I had that problem some time ago. The best I could figure was that it was an issue with my display resolution settings. I was using an older monitor, and when I switched to a newer one that could handle a higher resolution, the problem went away. If this has just suddenly happened without any changes to your system, I'm not sure what the problem could be. As a work-around, adjusting the size of your browser window might force it to display correctly - making the window narrower seemed to work for my old monitor.