anchor.diff:                        (point< start view-end-point))
                    (disp-set-line screen
                                   (overlay-attribute overlay)
  -                                (count-lines view-point start)
  +                                (calc-row start)
                                   (point-charpos start)
                                   (point-charpos end)))
                   ((and (point<= view-point start)
                         (point< end view-end-point))
                    (disp-set-overlay screen
                                      (overlay-attribute overlay)
  -                                   view-point
  +                                   (calc-row start)
                                      start
                                      end))
                   ((and (point<= start view-point)
  
positive.code:                 ((overlay-get overlay :display-left)
                   (when (and (point<= view-point start)
                              (point<= end view-end-point))
                     (let ((i (count-lines view-point start)))
                       (when (< i (length (screen-left-lines screen)))
                         (let ((str (overlay-get overlay :text)))
                           (setf left-width (max left-width (length str)))
                           (setf (aref (screen-left-lines screen) i)
  
negative.code:                       (point<= end view-end-point))
                   (disp-set-overlay screen
                                     (overlay-attribute overlay)
                                     0
                                     view-point
                                     end))
                  ((point<= view-point start)
                   (disp-set-overlay screen
                                     (overlay-attribute overlay)
                                     (calc-row start)
                                     start
                                     view-end-point))))
      (setf (screen-left-width screen) left-width)))
  