anchor.diff:     (display-height
       :initarg :height
       :accessor virtual-frame-height)
  -   (changed
  -    :initform t
  -    :accessor virtual-frame-changed)
      (buffer
       :initarg :buffer
  -    :accessor virtual-frame-header-buffer)))
  +    :accessor virtual-frame-header-buffer)
  +   (last-displayed-tabs
  +    :initform nil
  +    :accessor virtual-frame-last-displayed-tabs)))
   
   (defun make-virtual-frame (impl frame)
     (declare (type frame frame))
  
positive.code:     (if value
          (frame-multiplexer-on)
          (frame-multiplexer-off))))
  
  (defclass virtual-frame (header-window)
    ((implementation
  
negative.code:               (lem::window-buffer-point (current-window)))
  
    (setf (virtual-frame-current virtual-frame) frame)
    (notify-frame-redisplay-required frame)
    (map-frame (implementation) frame)
  
  