My xubuntu couldn't extend the display, nor checkbox about it nor dropdown list, just "use this" checkbox.
I tried this (http://www.webupd8.org/2012/11/how-to-use-multiple-monitors-in-xubuntu.html), first the first point (I.) then the second (II.). The second solution almost worked, but windows get a black overlay. Then I switch of the display extension, and reboot computer.
After reboot my nice blue xubuntu desktop goes away, I got a grey desktop without icons and context menu. OK, I used the google, but couldn't find any solution (tried to install xfdesktop4 any versions etc...). Then upgraded the Xfce I think, from 4.8 to 4.10. (It said, cant install xfdesktop) I got a blue desktop, I can make files on it, but no mounted icons or background, and there's no setting for desktop in the setting manager.
Finally got checkbox and dropdown list on display menu, but when I extend the display, windows get black overlay, like before:
So I'm sad about it, and searching solutions for my problems. If you can really help me, please tell it to me! Very thanks!