IE removes a scrollbar area if, the page is smaller than the viewport height, so when you click into a page which is longer, it appears as though there is a pixel shift but only a scrollbar appears on the right. So, overflow-y: scroll forces it to display the scrollbar even on pages which are smaller than the viewport height.
Inspired by http://html5boilerplate.com/docs/#The-style
IE removes a scrollbar area if, the page is smaller than the viewport height, so when you click into a page which is longer, it appears as though there is a pixel shift but only a scrollbar appears on the right. So, overflow-y: scroll forces it to display the scrollbar even on pages which are smaller than the viewport height.