bug#16129: 24.3.50; Emacs slow with follow-mode when buffer ends before last window

[Stefan Monnier]

> 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