The configuration dialogs attached to the main application window of some applications, e.g. managing my identities for Thunderbird, didn't fit on the screen of small displays (e.g. a 13" notebook). This makes it impossible to change the settings since you can't see all options and can't reach the "OK" and "Cancel" buttons. On GNOME2 I could use alt+left-mouse-button to move the configuration dialog beyond the top border of the screen to reach the options at the bottom of the dialog but this doesn't work for GNOME3 where the dialog is attached to the main window and can only be moved within the borders of the screen.