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

[Eli Zaretskii <eliz@gnu.org>]

> From: Po Lu <luangruo@yahoo.com>
> Cc: shingo.fg8@gmail.com,  65803@debbugs.gnu.org
> Date: Sun, 10 Sep 2023 09:00:45 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > 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

AFAICT, we never use this for anything but FRAME_COLUMN_WIDTH.  So
when you talk about "average width of ASCII characters", I don't think
I understand what is that property, since we never call it like that
and never use it for ASCII characters.

> 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.

I don't understand what you are trying to say here.  Who suggested to
calculate the value of the average width for each glyph in the font at
load time?