I was using LTS for a long time, and when a new monitor was getting connected a dialog would pop up asking me what to do. Now i'm on 17.10, the dialog also comes up when i connect the monitor, but by that time the system already chooses "mirror displays" for me.
I want to either change it to "use first display" or "extend to the right". How can i do that?
There's a small bug on XFCE concerning this subject. Go to Settings->Displays and click on Mirror Displays to activate it and click on it again to deactivate it. Now your secondary display should be placed as an extension of the primary one and you should see two monitors on the left. Drag and place them wherever you like and apply the changes afterwards.