anchor.diff:                   (window-vtop-linum window))
                 (< 0 (window-vtop-charpos window)))
            -1)
  -        ((let ((n (- (window-cursor-y window)
  +        ((let ((n (- (window-cursor-y-if-wrapping window)
                        (- (window-height window) 2))))
              (when (< 0 n) n)))
           (t
  
positive.code:   (if (minusp n)
        (scroll-up (- n))
        (dotimes (_ n t)
          (when (= (window-cursor-y (current-window)) 0)
            (unless (forward-line n)
              (return nil)))
          (let ((prev-linum (window-vtop-linum))
  
negative.code:     (charms/ll:wattroff (window-win window)
                          attr)))
  
  (defun window-cursor-y (window)
    (- (window-current-linum window)
       (window-vtop-linum window)))
  
  (defun window-cursor-y-if-wrapping (window)
    (if (buffer-truncate-lines (window-buffer window))
        (+ (- (window-current-linum window)
              (window-vtop-linum window))
           (window-wrapping-offset window))
        (window-cursor-y window)))
  
  (defun window-print-line (window y str &key (start-x 0) (string-start 0) string-end)
    (check-type str fatstring)
  