bug#64911: 30.0.50; switch-to-buffer-preserve-window-point not respected by switch-to-(next|prev)-buffer

[Eli Zaretskii]

> Cc: Adam Porter <adam@alphapapa.net>
> Date: Fri, 28 Jul 2023 17:24:36 +1200
> From: Phil Sainty <psainty@orcon.net.nz>
> 
> The `switch-to-buffer-preserve-window-point' variable determines
> whether the (BUFFER WINDOW-START POS) data returned by
> `window-prev-buffers' will be used by `switch-to-buffer' to set
> the window's start and point positions.
> 
> I think this variable should additionally be respected by both:
> 
> - `switch-to-prev-buffer'
> - `switch-to-next-buffer'
> 
> (If not others?  These are the cases I'm presently aware of.)

Please provide a significant rationale for this change in behavior.

This option has proved problematic in quite a few cases since it was
introduced: see its uses in the Emacs tree as the evidence of the
subtle issues it introduces.  So I'm not excited with making it affect
even more use cases.  We need a sound rationale for such a change (and
no, "consistency" is not a sound rationale in my book), and we need to
try to audit all the calls to these functions in the tree to
understand and anticipate the impacts.

> In practice this is a problem because the position stored in the
> `window-prev-buffers' data is a marker (or at least that is the case
> in the scenario I am dealing with), and the buffer in question is
> periodically erased and regenerated.  Erasing the buffer causes all
> its markers to be moved to position 1, so the end result is that the
> user loses their place.  (The buffer contents are rebuilt, but the
> new content is typically similar if not identical to the old content,
> and so maintaining the original position is desirable).

This sounds like quite a unique use case to justify a global behavior
change of 2 commands.  Can't you achieve what you need by other means?
Stashing the value of point or window-point somewhere and then
restoring it doesn't sound too complicated to me.

Adding martin, in case he has comments and suggestions.