[Loren]

I just stumbled upon a way to make it disappear and the game appear unresponsive.

The problem is that the pop-up box showing unit actions appears at a location some distance to the right of the cursor no matter where that would put it.

I apparently brushed the space bar. The mouse had been over on the menu on the right--there goes everything, it's on that box which is probably 2" beyond the right edge and thus entirely invisible. ESC gets things back to normal but if you don't realize what happened or don't know about the box positioning problem it could be an issue. This is probably behind some of the posts here about a vanishing UI.

That box location should be checked to ensure it's visible!

(I've hit this before when I knew I was calling up the box and thus I got out if it without thinking about it. This time I hadn't intended to call it up and it got me for a bit. A 3:45am alarm clock {inconvenient airline schedule} probably didn't help, either.)