Hi,
Could you please provide us information about your component version and the used service release. I tried to reproduce your issue with version 10.3 and SR 10.3.20103.2094, but without success. You could download the latest service release from our web site: Infragistics.com -> MyIG -> MyKeys & Downloads.
Please take a look at the attached sample, and if you think that I didn`t reproduce exactly your scenario, please feel free to modify my sample and revert it back to me.
Regards
Are you assigning images to any of the items in your list? If you have images one some items and not others, this may cause the item height to vary between items, in which case the combo has to allow scrolling past the last item into empty space because there's no way to calculate the total item height.