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

[Kévin Le Gouguec]

Eli Zaretskii <eliz@gnu.org> writes:

>> > 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.

Right, that covers issue (1); do you have an opinion on issue (2)?
Namely, that ^L being part of outline-regexp causes navigation commands
(e.g. outline-forward-same-level) to treat them the same as top-level
headings?

>                                                         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.

FWIW, I don't think issue (2) pertains to aesthetics.  And the behaviour
change I can see (C-c C-f skipping over form feeds) looks like a net
positive.

(Also FWIW, I'm not surprised such radical-sounding changes are
surfacing today; given how often outline-mode has made the headlines
(heh) in NEWS, my impression is that there's been a recent renewal of
interest in this mode which had not seen much love for the past… however
long it's been since it has been introduced[1].

One interpretation is that outline-mode is feature-complete and its
defaults should not be questioned; another interpretation is that
innovation happened elsewhere, and with the benefits of hindsight, some
people are now trying to tune the venerable outline(-minor)-mode to make
it more ergonomic)


[1] Starting from…

$ grep -c '^\*\+ .*outlin' etc/NEWS* | sort -r -k2 -t:

… and checking the results, AFAICT Emacs 28 comes on top, then Emacs 29,
then 20 and 21, and then… basically nothing (except some false positives
in 26 and 24).