bug#19576: write-file writes the wrong buffer

[martin rudalics <rudalics@gmx.at>]

 > Thanks for working on this, Martin.  However, I don't think we should
 > install this change.  We call Lisp hooks from many places, including
 > maybe a dozen in the display engine.  It makes little sense to make
 > only one of them resistant to this kind of problems.  OTOH, if we do
 > this everywhere, I feel that we will unduly punish 99.999% percent of
 > legitimate users of these hooks just because one of them had a bug.

100% agreed.  But run_window_configuration_change_hook goes a long way
saving and restoring current buffer and selected window around each call
to a function on ‘window-configuration-change-hook’.

People who put their functions on ‘window-size-change-functions’ and
‘window-configuration-change-hook’ usually don't care about the precise
reason why these function get called.  They simply want to cover all
cases where a new window appears or a specific window changes size.  Do
we really expect them to add a ‘save-window-excursion’ in one case and
avoid it in the other because it would mean unnecessary extra work?

 > I think this is a clear bug in follow.el, and should be fixed there,
 > and nowhere else.  Perhaps we should also have some prominent warnings
 > in the documentation about this gotcha, so that the probability this
 > will happen again becomes lower.

We've broken the taboo in ‘window-configuration-change-hook’.  Anything
we add now will only increase confusion.

martin