I think it depends on the rendering systems. When using Direct, the screen pauses whenever you tab out until you click back in and there's a little 'jump' per say because then you're getting caught up with what the chat.
When you use OpenGL, the screen keeps going.
( At least this is what happened me ). Keep in mind that while using Open & depending on the computer and such, you might crash more frequently, or, nothing might happened at all ^^