This might be due to me running the gnome remix of Ubuntu 14.04. I'm not sure if it happens in the Unity interface, as I only run gnome. Nvidia drivers 343.22, Geforce GTX 980.
1. Press Alt-F4 in game.
*Observed* The message "Are you sure you want to quit? Boo will miss you." pops up. The mouse cursor still is using the Baldur's Gate cursor and I am unable to click on anything. Eventually the mutter dialog pops up, saying "Baldur's Gate Enhanced Edition has stopped responding, Force Quit/Wait. Waiting doesn't restore the mouse cursor or allow anything to be clicked on. Obviously Force Quitting kills the game and the Boo will miss you dialog window.
*Expected* Game doesn't stop responding, and options in Boo will miss you dialog to be clickable.