Bar drawing in display engine, master indent-spacing variable

[JD Smith <jdtsmith@gmail.com>]

> On Sep 14, 2024, at 10:38 AM, Stefan Kangas <stefankangas@gmail.com> wrote:
> 
> JD Smith <jdtsmith@gmail.com> writes:
> 
>>> On Jul 24, 2024, at 2:33 PM, JD Smith <jdtsmith@gmail.com> wrote:
>>> 
>>> 
>>> I propose the addition of indent-bars to ELPA.
>>> 
>>> https://github.com/jdtsmith/indent-bars
>>> 
>> 
>> Following up this request to add indent-bars to ELPA.  The display issue in RTL mode which Eli mentioned was an Emacs bug, fixed in #72287.
> 
> Philip, Stefan, WDYT?
> 
> Meanwhile, I can only echo Eli's sentiment that we should really have
> this built-in and implemented in the display engine. 

Perhaps ideal would be to have low level drawing primitives available in the display engine (and automatically handling space vs. tab, blank/too-short lines, etc.).  Then lighter weight lisp packages can drive them.  Such packages can then get entirely out of the overlay/font-lock/stipple/replacing display-char business, and focus on the interesting logic (like treesitter scopes).  If these draw primitives are sufficiently generic, I suspect lots of different applications would occur to people, beyond just indentation guides.

One other very simple thing which would help here would be to have one master variable across all modes that sets (or is set to) the indentation spacing (`indent-spacing' ?).  Each mode pretty much goes its own way now.