bug#51016: 28.0.50; 'diff-font-lock-prettify' breaks display of outline headers

[Eli Zaretskii]

> From: Kévin Le Gouguec <kevin.legouguec@gmail.com>
> Cc: 51016@debbugs.gnu.org,  larsi@gnus.org,  orontee@gmail.com,
>   juri@linkov.net
> Date: Fri, 17 Dec 2021 08:07:58 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> https://debbugs.gnu.org/cgi/bugreport.cgi?bug=51016;msg=81;att=1;filename=outline_NEWS.png
> >> 
> >> On the left is NEWS with the default value of outline-regexp, on the
> >> right with ^L taken out.  FWIW I agree that the latter is easier on the
> >> eyes.
> >
> > And I disagree.  The ^L separators are there for a reason, and even in
> > the image they do their job.
> 
> I'd agree if
> 
> (1) Emacs used some display tricks to make the ^L separators look more
>     like actual separators (e.g. a full-width horizontal line, as is
>     done in C-h o; the page-break-lines package on MELPA tries to make
>     FORM FEEDs look a bit like that),
> 
> (2) outline-mode treated them as purely visual separators, and not as
>     actual section headings reachable with navigation keys.
> 
> As things stand, (1) the separators look noisy to me, sort of like stray
> control characters, (2) outline-forward-same-level will pause on them
> instead of skipping over them and jumping to the next, "actual" heading
> (i.e. a heading with actual subsections to show/hide).
> 
> It's clear that (1) is purely a visual preference; IMO (2) hints that
> outline-regexp conflates characters that are used to define the heading
> hierarchy (*) with characters that are used to delimit top-level
> sections (^L).

I have nothing against enhancing Outline mode to display ^L as we did
in "C-h o", provided that will be an optional feature.  But please
don't argue for _unconditional_ backward-incompatible changes in how
Outline buffers look and behave, for purely subjective aesthetic
reasons, and on top of that for hard-wiring such changes in defvar's.
That is as un-Emacsy as it gets, and frankly I'm surprised something
like that is even being put on the table.