IsEditable=True and ReadOnly=True
The current case does not showcase a bug but rather a not implemented custom logic. The desired behavior can be achieved by setting OpenDropDownOnFocus="True" and StaysOpenOnEdit="True", then check if the drop down is open and add a desired logic in the MouseLeftButtonDown event handler.
Nothing have been done since Mar 14 !!! I'm sure it's a really little bug into your template... Could you please fix this ? Thanks