Comment 2 for bug 1046722

This could potentially become a problem in games if hovering the mouse to the top region of the game window would bring down the wingpanel on top of the game. Some games rely on scrolling this way.

Besides, if we're talking about "true" fullscreen, doesn't that essentially give the application full control over the rendering and disable everything that has to do with the desktop? If so, then "borderless fullscreen" (http://pcgamingwiki.com/wiki/Borderless_fullscreen_windowed) would likely be the only way to achieve this.