The same thing happens to me all the time. This is because controlP5 has the default behavior of letting you drag elements when ALT is held down. Usually what happens is that you press ALT+TAB to switch in/out of the window, but because of the way focusing works, Processing doesn't see that you released ALT, so it responds as if ALT is being held down the whole time. To fix, simply tap the ALT button again while in the Processing window. This will trigger a "released" event and return things to normal. It's really a pain in controlP5, and the fact that it's not very well documented only creates more headaches.