bug#38485: Customizing glyph widths

[Clément Pit-Claudel]

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.

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 ↝ = ~ + >).

Clément.