I was having this same problem this morning. I muddled around with it, finally found it.
Go to your .minecraft folder. If you're on Windows, you'll probably have to do the whole %appdata% to pull it up. Once you open it, there's a notepad text document labeled "Options". Open that up, and change to value for fullscreen from true to false. Save the document, exit it. Start up Minecraft and you should be good to go. You may want to force an update from the login screen afterwards to be on the safe side.
I did all this and it cleared up for me instantly. Hope this helps.
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Dude, you are a lifesaver. Thanks so much
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