anchor.diff:  
   (defmethod allocate-space ((pane tab-layout-pane) width height)
     (let* ((header (tab-layout-header-pane pane))
  -	 (y (space-requirement-height (compose-space header))))
  +         (y (space-requirement-height (compose-space header))))
       (move-and-resize-sheet header 0 0 width y)
       (allocate-space header width y)
       (dolist (page (tab-layout-pages pane))
         (let ((child (tab-page-pane page)))
  -	(move-and-resize-sheet child 0 y width (- height y))
  -	(allocate-space child width (- height y))))))
  +        (move-and-resize-sheet child 0 y width (- height y))
  +        (allocate-space child width (- height y))))))
   
   (defmethod note-tab-page-changed
       ((layout tab-layout-pane) page)
     (redisplay-frame-pane (pane-frame layout)
  -			(tab-layout-header-pane layout)
  -			:force-p t))
  +                        (tab-layout-header-pane layout)
  +                        :force-p t))
  
positive.code:     (dolist (page pages)
        (setf (sheet-enabled-p (tab-page-pane page)) (eq page current))))
    (let ((header
  	 (make-pane 'tab-bar-pane
  	  :display-time :command-loop
  	  :display-function
  	  (lambda (frame pane)
  	    (declare (ignore frame))
  	    (funcall (header-display-function instance) instance pane)))))
      (setf (tab-layout-header-pane instance) header)
      (sheet-adopt-child instance header)
      (setf (sheet-enabled-p header) t)))
  
negative.code:   ;; do the call only after the change has been done:
    (let ((old-page (tab-layout-enabled-page parent)))
      (prog1
          (call-next-method)
        (when (and page (not (equal page old-page)))
          (note-tab-page-enabled page)))))
  
  (defmethod (setf tab-layout-pages) (newval (parent tab-layout))
    (unless (equal newval (remove-duplicates newval))
      (error "page list must not contain duplicates: ~A" newval))
    (let* ((oldval (tab-layout-pages parent))
           (add (set-difference newval oldval))
           (remove (set-difference oldval newval)))
      ;; check for errors
      (dolist (page add)
        (unless (null (tab-page-tab-layout page))
          (error "~A has already been added to a different tab layout" page)))
      ;; remove old pages first, because sheet-disown-child still needs access
      ;; to the original page list:
      (dolist (page remove)
  