Open picklist from ComboBox dropdown

[Andrus]

SWF ComboBox dropdown menu first item selection must open picklist.
For this I use the following code:

protected override void OnSelectedIndexChanged(System.EventArgs e) {
if (SelectedIndex != 0 || DropDownStyle != ComboBoxStyle.DropDown) {
base.OnSelectedIndexChanged(e);
return;
}
OpenPickList();
}

This causes picklist to open immediately if keyboard arrow key *activates*
to this item.
How to force OpenPickList() call only if user *selects* first menu item from
keyboard using enter or closing menu by F4 ?
Using mouse this works OK: OnSelectedIndexChanged does not occur if mouse
activates first item.
Why OnSelectedIndexChanged when first item is selected from keyboard ? How
to postpone this event until dropdown menu is closed ?

Andrus.