (1980x1200) monitor attached. In prior versions of OpenSuSE, I had managed* to configure this so the large monitor was the only display. The laptop screen was turned off. (But if turned on, was a duplicate of the other display.) After the new install, the system is configured as a single display extending over both screens. (And it has the laptop display logically to the left of the main one, but it is physically on the right - IOW, if I move the mouse cursor off the left edge of the main screen, it appears on the far right side of the laptop screen.)

I could probably use xrandr to change this after login, but would rather have it set on startup. How can I do so?

I’m currently using the LightDM display manage.

*Somehow: I’ve forgotten exactly how I did it, and the old config files have been overwritten by the new install.

2

My suggestion would be to use xrandr to configure it as you like via a config file in the /etc/lightdm/ directory eg lightdm.conf

You’ll need to get the appropriate output name for you your laptop with xrandr first. You could turn the laptop display off with something like

display-setup-script=xrandr --output LVDS-1 --off

It should then take effect when LightDM next starts.

Reference (just to give you a better idea here)
https://wiki.archlinux.org/index.php/LightDM#LightDM_displaying_in_wrong_monitor

3

In many cases, mostly for testing, I put a script with xrandr in it in /etc/X11/xinit/xinitrc.d/.

xrandr --output DP-1 --primary --output eDP-1 --right-of DP-1

should extend the desktop to the right using the internal display if it is turned on. My preference though is to do the setup in xorg.conf, which has overall proven to me to be more reliable among the various display managers. Depending on your actual outputs’ names, something like the following should do the same:

Section "Device"
	Identifier	"DefDev"
	Option	"monitor-DP-1"	"DigiP"	# DP
	Option	"monitor-eDP-1"	"DigiS"	# eDP
EndSection

Section "Monitor"
	Identifier	"DigiP"
	Option		"Primary"	"true"
EndSection

Section "Monitor"
	Identifier	"DigiS"
	Option		"RightOf"	"DigiS"
EndSection

LightDM is one I have yet to try to use with multiple displays.

4

That will do for testing, but when I get around to it, LightDM will be going away in favor of XDM. I think the xorg.conf method is what I did last time…

Reference (just to give you a better idea here)
LightDM - ArchWiki

Thanks, lots of useful information there, and in the links.