I had an issue with this too, which Brogan's post solved. In my case I have expanded the forum width to fill the full screen (with no borders) but when a new post is made, the ribbon goes beyond this and the browser then creates a slider so you can scroll right to see the edge of the "New" ribbon. I changed the right border to 0px and then added the CSS above using 0px again as the right margin. This brings the whole ribbon within the forum page.