anchor.diff:    (repaint-sheet sheet region))
   
   (defmethod handle-event ((sheet immediate-repainting-mixin)
  -			 (event window-repaint-event))
  +                         (event window-repaint-event))
     (repaint-sheet sheet (window-event-region event)))
   
   ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  
positive.code:   (when (typep sheet 'never-repaint-background-mixin)
      (return-from handle-repaint))
    (labels ((effective-repaint-region (mirrored-sheet sheet region)
  	     (if (eq mirrored-sheet sheet)
  		 (region-intersection (sheet-region mirrored-sheet) region)
  		 (effective-repaint-region mirrored-sheet
  					   (sheet-parent sheet)
  					   (transform-region (sheet-transformation sheet)
                                                               (region-intersection region
                                                                                    (sheet-region sheet)))))))
      (let* ((parent (sheet-mirrored-ancestor sheet))
             (native-sheet-region (effective-repaint-region parent sheet region)))
  	  (with-sheet-medium (medium parent)
  	    (with-drawing-options (medium :clipping-region native-sheet-region
  					  :ink (pane-background sheet)
  					  :transformation +identity-transformation+)
  	      (with-bounding-rectangle* (left top right bottom)
  		  native-sheet-region
  		(medium-draw-rectangle* medium left top right bottom t)))))))
  
negative.code: (defclass standard-repainting-mixin () ())
  
  (defmethod dispatch-event
      ((sheet standard-repainting-mixin) (event window-repaint-event))
    (queue-repaint sheet event))
  
  (defmethod dispatch-repaint ((sheet standard-repainting-mixin) region)
  