First off, it looks like you are in our Private Beta program. When using the Private Beta build, you should post issues you are having to the Private Beta Forums instead. The community there is better able to answer your questions, and help resolve issues, and it can help in bringing attention to problems sooner.
In regards to this problem, you can look at this thread which looks to be the same problem. The fix is to change the position of your browser background to something other than the middle (middle right, or middle left work fine). Since your browser background is set to Scale To Fit, this shouldn't have much effect, and you'll be able to change it back later. If you'd like more information, or if this fix does not work for you, please start a new thread in the Private Beta Forums.