How to scroll to selected item in a Combo

How do I get a Combo to scroll to the item that has been selected so that it always appears in the drop down list?

I haven't been able to find this anywhere in the documentation.

Roger,

Based on other examples I found here in the forum, I came up with the following code to override the showPopup function in the combo and set the current item to the selected item:

combo.AW_showPopup = combo.showPopup;
combo.showPopup = function() {
   this.AW_showPopup();
   var selected_item = this.getSelectedItems();
   var AW_onCurrentItemChanged = this.onCurrentItemChanged;
   this.onCurrentItemChanged = null;
   this.setCurrentItem(selected_item);
   this.onCurrentItemChanged = AW_onCurrentItemChanged;
}