Re: something like linum.el ought to be added

[Richard Stallman]

    Yes, and I also found it's not enough to update line numbers in
    window-size-change-functions, configuration-change-hook, and
    after-change-functions; one should also do it in hs-...-hook,
    allout-..., outline-..., and many others.

I think that means we need to add a suitable hook which redisplay can
call to inform Lisp code that visibility changes may have altered the
position of the bottom of the window.

The hook could be called by redisplay_window,
if current_matrix_up_to_date_p is zero.

Would someone like to try implementing this?