anchor.diff:                                (let* ((relative-x-position (offset-in-stroke-position pane view stroke offset))
                                        (absolute-x-position (+ (x1 stroke-dimensions) relative-x-position)))
                                   (values absolute-x-position (y1 stroke-dimensions)
  -                                        (dimensions-height line-dimensions)
  +                                        (dimensions-height stroke-dimensions)
                                           (if (= (1+ offset) end-offset)
                                               (- (x2 stroke-dimensions) absolute-x-position)
                                               (- (offset-in-stroke-position pane view stroke (1+ offset))
  
positive.code: (defmethod display-drei-view-cursor ((stream extended-output-stream)
                                       (view drei-buffer-view)
                                       (cursor drei-cursor))
    (multiple-value-bind (cursor-x cursor-y line-height object-width)
        (offset-to-screen-position stream view (offset (mark cursor)))
      (letf (((stream-current-output-record stream) cursor))
        (draw-rectangle* stream
                         cursor-x cursor-y
                         (+ cursor-x object-width) (+ cursor-y line-height)
                         :ink (ink cursor)))))
  
  (defmethod display-drei-view-cursor :after ((stream extended-output-stream) (view drei-view)
  
negative.code:                                  (/= start-offset end-offset))
                              (return-from worker
                                (values (x1 stroke-dimensions) (y1 stroke-dimensions)
                                        (dimensions-height stroke-dimensions)
                                        (if (= end-offset (1+ start-offset))
                                            (dimensions-width stroke-dimensions)
                                            (offset-in-stroke-position pane view stroke (1+ offset))))))
  