I was in a 1.12.1 server when this bug popped up. I couldn't click on anything ingame, and as I was writing a sign when this happened I had to close the game. After that, I couldn't click on any of the buttons on the main menu. I could highlight them for a brief second, but they wouldn't respond to my mouse.

1:01: I've checked through all the versions, and this seems to affect versions 1.8.9 and up, even the snapshots. It first seemed to affect versions 1.10 and up, but after a few minutes it started to affect 1.8.9 and up.

1:21: It's something to do with my "versions" or "libraries" folder in .minecraft., but I'm not sure what. I deleted all my folders in .minecraft., and the same thing happened.