Subject: Re: bug#32839: 27.0.50; recenter doesn't redisplay

From: Juri Linkov <[email-masked]>
Date: Sun, 30 Sep 2018 02:38:03 +0300

>> The docstring of window-scroll-functions says:
>>
>>   These functions are called whenever the ‘window-start’ marker is modified,
>>   either to point into another buffer (e.g. via ‘set-window-buffer’) or another
>>   place in the same buffer.
>
> You've omitted the first line of the doc string:
>
>   List of functions to call before redisplaying a window with scrolling.
>                                                          ^^^^^^^^^^^^^^

But (info "(emacs) Recentering") says that recentering is scrolling:

     Typing ‘C-l’ twice in a row (‘C-l C-l’) scrolls the window so that
                                             ^^^^^^^
  point is on the topmost screen line.  Typing a third ‘C-l’ scrolls the
                                                             ^^^^^^^
  window so that point is on the bottom-most screen line.  Each successive
  ‘C-l’ cycles through these three positions.

So 'C-l C-l C-l' is eligible for the calls of window-scroll-functions.

>> Maybe it will be called in the new hook window-state-change-functions
>> proposed by Martin.
>
> I hope not.

I grepped for window-scroll-functions, and see that the current situation
is quite bad:

1. tabulated-list-window-scroll-function is not called on 'C-u -1 C-l'
   when the last window line is fully visible, so it doesn't adjust
   the width for display-line-numbers in this case.

2. linum-mode relies more on post-command-hook because
   window-scroll-functions is not reliable.

3. erc-scroll-to-bottom was forced to replace window-scroll-functions
   with post-command-hook because window-scroll-functions doesn't
   support altering the way the window is scrolled.

The only hope to fix these problems and to close this report is to call
the new hook window-state-change-functions at the very end when the
redisplay is completely finished, probably at the same time when
post-command-hook is called.