Work around reporting a dpi change in apply_xft_settings

* src/xsettings.c (apply_xft_settings): Don't report a change
when dpi settings do not differ substantially.

src/xsettings.c

@@ -667,8 +667,23 @@ apply_xft_settings (struct x_display_info *dpyinfo,
     }
 #endif
 
-  if ((settings->seen & SEEN_DPI) != 0 && oldsettings.dpi != settings->dpi
-      && settings->dpi > 0)
+  if ((settings->seen & SEEN_DPI) != 0
+      && settings->dpi > 0
+      /* The following conjunct avoids setting `changed' to true when
+	 old and new dpi settings do not differ "substantially".
+	 Otherwise, the dynamic-setting Elisp code may process all sorts
+	 of unrelated settings that override users' font customizations,
+	 among others.  Compare:
+
+	 http://lists.gnu.org/archive/html/emacs-devel/2016-05/msg00557.html
+	 http://lists.gnu.org/archive/html/bug-gnu-emacs/2016-12/msg00820.html
+
+	 As soon as the dynamic-settings code has been tested and
+	 verified, this Emacs 25.2 workaround should be removed.  */
+      && ((oldsettings.dpi >= settings->dpi
+	   && (oldsettings.dpi - settings->dpi) > 2)
+	  || ((settings->dpi > oldsettings.dpi)
+	      && (settings->dpi - oldsettings.dpi) > 2)))
     {
       FcPatternDel (pat, FC_DPI);
       FcPatternAddDouble (pat, FC_DPI, settings->dpi);