bug#43128: Acknowledgement (27.1; DPI change not detected after switching to Emacs 27.1)

[Vincent Bernat <bernat@luffy.cx>]

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On 2021-07-19 14:09, Vincent Bernat wrote:
>>>>>> As I mentioned in https://debbugs.gnu.org/cgi/bugreport.cgi?bug=36284#8,
>>>>>> a proper fix would be to implement replacements for
>>>>>> XftDefaultSubstitute and XftDefaultSet.
>>>>>
>>>>> How complicate will such a change be?  Will it be simple and safe
>>>>> enough to allow installing such a change on the emacs-27 branch?
>>>>
>>>> Ping!  Could we please try solving this for Emacs 27.2?  Mitsuharu,
>>>> could you please show the fix you had in mind back then?
>>>
>>> Hey! Is there any news around this? The XFT backend seems to have some
>>> issues with TTC fonts that the Cairo backend does not.
>>
>> Sadly, no news yet.  Patches or ideas how to fix this are welcome.
> 
> I'll try to do something, but I have no experience with either Xft or
> Cairo, so if that's something clever to do, I am likely to fail.

I was able to build this patch. I did replace XftDefaultSubstitute() by 
something that looks correct. However, for XftDefaultSet(), it seems 
there is no mechanism to override the defaults for fontconfig or cairo, 
so it seems not needed to find a substitute.

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--- source/src/xsettings.c	1970-01-01 01:00:01.000000000 +0100
+++ source.new/src/xsettings.c	2022-10-25 22:50:26.350635015 +0200
@@ -608,16 +608,28 @@
 apply_xft_settings (struct x_display_info *dpyinfo,
                     struct xsettings *settings)
 {
-#ifdef HAVE_XFT
+#if defined USE_CAIRO || defined HAVE_XFT
   FcPattern *pat;
   struct xsettings oldsettings;
   bool changed = false;
+#ifndef HAVE_XFT
+  cairo_font_options_t *options;
+#endif
+
 
   memset (&oldsettings, 0, sizeof (oldsettings));
   pat = FcPatternCreate ();
+#ifdef HAVE_XFT
   XftDefaultSubstitute (dpyinfo->display,
                         XScreenNumberOfScreen (dpyinfo->screen),
                         pat);
+#else
+  FcConfigSubstitute (NULL, pat, FcMatchPattern);
+  options = cairo_font_options_create ();
+  cairo_ft_font_options_substitute (options, pat);
+  cairo_font_options_destroy (options);
+  FcDefaultSubstitute (pat);
+#endif
   FcPatternGetBool (pat, FC_ANTIALIAS, 0, &oldsettings.aa);
   FcPatternGetBool (pat, FC_HINTING, 0, &oldsettings.hinting);
 #ifdef FC_HINT_STYLE
@@ -716,8 +728,11 @@
 		     - sizeof "%f")
       };
       char buf[sizeof format + d_formats * d_growth + lf_formats * lf_growth];
-
+#ifdef HAVE_XFT
       XftDefaultSet (dpyinfo->display, pat);
+#else
+      FcPatternDestroy (pat);
+#endif
       store_config_changed_event (Qfont_render,
 				  XCAR (dpyinfo->name_list_element));
       Vxft_settings