That's happening for me on my iPhone 5 in safari with the latest iOS.
I just took another look and now it's back to normal. But I have seen it happen a few times now. Not sure on the exact steps yet to reproduce it but it has happened several times.
I think this is more of a browser issue (its not properly updating the media queries its using), than an XF issue, but Mike may be able to come up with a solution.