That page is already using responsive design, but there are some parts about it that might be done better. If you notice a bug in the style, that's what it is. It's possible the wrong width-threshold was chosen for a change, or perhaps a width % is off by 1 or 2.
I did some research and updated the info about this request. It was actually not public release at the time Beta 2 was out. For the version 4.0.0 Gamma 6, it was treated as a bug that the behavior was missing and was actually added in that version.