Comment 1 for bug 1377738

Resize your screen (click on the square on the top right of the screen) it should allow you to cancel or select and get out of it. If it doesn't, take your cursor over one side of the window until you get a double arrow, click and drag that screen size to smaller or larger. Once you're able to click out of that task, you should be able to click on that box again and go full screen. This has worked for me 100% of the time so far; and I've done it quite a bit.

Also, if you can't do that, just wait it out (get another cup of coffee or something) the scene will time out and automatically disconnect you from the hq clerk.

:D hope this helps you. :D