Went to share with Twitter, but decided to just take a screen shot instead, so didn't share my account details and hit cancel. The game then left the window that had opened in the game with no ability to close it. Had to save and exit (that part of the UI was thankfully visible) and reload to clear it.
El Tea writes:
http://steamcommunity.com/app/379530/discussions/1/517141882710717266/