Re: The unwarranted scrolling assumption

[Eli Zaretskii <eliz@gnu.org>]

> From: Lennart Borgman <lennart.borgman@gmail.com>
> Date: Wed, 16 Jun 2010 03:40:11 +0200
> Cc: emacs-devel@gnu.org
> 
> I wonder if this happens in redisplay_window in xdisp.c. This
> functions has a labeled called "recenter":
> 
>   /* Finally, just choose place to start which centers point */
> 
>  recenter:
>   if (centering_position < 0)
>     centering_position = window_box_height (w) / 2;
> 
> The intent of that label seems to be just trying to came up to speed
> with the input.

No, it's not just to keep up.  Recentering is the generic redisplay
method, used as the last resort if Emacs is unable to be smarter than
that.

> However I dislike the way the problem is solved. I would rather wish
> that the display engine instead calculated a new window start point
> from the input command queue. I have no idea if that is actually
> possible.

I think it's impossible.  How can you guess the effect of list of
arbitrary input events on the display, without actually redisplaying?

> What I can see is that redisplay_window can restart through
> redisplay_internal (the comment at the label "need_larger_matrices"
> says that).

need_larger_matrices means that the dimensions of the window changed
drastically, e.g. if the text uses smaller fonts.  In that case, the
current redisplay cycle is aborted, and Emacs reallocates the glyph
matrices to fit the new dimensions.

> Perhaps this possibility to restart can be used instead.

You will end up with the same recenter anyway, after redisplay is
re-entered.