For some reason, there is a scroll bar on the left side of #mainView div inside this page. I really don't understand why. The problem seems to occur in pretty much all browsers. The scroll bar on the left side disappears once the right one is triggered.
Before #mainView content is loaded, its width is null and its overflow property is set to scroll. So you have a 20px wide div with a lone scroll while waiting for content loading.
You can either a width for #mainView or try overflow: auto.