bug#57082: 29.0.50; emacs-news-view-mode breakage

[Juri Linkov <juri@linkov.net>]

>>>> `outline-minor-mode-highlight-buffer' is intended only for buffers
>>>> that don't support font-lock highlighting.
>>>
>>> Yes, but with that change, arrows are displayed on first visiting the
>>> NEWS buffer; without it, they only appear when typing TAB on an outline
>>> heading.
>>
>> This change broke fontification e.g. in diff buffers that now
>> add outline faces on diff hunks.  So we need another solution.
>
> Oh, dear (as I noted in my OP, I only checked NEWS and *Help* with
> describe-binding).  But do you have a recipe to see this?  When I create
> a diff with vc-diff I don't see outline faces on the hunks.

The minimal test case is this:

  (setq-default outline-minor-mode-highlight t)
  (add-hook 'diff-mode-hook 'outline-minor-mode)

Then the first lines of diff is highlighted with blue outline-1 face,
the @@-lines with outline-2 face, etc.

This is because outline-font-lock-keywords and
outline-minor-mode-highlight-buffer have slightly different
interpretations of the values for outline-minor-mode-highlight.

This could be improved, but in any case both these functions
should not be applied at the same time.