bug#46878: 27.1; lisp-outline-level returns imprecise level number

[Gabriel]

Juri Linkov <juri@linkov.net> writes:

>>> M-x outline-cycle-buffer RET
>>>
>>> It consistently alternates between only two states:
>>>
>>> Show all
>>> All headings
>>>
>>> But after evaluating your version of lisp-outline-level,
>>> it alternates between three states:
>>>
>>> Show all
>>> Top level headings
>>> All headings
>>
>> That is strange.  Is outline-regexp changing?  (I'm not on Emacs 28 so I 
>> can't check.)
>
> I don't think that outline-regexp is changing.  But I can't check
> in Emacs 27 because outline-cycle-buffer is only in Emacs 28.

So I checked this "apparent strange behavior" on master branch and it
seems to be working as expected. The commit
5a823a2a0c3eca60dd3939fba67df1bbe5a1b715 (Lars on 24 Nov 2020) added a
new condition ('has-top-level') to 'outline-cycle-buffer' to check,
well, if there is a top level heading, which is used to decide to hide
or not sublevels, in case they exist. For emacs-lisp-mode, it fails to
identify a top-level because 'outline-level' for a line starting with
";;; " returns a number greater than 1, which the code thinks its a
child level:

(when (= (funcall outline-level) 1)
  (setq has-top-level t))

Even though the outline level is used to compare relatively the headings
to decide which ones are parents and which ones are children (level 5 is
parent of level 6 and subsequent levels), there are cases (like the one
above) where it's expected to have an ordered levelling starting from
1. It's more intuitive, and otherwise it's impossible to know what is a
top-level or to set faces according to is level (level 1 with face
outline-1 and so on).

A simpler approach to avoid rewriting the existing functions and regexps
would be to create a new variable called 'outline-level-offset', which
could be used inside 'outline-level' to subtract from the original
value. For example, if today the outline level for ";;; " in emacs-lisp
returns 3, the elisp-mode.el (or lisp.mode.el) could set
'outline-level-offset' to 2, so 3 - 2 = 1, the expected value.

Regards,
Gabriel