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

[Kévin Le Gouguec]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Juri Linkov <juri@linkov.net>
>> Cc: Eli Zaretskii <eliz@gnu.org>,  51016@debbugs.gnu.org,  larsi@gnus.org,
>>   orontee@gmail.com
>> Date: Sat, 18 Dec 2021 19:05:52 +0200
>> 
>> > As a user, I still consider that Emacs tripping over form feeds
>> > when I ask it to go to the next heading is a plain bug
>> 
>> Indeed, it's a plain bug: when the new variable 
>> outline-minor-mode-use-buttons
>> is enabled, ^L lines displayed as outline headings are making a mess:
>
> I guess I'm blind, since I see neither a bug nor a mess.

I guess the assumption Juri and I are working with (and maybe Lars too,
since IIUC he added these buttons to act as visual cues to indicate
places which can be expanded or collapsed) is that the purpose of
outline-regexp is to capture "heading" lines, by which we mean titles
and subtitles which define a hierarchy of things to show and hide.

These form feeds do not contribute to the document's hierarchy.  They do
not have subsections to expand or collapse.  Despite this, they are
treated as level-1 headings.

That leads to what Juri and I consider "absurd" results:

- outline-forward-same-level pauses on them: why?  There's nothing for
  a user to expand or collapse there;

- outline-minor-mode-use-buttons adds these clickable buttons: why?
  There's nothing for a user to expand or collapse there.