Hi!

I agree that we would need to find out why the patch makes Emacs slow. (In
fact, I only supplied the information about the internals of follow-mode to
help you track down the problems with the slowdown.)

However, I don't agree with Eli -- it is possible to place window-start at
point-max! However, there is code in the display engine that explicitly
recenters such windows, after a while, or when something happens. For
example:

     emacs -Q
     C-x 3
     C-x o
     M-: (set-window-start (selected-window) (point-max)) RET
     C-x o
     M-<
     blablabla     (type some text)

As you type text in the left window at the beginning of the scratch buffer,
the right window is recentered. Follow-mode needs its windows to stay put
(even the empty ones), as this is essential in creating the illusion that a
number of windows make up a very tall virtual window.

When I originally wrote follow-mode (some 18 years ago), I suggested to the
Emacs maintainers to add a feature to make the recentering of empty windows
conditional, so that follow-mode could control this. However, at the time
they were not interested so I continued with the current system, which has
worked flawlessly since then.

If you are interested in making the change in the display engine,
follow-mode should of course be rewritten to use it. Otherwise, I suggest
that we keep it as it is today -- solutions using overlays etc. don't
appeal to me at all.

    -- Anders



On Fri, Dec 13, 2013 at 5:38 PM, Stefan Monnier <monnier@iro.umontreal.ca>wrote:

> > I am the original author of follow-mode, so I can share one interesting
> > implementation detail. When the viewed buffer ends before the last
> window,
> > follow-mode tries to display this window without any content (by setting
> > the window start to point-max). Unfortunately, the Emacs display engine
> > always tries ensure that windows are not empty so it repositions it...
> So,
> > follow-mode hammers in its view of the world every chance it gets,
> > currrently in post-command hook and window-scroll-functions.
>
> Hmm.. so we have 2 things to do:
> 1- figure out why my patch slowed things down so much.
> 2- change follow-mode to use a different approach.  Maybe a good way is
>    to do the following: put window-point at point-max, and add an overlay
>    on window-start...point-max that makes the text invisible (with
>    a `window' property, so it's only invisible in that window).
>    Of course, maybe that won't work either.  But hooking everywhere
>    doesn't sound like a good idea.
>
>
> -- Stefab
>