anchor.diff:                         :min-value 0
                          :max-value 1))
         (sheet-adopt-child pane vscrollbar))
  -    (when (member scroll-bar '(:horizontal t))
  +    (when (member scroll-bar '(:horizontal :both t))
         (setq hscrollbar
               (make-pane 'scroll-bar-pane
                          :orientation :horizontal
  
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:                             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)
  