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Re: [Patch]: Allow overlay arrows to be inserted before the text at colu

From: Stefan Monnier
Subject: Re: [Patch]: Allow overlay arrows to be inserted before the text at column zero rather than splatting it.
Date: Mon, 19 Aug 2019 05:30:24 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

>> 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.

I didn't know there's a consensus around that, but I'm happy to
hear it and I'm definitely in agreement with that consensus.

> I don't think there's anything else in Emacs at the moment that could
> take its place.

Strictly speaking, you can kludge something with a mix of `display` and
`after-string` properties, but it'd likely be fiddly.

> The essence here is being able to insert "=>" onto the
> display _without_ disturbing the horizontal alignment, as needed by
> debuggers.  Maybe, in addition to before-strings and after-strings we
> need "overlay-strings" (with a better name than that), which would cover
> up the underlying text.

Indeed.  Tho, IMO the implementation shouldn't be done in the glyph
matrix but in an additional layer drawn on top of the rending of the
glyph matrix (this way it can work even when the "hidden" text uses
proportional fonts, or a different size, or an image, ...).


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