WPF Lookless control that inherits ComboBox - IsEditable problem

I've built a "lookless" control that inherits ComboBox. It has three
dependancy properties (CountryCode, Label, and ListByName), and a ReadOnly
property of DistrictList.

Whenever CountryCode changes, the Label is changed, and the DistrictList is
rebuilt. (eg. Country changes from CAN to USA, Label will change from
Province to State, and the list contents will change from provinces to
states). When the ListByName property changes, the items in the list will
flip from names, to two character...

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