Currently the ListBox assumes that all the items it manages have the same height. This is done to simplify the internal logic a bit. Internally the ListBox generates Nifty elements for the number of visible elements and only replaces the content of those elements automatically when the scrolling position is changed. This obviously only works when all of the elements have the same height.
In this feature request the ListBox should be extended in a way so that the actual items can be any size. The Listbox should still be able to generate only the visible items on the fly even with different sized elements. Additionally the Listbox should support soft scrolling, e.g. it should be possible to scroll the content only a single pixel when necessary instead of whole elements like it does now.