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: 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.

> 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.