BTW could you please look on z-ordering and drawing order? This might be a Lazarus/LCL bug, but if you put a RxLookupDBComboBox on top of eg: TDBGrid, the "button" and popup will get drawn behind the grid (so user can't click the arrow and can't select). Only the "edit" area will be visible.
I think this is a LCL/widgetset bug but can you confirm please?