A TreeView in ComboBox would be most helpful as both a stand-alone control and when editing within the Grid. something like this: http://www.brad-smith.info/blog/archives/193 (example from .NET Windows forms world) * Used for relative small tree data structure. * User may select one (many) item(s), without _loosing context_ (no interrupting dialog box, 'flat' interface). * User may accept selection by click on item, or pressing 'enter'. * User may cancel (hide) popup when click outside control/popup (ie in another control, that received focus) or pressing escape - like in combobox/dropdown. * It will be nice if user can select items using keyboard arrows. * After opening popup or selecting item, widget still has focus * When user types in combo (with open popup), data should be filtered. * No additional 'ok' or 'cancel' buttons to accept/dismiss selection.