custom combobox problem(solved)

I want to create a combobox that popups from top.
But I am unsuccessful.
My idea is to amend the ComboBox script.
But I find that there is not effect to set script as follows:

<Child type="List" style="Popup" skin="List" offset = "0 -50 0 0" align = "Top" layer="Popup" name = "List"/>

So I think the key must exist in other place.
The following image is made by photoshop.

task added

compete in trunk 3
https://my-gui.svn.sourceforge.net/svnr ... /trunk_3.0
revision 3056

combo->setFlowDirection(FlowDirection::BottomToTop);

Thanks.
It's OK.