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

[Alan Mackenzie <acm@muc.de>]

Hello, Eli.

On Sun, Aug 18, 2019 at 17:34:09 +0300, Eli Zaretskii wrote:
> > Date: Sun, 18 Aug 2019 13:48:04 +0000
> > From: Alan Mackenzie <acm@muc.de>

> > Currently, when there are no fringes, inserting an overlay arrow
> > overwrites the first two characters on the pertinent line.  This is fine
> > for things like edebug, where the text is usually indented many columns
> > anyway, and only whitespace gets overwritten.

> > This is less good for things like compilation-mode, where this arrow
> > would obliterate the first two characters of a file name.

> > The following amendment fixes this by allowing the arrow to be displayed
> > at the BOL, displacing the rest of the line two characters to the right.
> > To do this, a new buffer local boolean variable, overlay-arrow-insert is
> > introduced.

> 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.

> 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.

> What am I missing?

Probably not a lot.  Extending the overlay arrow functionality using
before-strings would certainly have been easier than changing the
display engine.  But it may be worthwhile to be able to use the overlay
arrow interface for "insertion type" arrows.

-- 
Alan Mackenzie (Nuremberg, Germany).