Remember the classic Win9x / NT / XP Windows themes? Of course you do, many including me are still running such operating systems. One of the largest differences between these old themes and the new Vista Aero theme (apart from transparency effects and desktop compositing) is the lack of a control icon in some dialogs and Windows. This in itself is unremarkable, after all, new theme, new design right? Wrong.
I appears Windows Vista Aero still has an invisible icon in the top left hand corner of the window. Windows without control icons like Windows Explorer still have this invisible area where, if clicked (or double clicked) will react. Try it yourself!
Of course this is not particularly useful. We have not been double clicking control icons to close windows since the days of Windows 3.x, even still I find it amusing that the functionality remains if only to support the legacy themes that can be used in Windows Vista.