Just a message floating in the middle of or at the top of the screen like the room in Asteroid where it does the countdown ("30 seconds to...." "1 second and have a nice day"). You could still see what's going on and it would go away on it's own after a brief time rather than requiring an action to close it. It's really annoying on my iPad because if you click anywhere near the door controls, the "The blast door is already open" message pops up and has to be closed.