Finally fix font rendering properly

assets/prefs.js

@@ -8,7 +8,7 @@ lockPref("browser.startup.homepage", "about:blank");
 lockPref("toolkit.legacyUserProfileCustomizations.stylesheets", true);
 lockPref("browser.uidensity", 1);
 lockPref("browser.startup.page", 3);
-lockPref("layout.css.system-ui.enabled", false);
+//lockPref("layout.css.system-ui.enabled", false);
 lockPref("privacy.fingerprintingProtection", false);
 lockPref("signon.rememberSignons", false);
 lockPref("browser.download.viewableInternally.typeWasRegistered.jxl", true);

assets/userChrome.css

@@ -85,18 +85,6 @@
     display: none;
 }
 
-/* Workaround for https://bugzilla.mozilla.org/show_bug.cgi?id=1721612 */
-
-@font-face {
-  font-family: system-ui;
-  src: local('SF Pro Display');
-}
-
-@font-face {
-  font-family: -apple-system;
-  src: local('SF Pro Display');
-}
-
 * {
   font-family: system-ui;
 }

assets/userContent.css

@@ -1,9 +0,0 @@
-@font-face {
-  font-family: system-ui;
-  src: local('SF Pro Display');
-}
-
-@font-face {
-  font-family: -apple-system;
-  src: local('SF Pro Display');
-}

patches/0015-fix-system-ui-font.patch

@@ -0,0 +1,17 @@
+diff --git a/gfx/thebes/gfxFont.cpp b/gfx/thebes/gfxFont.cpp
+--- a/gfx/thebes/gfxFont.cpp
++++ b/gfx/thebes/gfxFont.cpp
+@@ -841,11 +841,11 @@
+ }
+ 
+ void gfxShapedText::AdjustAdvancesForSyntheticBold(float aSynBoldOffset,
+                                                    uint32_t aOffset,
+                                                    uint32_t aLength) {
+-  uint32_t synAppUnitOffset = aSynBoldOffset * mAppUnitsPerDevUnit;
++  int32_t synAppUnitOffset = aSynBoldOffset * mAppUnitsPerDevUnit;
+   CompressedGlyph* charGlyphs = GetCharacterGlyphs();
+   for (uint32_t i = aOffset; i < aOffset + aLength; ++i) {
+     CompressedGlyph* glyphData = charGlyphs + i;
+     if (glyphData->IsSimpleGlyph()) {
+       // simple glyphs ==> just add the advance
+