bug#38485: Customizing glyph widths

[Eli Zaretskii]

> From: Clément Pit-Claudel <cpitclaudel@gmail.com>
> Date: Tue, 3 Dec 2019 23:22:35 -0500
> 
> Interestingly, both of these cases are handled quite nicely in emacs -nw, 
> because there the display is purely grid-like.

FTR, I'd like to clarify that Emacs doesn't "handle" the -nw case in
any way, it's taken care for us of by the terminal itself.  Assuming
that the terminal has the same notion of character width as Emacs
does, that is; if they use different tables, the characters will not
align.

> For my use case, it would be enough to have this available as a minor mode, 
> but Yuan Fu mentioned the following on emacs-devel: "I’d like to see it as a 
> face attribute instead of a mode. (Because I want to align my org table). 
> Maybe there could be a face attribute (:grid WIDTH) that instructs the 
> display engine to pad each glyph to have width that is a multiple of WIDTH in 
> pixel, and if WIDTH is t, default to “base character width”?  I remembered 
> that ‘window-width’ gives width in char widths and had a look at its source. 
> It knows the character width from FRAME_COLUMN_WIDTH; the comment says the 
> value currently equals to the average width of the default font of the frame. 
> I think this value can be used as the “base character width”."

I don't think I understand the rationale for using a face.  It sounds
like a subset of the general case, and why would someone want this
alignment only for one special face?

Using a face also means that the same characters from a larger font
will not be aligned.

> 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?

Thanks.