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 13:55:47 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > No, the purpose of FRAME_COLUMN_WIDTH is much more than just "the
> > width of ASCII characters".  It is used as the canonical character
> > width of the frame, for gazillion purposes.  One example which
> > triggered this bug is :align-to display spec, something utterly
> > unrelated to ASCII characters.
> 
> However, the column width has hitherto been defined to the average width
> of the frame font's ASCII characters.  At least outside W32, that is.

No!  Once again, for fixed-pitch fonts the average width is taken from
the font.  Here, from ftfont.c:

    if (spacing != FC_PROPORTIONAL
  #ifdef FC_DUAL
	&& spacing != FC_DUAL
  #endif	/* FC_DUAL */
	)
      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);
    else
      {
	int n;

	font->min_width = font->average_width = font->space_width = 0;
	for (i = 32, n = 0; i < 127; i++)
	  if (FT_Load_Char (ft_face, i, FT_LOAD_DEFAULT) == 0)
	    {
	      int this_width = ft_face->glyph->metrics.horiAdvance >> 6;

	      if (this_width > 0
		  && (! font->min_width || font->min_width > this_width))
		font->min_width = this_width;
	      if (i == 32)
		font->space_width = this_width;
	      font->average_width += this_width;
	      n++;
	    }
	if (n > 0)
	  font->average_width /= n;
      }

This clearly only calculates the average width for proportional fonts,
and otherwise takes the average width from the font's max_advance
width without calculating anything.  Or what am I missing?

> > But other font back-ends don't compute average_width for fixed-pitch
> > fonts, so are you only talking about proportional fonts here?
> 
> I'm talking about fonts in general: since fixed pitch fonts are meant to
> incorporate uniformly sized glyphs, the width of the space glyph should
> represent the average width of any subset of the font's glyphs.  In this
> particular case, Fontconfig doesn't deem the font in question a fixed
> pitch font, and thus Emacs measures the average width of each ASCII
> character itself.

See above: other backends only calculate the average width for
proportional fonts.  So what you say doesn't fit my reading of the
code.