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Re: [Emacs-diffs] master 51f0ccc: Put follow-mode's engine on pre-redisp

From: Alan Mackenzie
Subject: Re: [Emacs-diffs] master 51f0ccc: Put follow-mode's engine on pre-redisplay-hook instead of post-command-hook
Date: Wed, 3 Oct 2018 13:14:11 +0000
User-agent: Mutt/1.10.1 (2018-07-13)

Hello, Stefan.

On Wed, Oct 03, 2018 at 08:20:30 -0400, Stefan Monnier wrote:
> Hi Alan,

> >     Put follow-mode's engine on pre-redisplay-hook instead of
> >     post-command-hook

> Yay!

> > +(defun follow-pre-redisplay-function (wins)
> > +  (if (or (eq wins t)
> > +          (null wins)
> > +          (and (listp wins)
> > +               (memq (selected-window) wins)))
> > +      (follow-post-command-hook)))

> I'm not sure it's possible for (selected-window) not to be memq of wins,
> but even if it is, it's likely to be extremely rare.  So the above (or ...)
> is probably just a complicated way to say `t`.

Maybe.  But it looks safe, and keeps rigidly to the contract of

> > +;; The magic little box. This function was formerly called after every
> > +;; command.  It is now called before each redisplay operation (see
> > +;; `follow-pre-redisplay-function' above), and at the end of several
> > +;; search/replace commands.  It retains its historical name.

> I wonder why this is still needed.

There are interfaces like `isearch-update-post-hook' that need a
function with no arguments.  Isearch calls this hook at the end of a
search operation, and this allows follow-mode to resynchronise the
windows before a redisplay.

I tried removing this interface, hoping that the pre-redisplay-function
would do everything properly anyway, but it didn't - a search caused the
LH window to scroll rather than moving point to the RH window.  Also the
windows have to be aligned before lazy highlighting takes place.  So
`isearch-update-post-hook' and friends stay.

Also, for debugging follow-adjust-window, it is helpful to put
follow-pre-redisplay-function temporarily on post-command-hook, allowing
edebug to work.

Let me save you the bother of telling me I could just make the wins
argument &optional.  ;-)  I've just noticed that.

>         Stefan

Alan Mackenzie (Nuremberg, Germany).

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