force-mode-line-update in post-command-hook renders scroll-conservatively worthless

[Juanma Barranquero]

emacs -q --no-site-file
M-x set-variable scroll-conservatively 1000000
C-x C-n
<down>...                ; Scrolls one line at a time
M-x ruler-mode
<down>...                ; Cursor jumps to the middle of screen
                         ; while scrolling the window

The reason is not ruler-mode, but the fact that it does:

(add-hook 'post-command-hook #'force-mode-line-update nil t)

AFAICS, adding `force-mode-line-update' (or in fact any function that
calls `set-buffer-modified-p') to post-command-hook makes
`scroll-conservatively' useless in all 21.X releases. It worked fine in
20.7.


                                                           /L/e/k/t/u