I'm programming a modal window and it can be popped up. I can't think of a good word for the opposite state. Similarly, if the window can pop up, what should be call the act of retracting to the previous state?
I googled but found no antonyms.
The term "pop-up" originally reflected the fact that these windows seem to appear quickly out of nowhere when opened in response to some interaction; they are superimposed upon another window and are usually smaller than the underlying window and originally had drop-shadows which made them appear to float above it.
The act of dismissing them has always been called "closing" them. There is no specific antonym that conveys the idea of closing just as rapidly as they had appeared.