anchor.diff:  
   ;; VIEWPORT
   
  -(defclass viewport-pane (sheet-single-child-mixin composite-pane) ())
  +(defclass viewport-pane (single-child-composite-pane) ())
   
   
   ;; SCROLLER-PANE
  
positive.code: (defmacro scrolling ((&rest options) &body contents)
    `(let ((viewport (make-pane 'viewport-pane :contents (list ,@contents))))
       (make-pane 'scroller-pane ,@options :contents (list viewport))))
  
  (defmethod pane-viewport ((scroller scroller-pane))
    (first (sheet-children scroller)))
  
negative.code:     (setf (pane-space-requirement bp) space)
      space))
  
  (defmethod allocate-space ((bp border-pane) width height)
    (set-width-and-height bp width height)
    (let ((border-width (border-pane-width bp))
  	(child (first (sheet-children bp))))
      (setf (sheet-region child)
        (make-bounding-rectangle border-width border-width
  			       (- width border-width) (- height border-width)))
      (allocate-space child (- width (* 2 border-width)) (- height (* 2 border-width)))))
  
  
  ;; RESTRAINING PANE
  