This is not a problem that I have seen before, I'm not sure if it is server specific or has something to do with the css/js on the pages. When I try the same thing on my local server I do not get any pages highlighted with the default document, then when I visit the index page I see just the home page as highlighted.
If you see this again or on a different site you design or test let us know and we can try to look into the cause a little further.