Hi,

The fix for the Office2016 and VisualStudio2013 themes will be available in next official version – R2 2018, scheduled for the middle of May.

Best regards,
Ralitsa Kumanova

Generally the control behaves the same in all themes and this gap is related to the default ComboBox MaxDropDownHeight which is 200px and the ComboBoxItems Height in Office2016 and VisualStudio2013 themes. For example in Office2016 the size of the items is 24 px + 1px transparent bottom border. In this case when having 8 items in the Popup, you need 8x25 = 200px, but actually you have only 198px available because of the Border inside the Popup with 1px thickness. That is why when you MouseOver the last item in the DropDown, it is brought into view by automatic scrolling as it is not fully visible.  We will fix this by changing the default MaxDropDownHeight in Office2016 and VisualStudio2013. Meanwhile I would suggest as a workaround to change it to be 201px locally in your application. If you customize the default size of the ComboBoxItems, you will need to take care of the MaxDropDownHeight as well.