ListViews' positionViewAtIndex doesn't take an overlay header into account

One would expect that positionViewAtIndex would function exactly the same as scrolling a view, but that doesn't seem to be the case with an overlay header (maybe other header types, but that's the one I've tested right now). I guess that the header item's size needs to be taken into account when doing the positioning.

To reproduce, open the attached testcase. Note that item 0 is visible. Press the red "+" button, followed by the red "-" button. Note that item 0 is now occluded by the header until scrolling is done with some other mechanism (e.g. mouse panning).