anchor.diff:                              mouse-wheel-scroll-mixin
                               cut-and-paste-mixin)
     ((redisplay-needed :initarg :display-time) 
  -   (scroll-bars :type (member t :vertical :horizontal nil)
  +   (scroll-bars :type scroll-bar-spec ; (member t :vertical :horizontal nil)
   		:initform nil
   		:initarg :scroll-bars
   		:accessor pane-scroll-bars)
  
positive.code:     (when (first (sheet-children viewport))
        (setf (slot-value pane 'background)  ;### hmm ...
              (pane-background (first (sheet-children viewport)))))
      ;;
      (when (member scroll-bar '(:vertical t))
        (setq vscrollbar
              (make-pane 'scroll-bar-pane
                         :orientation :vertical
  
negative.code:                        :min-value 0
                         :max-value 1))
        (sheet-adopt-child pane vscrollbar))
      (when (member scroll-bar '(:horizontal :both t))
        (setq hscrollbar
              (make-pane 'scroll-bar-pane
                         :orientation :horizontal
  