I have seen this too. It is because the CSS is doing some funky 'work out the window size' incorrectly. Basically the X to close the window is above the top of the screen. I have found if you go to full screen mode (F11 in Internet Explorer) the CSS code works better and you can see the X and hence close the window.
Hope that helps.
...find my happy place... find my happy place...find my happy place...I love v4.5
