bug#32874: Unwanted scrolling in edebug `f' command when follow-mode is active

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 29 Sep 2018 20:41:13 +0000
> Cc: 32874@debbugs.gnu.org
> From: Alan Mackenzie <acm@muc.de>
> 
> > I'd actually urge you to have a good look at window-scroll-functions
> > as well.  (Follow mode already uses it, but I think it could use it
> > for quite a lot more.)  This hook is called when Emacs concludes that
> > a window may need to be scrolled to bring point into view.  This is
> > exactly where Follow mode wants to be able to affect the decision of
> > the display engine, right?  I think by making a few simple
> > changes/extensions where this hook is called, we could make the work
> > of Follow mode quite a lot easier, by letting it rely on the display
> > engine instead of trying to maneuver the display engine to do what it
> > wants.
> 
> I've had a look at window-scroll-functions, but I can't see what you
> must be seeing.  Currently, the documentation warns against trying to
> influence the scrolling, saying "it probably won't work anyway".

But you don't want to scroll yourself, you just want to switch the
selected window and move point so that Emacs won't need to scroll.

AFAIU, follow-mode wants to kick in when point goes off the selected
window.  And the call to window-scroll-functions is exactly the place
where the display engine decides it needs to scroll the window, but
didn't actually scroll it yet.  So that looks like a good place to
have follow-mode do its thing.  We might need to add some simple
facility for follow-mode to use, so that it could signal the display
engine not to scroll the window.  Other than that, I think this
possibility is worth exploring.

The advantage of using window-scroll-functions is that
pre-redisplay-function is called much more frequently, in most cases
follow-mode will need to do nothing at all.  You probably already have
a logic for detecting when it should do something, but if you are
invoked from window-scroll-functions, most if not all of that logic
will be redundant.

> Maybe it would be relatively simple to introduce new functionality.
> Something like "scroll window so that window-end gets the given value".

I'm not sure I understand how this could help follow-mode.  Please
elaborate.