GNU bug report logs - #56808
29.0.50; Elusive display problem on macOS

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Message #56 received at 56808 <at> debbugs.gnu.org (full text, mbox):

From: YAMAMOTO Mitsuharu <mituharu <at> math.s.chiba-u.ac.jp>
To: Gerd Möllmann <gerd.moellmann <at> gmail.com>
Cc: Eli Zaretskii <eliz <at> gnu.org>, 56808 <at> debbugs.gnu.org
Subject: Re: bug#56808: 29.0.50; Elusive display problem on macOS
Date: Sat, 30 Jul 2022 16:49:46 +0900

On Fri, 29 Jul 2022 23:32:03 +0900,
Gerd Möllmann wrote:
> 
> YAMAMOTO Mitsuharu <mituharu <at> 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 <at> math.s.chiba-u.ac.jp

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