text-input-choice :read-only and :completion-function

I would like to have a drop-down list for picking a state. I have tried 
implementing this using text-input-choice, but neither setting :enabled 
to :read-only nor specifying a completion-function works the way I expected.

I am able to type text into the pane even though it is read-only and my 
completion-function is never called when I tab out of the pane.

Thanks for any advice.

Mitch

(defconstant +state-abbreviations+
   (list "AL" "AK" "AR" "AZ" "CA"
         "CO" "CT" "DE" "DC" "FL"))

(defun completion-fn (pane text)
   (print "completion-fn called"))

(capi:contain
  (make-instance 'capi:column-layout
                 :description
                 (list (make-instance
                        'capi:text-input-choice
                        :items +state-abbreviations+
                        :selection 0
                        :max-characters 2
                        :enabled :read-only
                        :completion-function 'completion-fn)
                       (make-instance
                        'capi:text-input-pane))))