bug#64420: string-width of … is 2 in CJK environments

[Eli Zaretskii]

> Date: Sun, 2 Jul 2023 15:57:07 +0300
> From: Dmitry Gutov <dmitry@gutov.dev>
> 
> Hi! This was reported to company-mode 
> (https://github.com/company-mode/company-mode/issues/1388), as a 
> scenario that makes the overlay-based completion popup misrender because 
> the columns are not computed right when that char is present.
> 
> To repro:
> 
>    (set-language-environment "Chinese-BIG5")
>    (string-width "…") ;; => 2
> 
> In the default language environment its width is reported to be 1.
> 
> This doesn't seem to make sense because it's rendered one column wide 
> either way.

On GUI frames Lisp programs that need to know the actual width of some
string should use string-pixel-width, not string-width.  The latter is
basically only for TTY frames.

   (progn
     (set-language-environment "Chinese-BIG5")
     (ceiling (/ (string-pixel-width "…")
                 (float (default-font-width))))) ;; => 1