bug#18793: 24.4; zero width rectangular selection displaces text

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: monnier@iro.umontreal.ca,  carlosjosepita@gmail.com,  
> 18793@debbugs.gnu.org
> Date: Fri, 06 May 2022 13:33:20 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > It might be better and easier to introduce a defcustom that disables
> > the visualization of zero-width rectangles.  Was that considered as a
> > solution to this issue?
> 
> I think we wanted to keep that?

I'm not sure.  This was supposed to be a nifty feature.  If some users
are annoyed by its effect, the easy way to handle that is to let those
users disable the cause of the annoyance.  After all, we don't show
anything comparable for non-rectangular regions, do we?

> I'm not suggesting that it's worth implementing something as complex as
> this just for this feature, but like I said -- I think something like
> this may be useful, in general, for marking things, and if we had it, it
> would be natural to use it here, too.

What other uses could this have, except fill-column indication, which
is another minor feature?

Adding capabilities to the Emacs display that operate on
sub-character-cell resolution will mean serious complications.  For
example, what would we do when displaying color Emoji at that place?
Or what about images?  Or what if the character in question is
currently highlighted by mouse-face?

IOW, I'm asking whether these marginal features are worth a serious
surgery and complications in the display engine, which currently has
the canvas-based design?