bug#65896: 30.0.50; folding text with text properties prevents background from extending past the newline

[Kévin Le Gouguec]

Eli Zaretskii <eliz@gnu.org> writes:

>                                        I cannot see how this proposal
> could be TRT.

Yeah, I believe there should be ways to scratch that itch without going
all the way down to the display engine.

FWIW, I would invite motivated hackers to check out magit-section and
see if outline-mode could be taught a new "folding style" that would use
the same folding principles.  My own wandering through the EIEIO maze
has been too brief to yield anything useful, but AFAICT the salient
points are:

* setting the 'invisible overlay's BEG at the start of the "section
body" (after the heading's newline),

* storing bookkeeping information (such as this beginning position) in a
'magit-section property applied to the heading, so that
magit-section-show can retrieve that information when invoked by the
user with point on that heading.

I would imagine outline.el could grow a user option to adjust overlay
boundaries this way, so the heading's newline would remain visible, and
so would any :extend property on that newline… although perhaps I'm
missing some key differences between outline-mode and magit-section-mode
that may derail this train of thought.