Re: [Patch]: Allow overlay arrows to be inserted before the text at column zero rather than splatting it.

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 18 Aug 2019 16:15:30 +0000
> Cc: emacs-devel@gnu.org
> From: Alan Mackenzie <acm@muc.de>
> 
> > If you want the arrow be displayed before the line's text, why didn't
> > you just put a before-string at the beginning of the line, instead of
> > implementing this in the display engine?
> 
> I think it was to be able to use the same interface that the overlay
> arrow already uses, without having to reimplement a lot of it using
> before-strings.

I think it's a general consensus that the "overlay arrow" feature
should be walked away of, and at some point should be deprecated.  I'd
prefer not to base new code on that kludge.

> > AFAIU, that would give you most of the patch for free, e.g. you
> > wouldn't need to mess with the set_cursor_from_row hair.
> 
> Yes, there was set_cursor_from_row which I had to change.  Somehow, only
> partially initialised glyphs got into it; they pointed to lisp strings,
> but with an offset of -1.  This caused an error to be thrown, and the
> surrounding internal_condition_case_1 reentered the redisplay code in a
> loop, causing Emacs to hang.  I'm not sure where they failed to get
> initialised, but the function is probably better with the workaround I
> put in.

This might mean there's a bug in the code that generates those glyphs.
One more reason not to implement this in the display code.

> But it may be worthwhile to be able to use the overlay arrow
> interface for "insertion type" arrows.

Any particular reason why this might be worth our while?  Because I
don't see any.

Thanks.