I started using debian jessie, AKA debian 8, and have been using gnome-fallback as the window manager. But I also use dual monitors which were working fine while using gnome 3. I tried to configure my displays using settings -> display but i was met with the message “could not get screen information” . This led me to https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=755048 which mentioned running arandr.
The solution to my issues was to run ‘arandr’ to configure my screen layout. I used that to adjust my layout, applied the configuration, and everything started working.