bug#65803: 29.1; Noto Sans Mono CJK JP has doubled-width on Windows

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Po Lu <luangruo@yahoo.com>
>> Cc: shingo.fg8@gmail.com,  65803@debbugs.gnu.org
>> Date: Sat, 09 Sep 2023 21:38:32 +0800
>> 
>>     font->min_width = font->average_width = font->space_width
>>       = (scalable ? ft_face->max_advance_width * size / upEM + 0.5
>> 	 : ft_face->size->metrics.max_advance >> 6);
>> 
>> That aside, Fontconfig does not judge Noto Sans Mono CJK JP a fixed
>> pitch font on my system.
>
> Basically, calculating our own estimate of the average width means we
> discard the attribute reported by the font, in effect backing up on
> the change made in OpenType spec v3, which deprecated the previous
> requirement to compute the average width based only on ASCII
> characters.  This seems to be justified only because Emacs uses the
> average width of the font for one purpose only: to calculate the
> default column width of a frame.  So this calculation is only relevant
> for when a font is used as the default face's font.  If we ever decide
> to use the average width for anything else, we might be bitten by
> this.
>
> So I think a cleaner solution would be to leave the average width
> attribute as the font reports it, and introduce a new attribute for
> the average width of the ASCII characters.  Not sure how urgent this
> is, but we should at least describe this subtlety in the comments.

However, the only function of the average width property is to provide
the average width of ASCII characters; the W32 font driver is AFAIK the
only exception to this rule.  Moreover, the average width attribute in
older TrueType fonts is that of each ASCII glyph, and several fonts
provide no average width attribute at all (given that an OS/2 table need
not be supplied in fonts that aren't designed to function under
MS-Windows), in which case calculating its value for each glyph at
load-time will prove prohibitively expensive.