bug#38485: Customizing glyph widths

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 38485@debbugs.gnu.org
> From: Clément Pit-Claudel <cpitclaudel@gmail.com>
> Date: Wed, 4 Dec 2019 11:57:12 -0500
> 
> On 2019-12-04 10:52, Eli Zaretskii wrote:
> > 
> >> Now that I think of it, though, I could see the use for a text property.  For example, prettify-symbols-mode could have an option to make each n-characters prettification n-characters wide, so that composing ~~> into ⟿ would produce a wide arrow occupying the exact same amount of space as the original uncomposed characters.
> > 
> > So what kind of text property would that be, and where and how will it
> > come into play in the above scenario?
> 
> I'm thinking something like `:display-width 3' or maybe `display-width "~~>"' (the former would mean "as wide as three spaces in the default font"; the later, "as wide as `~~>' in the default font").
> These properties would be applied by prettify-symbols-mode in addition to composition.

I don't understand why would prettify-symbols-mode want to do this via
a text property, instead of via a buffer-local variable.

> An interesting related feature is the ability to move the cursor inside composed characters.  For example, when composing -> into →, assuming a font such as Fira code with a two-characters wide → symbol, it would be nice to be able to position the point in the middle of the composed symbol.  I often have this problem in Emacs with ||; I compose it into ‖, but I sometimes want to type |a|, and in those cases I tend to type || then press left and type a, but this doesn't work when || has been composed into ‖.
> Other editors have this feature, but I'm not sure how they handle the distinction between a composition that shouldn't be "separable" (such as é = e + ') and one that should be (such as ↝ = ~ + >).

This is an unrelated feature.  We currently allow DEL to delete one
characters even if it's composed with adjacent characters, but we
don't allow moving inside the composition.  (You can always
toggle-auto-composition, though.)