scrollbar for aggrelist

Is there a way to add a v-scrollbar to an aggrelist? I have an aggrelist
of varying length modelled after the framed-text-list example in the
Aggre* Tutorial (1.5.2.6 An Aggrelist with an Interactor). As I don't
know how many items it will have, I'd like to make use of a v-scroll
when the list gets longer than a certain number. If this is possible,
how, or is there a better way to do this?

Here's the code for the function, and, yes, it's part of the umpteenth
incarnation of an mp3 encoder/id3 tagger frontend, the only difference
of this being that it's written in Common Lisp and aimed at helping me
encode music ripped from vinyl.

(defun get-names-gui ()
  "Display artist, track names, year, and genre, and let the user interactively
   change them."
  (let* ((names-win (create-instance NIL inter:interactor-window
                                     (:left 150) (:top 80) (:width 800) (:height 600)))
         (agg (create-instance NIL opal:aggregate))
         (header-box (create-instance NIL opal:rectangle
                                      (:top 2) (:left 2)
                                      (:width (o-formula (- (g-value names-win :width) 4)))
                                      (:height 40)
                                      (:filling-style opal:orange-fill)))
         (header (create-instance NIL opal:text
                                  (:left (o-formula (opal:gv-center-x-is-center-of names-win)))
                                  (:top 10) 
                                  (:font (opal:get-standard-font :fixed :bold :large))
                                  (:string "Album data")))
         (artist (create-instance NIL gg:scrolling-labeled-box
                                   (:label-string "Artist: ")
                                   (:label-font (create-instance NIL opal:font (:face :bold)))
                                   (:top 55) (:left 20) (:width 470)
                                   (:value (names-artist *alb*))
                                   (:selection-function
                                    #'(lambda (inter obj)
                                        (setf (names-artist *alb*) obj)
                                        (setf (names-artist-clean *alb*) (rem-trouble-char obj))
                                                   inter))))
         (titles-title
          (create-instance NIL opal:text
                           (:left (o-formula (opal:gv-center-x-is-center-of names-win)))
                           (:top 85)
                           (:font (opal:get-standard-font :sans-serif :bold :large))
                           (:string "List of Tracks:")))
         (titles
          (create-instance NIL opal:aggrelist
                           (:left 0)
                           (:top (o-formula (+ (gv titles-title :top) 25)))
                           (:height 100)
                           (:items (names-title-list *alb*))
                           (:item-prototype
                            `(,opal:aggregadget
                              (:parts
                               ((:frame ,opal:rectangle
                                        (:left ,(o-formula
                                                 (opal:gv-center-x-is-center-of names-win)))
                                        (:top  ,(o-formula (gvl :parent :top)))
                                        (:width 380)
                                        (:height ,(o-formula (+ (gvl :parent :text :height) 4))))
                                (:text  ,opal:text
                                        (:left ,(o-formula
                                                 (opal:gv-center-x-is-center-of names-win)))
                                        (:top  ,(o-formula (+ (gvl :parent :top) 2)))
                                        (:cursor-index nil)
                                        (:string ,(o-formula
                                                   (nth (gvl :parent :rank)
                                                        (gvl :parent :parent :items)))))))
                              (:interactors
                               ((:text-inter ,inter:text-interactor
                                             (:window ,(o-formula
                                                        (gv-local :self :operates-on :window)))
                                             (:feedback-obj nil)
                                             (:start-where ,(o-formula
                                                             (list :in (gvl :operates-on :text))))
                                             (:stop-event (:leftdown #\Return))
                                             (:final-function
                                              ,#'(lambda (inter text event string x y)
                                                   (let ((elem (gv inter :operates-on)))
                                                     (opal:change-item (gv elem :parent)
                                                                       string
                                                                       (gv elem :rank))
                                                     text event x y))))))))))
         (done-button (create-instance NIL gg:text-button
                                       (:left (o-formula
                                               (opal:gv-center-x-is-center-of names-win)))
                                       (:top 350)
                                       (:string "done")))
         (done-button-inter (create-instance NIL inter:button-interactor
                                             (:window names-win)
                                             (:start-where (list :in done-button))
                                             (:continuous t)
                                             (:final-function
                                              #'(lambda (done-button-inter obj)
                                                  (setf (names-title-list *alb*)
                                                        (gv titles :items))
                                                  (setf (names-title-list-clean *alb*)
                                                        (mapcar #'rem-trouble-char
                                                                (names-title-list *alb*)))
                                                  (opal:destroy names-win)
                                                  done-button-inter obj)))))
    (s-value names-win :aggregate agg)
    (opal:add-components agg header-box header artist
                         titles-title titles done-button done-button-inter)
    (opal:update names-win)))


-- 
Wolfgang Mederle

$BONMOT