Glitch with closed second window

Things have been much smoother since the last update, with far fewer oddities regarding tabs moving, extra window bars, etc.

However, I just had this happen - I opened an image in a new window, and when I closed it (the second window) I was left with this. The box pops up when I click on the 'Hibernated' label (don't worry about the strange button floating in the title bar - that's from something else):