When I try to close the game via alt-f4, the "Boo" message box pops up behind the game window so I can't click on it. Alt tabbing does not work - the mouse cursor changes as if I'm hovering over a different window but the game window is still rendered over everything so I can't see where I'm clicking. It looks like a graphics issue so if it helps I'm using an ATI laptop card.
I tried using the integrated graphics card on my laptop and when I alt-f4, the game window minimizes properly but the "Boo" message box appears behind all the other stuff rather than on top. It does allow me to close the game at least whereas I can't see anything in the first scenario.