Bug 1362599 - Remember which languages were used and force-cache when reset; r=heycam

layout/base/nsPresContext.cpp

@@ -237,6 +237,7 @@ nsPresContext::nsPresContext(nsIDocument* aDocument, nsPresContextType aType)
     mExistThrottledUpdates(false),
     // mImageAnimationMode is initialised below, in constructor body
     mImageAnimationModePref(imgIContainer::kNormalAnimMode),
+    mFontGroupCacheDirty(true),
     mInterruptChecksToSkip(0),
     mElementsRestyled(0),
     mFramesConstructed(0),
@@ -623,6 +624,7 @@ nsPresContext::GetUserPreferences()
   mPrefScrollbarSide = Preferences::GetInt("layout.scrollbar.side");
 
   mLangGroupFontPrefs.Reset();
+  mFontGroupCacheDirty = true;
   StaticPresData::Get()->ResetCachedFontPrefs();
 
   // * image animation
@@ -1066,6 +1068,7 @@ nsPresContext::UpdateCharSet(const nsCString& aCharSet)
       mLanguage = mLangService->GetLocaleLanguage();
     }
     mLangGroupFontPrefs.Reset();
+    mFontGroupCacheDirty = true;
   }
 
   switch (GET_BIDI_OPTION_TEXTTYPE(GetBidi())) {
@@ -1975,6 +1978,33 @@ void nsPresContext::StopEmulatingMedium()
   }
 }
 
+void
+nsPresContext::ForceCacheLang(nsIAtom *aLanguage)
+{
+  // force it to be cached
+  GetDefaultFont(kPresContext_DefaultVariableFont_ID, aLanguage);
+  if (!mLanguagesUsed.Contains(aLanguage)) {
+    mLanguagesUsed.PutEntry(aLanguage);
+  }
+}
+
+void
+nsPresContext::CacheAllLangs()
+{
+  if (mFontGroupCacheDirty) {
+    nsCOMPtr<nsIAtom> thisLang = nsStyleFont::GetLanguage(this);
+    GetDefaultFont(kPresContext_DefaultVariableFont_ID, thisLang.get());
+    GetDefaultFont(kPresContext_DefaultVariableFont_ID, nsGkAtoms::x_math);
+    // https://bugzilla.mozilla.org/show_bug.cgi?id=1362599#c12
+    GetDefaultFont(kPresContext_DefaultVariableFont_ID, nsGkAtoms::Unicode);
+    for (auto iter = mLanguagesUsed.Iter(); !iter.Done(); iter.Next()) {
+
+      GetDefaultFont(kPresContext_DefaultVariableFont_ID, iter.Get()->GetKey());
+    }
+  }
+  mFontGroupCacheDirty = false;
+}
+
 void
 nsPresContext::RebuildAllStyleData(nsChangeHint aExtraHint,
                                    nsRestyleHint aRestyleHint)

layout/base/nsPresContext.h

@@ -373,6 +373,9 @@ public:
                                                        GetFontPrefsForLang(lang));
   }
 
+  void ForceCacheLang(nsIAtom *aLanguage);
+  void CacheAllLangs();
+
   /** Get a cached boolean pref, by its type */
   // *  - initially created for bugs 31816, 20760, 22963
   bool GetCachedBoolPref(nsPresContext_CachedBoolPrefType aPrefType) const
@@ -1420,6 +1423,9 @@ protected:
   // link items.
   LangGroupFontPrefs    mLangGroupFontPrefs;
 
+  bool mFontGroupCacheDirty;
+  nsTHashtable<nsRefPtrHashKey<nsIAtom>> mLanguagesUsed;
+
   nscoord               mBorderWidthTable[3];
 
   uint32_t              mInterruptChecksToSkip;

layout/style/ServoSpecifiedValues.cpp

@@ -51,8 +51,7 @@ ServoSpecifiedValues::SetIdentStringValue(nsCSSPropertyID aId,
   if (aId == eCSSProperty__x_lang) {
     // This forces the lang prefs result to be cached
     // so that we can access them off main thread during traversal
-    mPresContext->GetDefaultFont(kPresContext_DefaultVariableFont_ID,
-                                 atom);
+    mPresContext->ForceCacheLang(atom);
   }
 }
 

layout/style/ServoStyleSet.cpp

@@ -267,6 +267,7 @@ ServoStyleSet::PreTraverseSync()
   mPresContext->Document()->GetUserFontSet();
 
   UpdateStylistIfNeeded();
+  mPresContext->CacheAllLangs();
 }
 
 void