I awoke this morning from uneasy dreams to find my Ubuntu 12.04's pop-ups transformed into a gigantic bug.

I'm not aware of having made any changes here, but the pop-ups look like this:

You can see there's no border around the pop-up, and no title menu allowing me to close or minimize it.

Same for Thunderbird password prompts, the menus in Gimp, etc., so it isn't localized to Browsers. These are pretty important for operating with my windows, especially in Gimp. What happened and how can I fix it?