ComboBox Customisation

Hey all

Is it possible to customise the behaviour of the PopUpMenu that is produced by a ComboBox, so that it stays exclusively below the combobox when the list is long enough to go below the bottom edge of the screen. I can make this happen if I set the target area appropriately, however if the item it needs to select is not in the first few items then the list is extended over the top of the combobox.