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

[Kévin Le Gouguec]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Kévin Le Gouguec <kevin.legouguec@gmail.com>
>> Cc: Juri Linkov <juri@linkov.net>,  51016@debbugs.gnu.org,  larsi@gnus.org,
>>   orontee@gmail.com
>> Date: Thu, 16 Dec 2021 22:02:05 +0100
>> 
>> Eli Zaretskii <eliz@gnu.org> writes:
>> 
>> >> The proposed change is to improve NEWS and similar files to make
>> >> their views more compact by fitting more lines on the screen.
>> >
>> > What does this have to do with removing support for ^L?
>> 
>> See this screenshot from one of Juri's previous messages:
>> (<86lf0l23u1.fsf@mail.linkov.net>)
>> 
>> 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).