bug#56808: 29.0.50; Elusive display problem on macOS

[YAMAMOTO Mitsuharu]

On Fri, 29 Jul 2022 23:32:03 +0900,
Gerd Möllmann wrote:
> 
> YAMAMOTO Mitsuharu <mituharu@math.s.chiba-u.ac.jp> writes:
> 
> > Does initializing font->space_width to 0 work for you?
> 
> Yes it works.  Thanks!

Thanks for checking.  Eli, is it OK to install the following change to
the emacs-28 branch?

diff --git a/src/macfont.m b/src/macfont.m
index dfdd9bc6ba..eec1707c3c 100644
--- a/src/macfont.m
+++ b/src/macfont.m
@@ -2651,6 +2651,9 @@ So we use CTFontDescriptorCreateMatchingFontDescriptor (no
   font->pixel_size = size;
   font->driver = &macfont_driver;
   font->encoding_charset = font->repertory_charset = -1;
+  /* Clear font->space_width so macfont_monospace_width_multiplier may
+     not be confused by an uninitialized value.  */
+  font->space_width = 0;
 
   block_input ();
 
> Can we be sure that other uses of space_width don't run into problems?
> For instance:
> 
>             case MACFONT_SPACING_MONO:
>             if (cache->width_frac)
>               bounds.origin.x += - ((cache->width_frac
>                                      / (CGFloat) (WIDTH_FRAC_SCALE * 2))
>                                     * (cache->width_int / font->space_width));
>               break;
> 
> That would be an integer division by 0.  And it's a bit strange because
> of the integer division in a float expression...

This part is inside `if (metrics) ...' where `metrics' is the third
arg of macfont_glyph_extents, so it is not executed in the following
call in macfont_open:

    font->space_width = macfont_glyph_extents (font, glyph, NULL, NULL, 0);

The reason for the integer division is that cache->width_int has
already been adjusted to a multiple of font->space_width if
macfont_info->spacing == MACFONT_SPACING_MONO and cache->width_frac != 0.

      if (macfont_info->spacing == MACFONT_SPACING_MONO)
        {
          /* Some monospace fonts for programming languages contain
             wider ligature glyphs consisting of multiple characters.
             For such glyphs, simply rounding the combined fractional
             width to an integer can result in a value that is not a
             multiple of the (rounded) font width.  */
          int multiplier = macfont_monospace_width_multiplier (font, fwidth);

          if (multiplier)
            {
              cache->width_int = font->space_width * multiplier;
              cache->width_frac = lround ((fwidth / multiplier
                                           - font->space_width)
                                          * WIDTH_FRAC_SCALE);
            }
          else
            {
              cache->width_int = lround (fwidth);
              cache->width_frac = 0;
            }
        }

                                     YAMAMOTO Mitsuharu
                                mituharu@math.s.chiba-u.ac.jp