Description of problem: gnome-shell seems to have trouble with vertical screens arrangements. Steps to Reproduce: 1. boot up F16 Desktop Live in a laptop 2. attach an external monitor 3. use System Settings -> Displays to arrange monitor above laptop display (without mirroring) 4. try to move a window from the laptop screen to monitor screen Actual results: 3. arrangement looks fine 4. windows can't be moved around or placed windows freely on the monitor screen (but they can on the laptop screen). In particular windows can only be placed on one or/and other side of the monitor screen or even not at all in some configurations, but not in the middle or other side. Horizontal arrangements work fine. Arranging the monitor screen below the laptop screen also works ok. Expected results: 4. To be able to move/place windows freely and normally on the upper screen. Additional info: I think this also doesn't work in F15 (gnome-shell-3.0) either or probably the behaviour is worse in 3.0. Sometimes after restarting or rebooting (would need to recheck) and the monitor screen has become the first screen, then vertical configuration works again - but this can also get lost again or reconnecting a monitor or rebooting, etc. FWIW I am testing with a Dell 23" monitor (1920x1080) and reproduced on two intel Thinkpads (T500 and T420s).
Ok, a workaround is to disable the laptop display and then turn it on again - so as to make the monitor screen the main gnome-shell screen. (I did this now using fn-F7 but Displays config might well work too.)
Still happening in F17.
*** This bug has been marked as a duplicate of bug 711479 ***