I feel dumb for needing to ask, but I've been annoyed by this several times in the past and have yet to come across the answer.
Sometimes, when rearranging tabs in IDA Pro, I accidentally detach the tab from the main window, leaving it floating in its own window. The problem is that I have no idea how to reattach it, so generally when that happens I have to close it and reopen that view in the main window, which isn't the best solution.
In other programs that do the whole "a tab can be detached as its own window" thing, generally there's something that appears on the main window to signal that's where you need to drag it to to reattach it. Browsers do a slightly different thing in that you just drag the tab back to the tab bar. Neither of those work in IDA, and I can't find anything in the menu.