anchor.diff:      (when (first (sheet-children viewport))
         (setf (slot-value pane 'background)  ;### hmm ...
               (pane-background (first (sheet-children viewport)))))
  -    ;;
  -    (when (member scroll-bar '(:vertical t))
  +    ;; make sure that we have ok options for the scroll-bar argument...
  +    (check-type scroll-bar scroll-bar-spec) ; (member :vertical :horizontal :both t nil))
  +    (when (member scroll-bar '(:vertical :both t))
         (setq vscrollbar
               (make-pane 'scroll-bar-pane
                          :orientation :vertical
  
positive.code: (defparameter *scrollbar-thickness* 17)
  
  (defclass scroller-pane (composite-pane)
    ((scroll-bar :type (member t :vertical :horizontal nil)
                 ;; ### Note: I added NIL here, so that the application
                 ;; programmer can switch off scroll bars alltogether. 
                 ;; The spec though has it neither in the description of
  
negative.code:                             mouse-wheel-scroll-mixin
                              cut-and-paste-mixin)
    ((redisplay-needed :initarg :display-time) 
     (scroll-bars :type scroll-bar-spec ; (member t :vertical :horizontal nil)
  		:initform nil
  		:initarg :scroll-bars
  		:accessor pane-scroll-bars)
  