Looks like we can't reproduce the issue or there's a problem in the test case provided.
List Group Header height problem
Prior to 2.1, I was able to set the height of a lists group header. Now in 2.1, height applied to the group header doesn't push the first list item down, it floats over the top of it. In fact, padding, margins, breaks do nothing but cover the first row below the group header instead of pushing it down.
It appears the group header container is absolutely positioned instead of relative which is used to achieve the 'pinned header' effect but it shouldnt cover up the first row if the list is scrolled up to the top.
I tried turning off variable heights, no header pin, still same overlap problems.