bug#17678: 24.4.50; Feature Request -- calculate new `window-start` & `window-end` before visual redisplay

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: esq@lawlist.com,  17678@debbugs.gnu.org
> Date: Fri, 13 Jun 2014 10:14:03 -0400
> 
> > Not necessarily: there are the w->optional_new_start and
> > w-> force_start flags, which determine what redisplay does with
> > window-start in these cases.
> > Also, the window-start could be set to a value that leaves point out
> > of the displayed area, in which case it won't be in effect.
> 
> Right, but these fall back into the case where redisplay performs
> a second pass trough the window/buffer, so it's similar to the scrolling
> case, right?

Not necessarily, AFAIR.  Sometimes these situations are detected and
handled on the fly.

> >> If OTOH redisplay decides to scroll, it won't re-execute
> >> pre-redisplay-function, so Elisp doesn't get a chance to react to this
> >> new window-start.
> > ??? Then what are those calls to run_window_scroll_functions that
> > redisplay issues?
> 
> Ah, right, I forgot about this.  So maybe window-scroll-functions is the
> answer to the OP's needs!

Or maybe we should call them in more places during redisplay.