Re: region-based face-remapping

[Eli Zaretskii <eliz@gnu.org>]

> From: JD Smith <jdtsmith@gmail.com>
> Date: Sat, 6 Jan 2024 09:56:08 -0500
> Cc: emacs-devel@gnu.org
> 
>  I cannot be of more help here without understanding better what kinds
>  of application-level features you want to support.  Until now, you
>  only described that in very general terms.
> 
> This is because I was discussing a general feature that would be useful for more than my own
> application.  

And I'm trying to get a grip on the specific instances of that general
feature which perhaps could be much easier to implement and have much
lower costs.

> To make it more concrete, what I had in mind is an update to indent-bars which would changes the
> appearance of the set of bars in a “scope” region via treesitter queries in a post-command hook.  As
> point changes, the TS “enclosing scope” is calculated, and if it has changed, all the existing indent
> bars in that region would be updated with “alternate” styling (and formerly highlighted text would be
> returned to normal styling).  See [1] for some images to give you the idea of how the normal styling
> can look.  Important to note are that:

Why cannot this be done by modifying faces or overlays in the affected
region?

Btw, my advice is to use an idle timer, not post-command-hook, if that
is possible.  A timer-based implementation will not slow down Emacs
when the user moves point quickly or scrolls through a portion of the
buffer by leaning on an arrow key.  Also, the timer will always run
with point corresponding to what is on the screen, whereas
post-command-hook runs before point adjustment, so could use
inaccurate value of point.

>  It could be a buffer-local variable, which defines the size of the
>  region around point where the faces should change their appearance,
>  and how to change the appearance.  The display engine then could take
>  that into consideration when processing buffer positions around point.
> 
>  Whether this makes sense depends on the applications you have in mind.
> 
> Since there are many small stretches of text (single character stretches) that would be impacted over
> a larger region, I’m afraid such a simple approach wouldn’t work.

If all you need is change the faces, I think it will work.

> I understand.  The question is whether it would be desirable, tractable, performant, and maintainable
> to add any such infrastructure.

I don't know.  I do know it will not be simple.