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: Sat, 18 Dec 2021 20:43:18 +0100
>> 
>> 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.
>
> You are treating NEWS as if it were an Org document.  It isn't.

I don't see where Org enters the picture from my description?  AFAICT
this "assumption" I described is just paraphrasing outline-mode's
docstring:

> Set major mode for editing outlines with selective display.
> Headings are lines which start with asterisks: one for major headings,
> two for subheadings, etc.  Lines not starting with asterisks are body lines.
> 
> Body text or subheadings under a heading can be made temporarily
> invisible, or visible again.

>> 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.
>
> These are _page_ delimiters.  They are conceptually _above_ level-1
> headings.

Right; unfortunately, in addition to this *conceptual* understanding of
form feeds, *functionally* they are treated as level-1 headings, with
the unfortunate consequences we highlighted.

As you point out, they are page delimiters; if we were to take them out
of outline-regexp, page commands (navigation, marking, narrowing) would
keep working just fine!