Re: string-width for emojis

[Eli Zaretskii]

> From: Evgeny Zajcev <lg.zevlg@gmail.com>
> Date: Fri, 28 Jan 2022 19:32:47 +0300
> 
> If I evaluate
>   (insert "ABCDEF\n"
>           "\x2734\xfe0f -> "
>           (format "%d" (string-width "\x2734\xfe0f")))
> directly in the scratch buffer, I'll get
>   ABCDEF
>   ✴️ -> 2
> as output.  However, if I evaluate
>   (with-current-buffer (get-buffer-create "*help2*")
>     (erase-buffer)
>     (insert "ABCDEF\n"
>             "\x2734\xfe0f -> "
>             (format "%d" (string-width "\x2734\xfe0f")))
>     (display-buffer (current-buffer)))
> I'll get
>   ABCDEF
>   ✴️ -> 1
> as output.

I get 1 in both cases.

> Documentation states, that "Width is measured by how many columns it occupies 
> on the screen."  I see
> visually that ✴️ occupies approximately 2 columns (see screenshot at
> http://lgarc.narod.ru/pics/emacs-string-width.png)
> 
> What is the correct use of `string-width` to always get 2 as width for the 
> "\x2734\xfe0f" ?

The correct answer is 1, not 2, since U+2434 is a single-column
character:

  (char-width #x2734) => 1

That it takes more pixels than a single canonical character depends on
the font used to display it.  Note that the doc string warns about
inaccuracies in the returned value, and advises to use
window-text-pixel-size instead.