anchor.diff:    (typep pane 'top-level-sheet-pane))
   
   (defmethod note-space-requirements-changed ((sheet graft) (pane top-level-sheet-pane))
  -  (let ((sr (compose-space pane)))
  -    (resize-sheet pane (space-requirement-width sr) (space-requirement-height sr))
  -    '(allocate-space pane (space-requirement-width sr) (space-requirement-height sr))))
  +  (when (sheet-grafted-p pane)
  +    (let ((sr (compose-space pane)))
  +      (resize-sheet pane (space-requirement-width sr) (space-requirement-height sr))
  +      ;; #+NIL
  +      (allocate-space pane (space-requirement-width sr) (space-requirement-height sr))
  +      nil)))
     
   (defmethod compose-space ((pane top-level-sheet-pane))
     (compose-space (first (sheet-children pane))))
  
positive.code: 
  (defclass viewport-pane (single-child-composite-pane) ())
  
  (defmethod allocate-space ((pane viewport-pane) w h)
    (allocate-space (first (sheet-children pane))
                    (space-requirement-width (compose-space (first (sheet-children pane))))
                    (space-requirement-height (compose-space (first (sheet-children pane))))))
  
  ;;;;
  ;;;; SCROLLER-PANE
  
negative.code:             (compose-space child)) )))
  
   (defmethod box-layout-mixin/xically-compose-space ((pane box-layout-mixin))
     (let ((n (length (sheet-enabled-children pane))))
       (with-slots (major-spacing) pane
         (loop
             for child in (sheet-enabled-children pane)
             for sr = (xically-content-sr** pane child)
             sum (space-requirement-major sr) into major
             sum (space-requirement-min-major sr) into min-major
  