anchor.diff:     (vscrollbar :initform nil)
      (hscrollbar :initform nil)))
   
  +(defun scroll-page-callback (scroll-bar direction)
  +  (with-slots (client orientation) scroll-bar
  +    (multiple-value-bind (old-x old-y)
  +        (untransform-position (sheet-transformation client)
  +                              0 0)
  +      (if (eq orientation :vertical)
  +          (scroll-extent client
  +                         old-x
  +                         (- old-y
  +                            (* direction
  +                               (bounding-rectangle-height
  +                                (pane-viewport-region client)))))
  +          (scroll-extent client
  +                         (- old-x
  +                            (* direction
  +                               (bounding-rectangle-width
  +                                (pane-viewport-region client))))
  +                         old-y)))))
  +
   (defmethod initialize-instance :after ((pane scroller-pane) &rest args)
     (declare (ignore args))
     (with-slots (scroll-bar viewport vscrollbar hscrollbar) pane
  
positive.code:                                             old-x (* new-value (scroll-bar-length gadget)))))
                         :scroll-up-page-callback
                         #'(lambda (scroll-bar)
                             (declare (ignore scroll-bar))
                             (format *debug-io* "To the top!~%"))
                         :scroll-down-page-callback
                         #'(lambda (scroll-bar)
                             (declare (ignore scroll-bar))
                             (format *debug-io* "To the bottom!~%"))
                         :foreground +grey40+
                         :background +grey+))
        (sheet-adopt-child pane vscrollbar))
  
negative.code:                                           (gadget-client gadget)))))
                               (scroll-extent (gadget-client gadget)
                                              (* new-value (scroll-bar-length gadget)) old-y)))
                         :scroll-up-page-callback
                         #'(lambda (scroll-bar)
                             (scroll-page-callback scroll-bar 1))
                         :scroll-down-page-callback
                         #'(lambda (scroll-bar)
                             (scroll-page-callback scroll-bar -1))
                         :foreground +grey40+
                         :background +grey+))
        (sheet-adopt-child pane hscrollbar))))
  