anchor.diff:          :for str :across (screen-lines screen)
           :while (< y (screen-height screen))
           :do
  -        (cond ((and (not flag)
  +        (cond ((and (not redraw-flag)
                       (aref (screen-old-lines screen) i)
                       str
                       (fat-equalp str (aref (screen-old-lines screen) i))
  
positive.code:          (if (buffer-truncate-lines buffer)
               #'disp-line-wrapping
               #'disp-line)))
      (let ((wrap-lines (screen-wrap-lines screen))
            (flag redraw-flag))
        (setf (screen-wrap-lines screen) nil)
        (loop
          :with y := 0
  
negative.code:                                                screen start-charpos curx cury pos-x y str))
                   (let ((offset (- y2 y)))
                     (cond ((< 0 offset)
                            (setq redraw-flag t)
                            (dotimes (_ offset)
                              (push i (screen-wrap-lines screen))))
                           ((and (= offset 0) (find i wrap-lines))
                            (setq redraw-flag t))))
                   (setf y y2)
                   (incf y)))
                (t
  