anchor.diff:  (defgeneric note-space-requirements-changed (sheet pane))
   
   ;; Panes
  +
  +(defconstant +fill+ :fill)
   
   (defclass space-requirement ()
     ((width :initform 0
  
positive.code:     (setf (slot-value region 'x1) x
  	  (slot-value region 'y1) y)))
  
  		    
  
  ;; INTERACTOR PANES 
  
negative.code:   (update-scrollbars (sheet-parent pane)))
  
  (defmethod update-scrollbars ((pane scroller-pane))
    (with-slots (viewport vscrollbar hscrollbar) pane
      (with-bounding-rectangle* (hminx hminy hmaxx hmaxy)
  	(stream-output-history (first (sheet-children viewport)))
        (with-bounding-rectangle* (rminx rminy rmaxx rmaxy)
  	  (sheet-region (first (sheet-children viewport)))
  	(when vscrollbar
  	  (setf (scrollbar-offset vscrollbar) (/ (- rminy hminy) (- hmaxy hminy)))
  	  (setf (scrollbar-length vscrollbar) (/ (- rmaxy rminy) (- hmaxy hminy)))
  	  (window-refresh vscrollbar))
  	(when hscrollbar
  	  (setf (scrollbar-offset hscrollbar) (/ (- rminx hminx) (- hmaxx hminx)))
  	  (setf (scrollbar-length hscrollbar) (/ (- rmaxx rminx) (- hmaxx hminx)))
  	  (window-refresh hscrollbar))))))
  
  
  ;; LABEL PANE
  