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

[Eli Zaretskii <eliz@gnu.org>]

> From: StrawberryTea <look@strawberrytea.xyz>
> Cc: juri@linkov.net, kevin.legouguec@gmail.com, 65896@debbugs.gnu.org,
>  yantar92@posteo.net
> Date: Sun, 01 Oct 2023 23:28:55 -0500
> 
> Sure. Basically, I want :extend to apply to a line with a folded region even
> though the newline at the end itself does not have the :extend property. So I
> want the extend property to somehow propagate across a line to the newline at
> the end, even if it does not have the :extend property. This would allow for
> :extend to work as expected when we have a a folded region that extends across
> multiple lines but the end of the folded region does not have the :extend
> property.

You say "propagate the :extend property", but you really mean
"propagate the face", right?  Because propagating :extend alone might
then show the wrong face extended, as the newline might have a
different face.

Anyway, this kind of thing can be easily done by the command that
folds the text: it could place a face with a suitably computed :extend
attribute on that newline.  Right?

Changing the display engine to do something like that will be much
harder, or even impossible, since the display engine currently
basically ignores the faces of invisible text, and the case of folded
text is not special in any way from the POV of the display engine.  So
making such a change will likely produce incompatible behavior changes
in other cases.