Trinity and Gnome son't see the same screen

I have a laptop (Thinkpad X220t) freshly reinstalled with latest openSuSE (I did not yet have time to check under other distributions). I installed Trinity over Gnome 3; If I boot and start Trinity (first user) I have nothing on the external screen (which is a beamer, 1024x768). Internal screen is at 1366*900. If I start a second user (under Gnome) the internal screen resizes to 1024x768 and I get an image on the beamer. Thierry