Return ActiveSheetsChanged when rulesets change in common prefix.

third_party/WebKit/Source/core/css/ActiveStyleSheets.cpp

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/css/ActiveStyleSheets.h"
 
 #include "core/css/CSSStyleSheet.h"
 #include "core/css/RuleSet.h"
 #include "core/css/resolver/ScopedStyleResolver.h"
 #include "core/dom/ContainerNode.h"
@@ skipping 20 matching lines @@
     if (newStyleSheets[index].second == oldStyleSheets[index].second)
       continue;
 
     if (newStyleSheets[index].second)
       changedRuleSets.add(newStyleSheets[index].second);
     if (oldStyleSheets[index].second)
       changedRuleSets.add(oldStyleSheets[index].second);
   }
 
   if (index == oldStyleSheetCount) {
-    if (index == newStyleSheetCount) {
-      return changedRuleSets.isEmpty() ? NoActiveSheetsChanged
-                                       : ActiveSheetsChanged;
-    }
-
-    // Sheets added at the end.
+    // The old stylesheet vector is a prefix of the new vector in terms of
+    // StyleSheets. If none of the RuleSets changed, we only need to add the new
+    // sheets to the ScopedStyleResolver (ActiveSheetsAppended).
+    bool ruleSetsChangedInCommonPrefix = !changedRuleSets.isEmpty();
     for (; index < newStyleSheetCount; index++) {
       if (newStyleSheets[index].second)
         changedRuleSets.add(newStyleSheets[index].second);
     }
-    return changedRuleSets.isEmpty() ? NoActiveSheetsChanged
-                                     : ActiveSheetsAppended;
+    if (ruleSetsChangedInCommonPrefix)
+      return ActiveSheetsChanged;
+    if (changedRuleSets.isEmpty())
+      return NoActiveSheetsChanged;
+    return ActiveSheetsAppended;
   }
 
   if (index == newStyleSheetCount) {
     // Sheets removed from the end.
     for (; index < oldStyleSheetCount; index++) {
       if (oldStyleSheets[index].second)
         changedRuleSets.add(oldStyleSheets[index].second);
     }
     return changedRuleSets.isEmpty() ? NoActiveSheetsChanged
                                      : ActiveSheetsChanged;
@@ skipping 37 matching lines @@
     if (sheet1.second)
       changedRuleSets.add(sheet1.second);
     if (sheet2.second)
       changedRuleSets.add(sheet2.second);
   }
   return changedRuleSets.isEmpty() ? NoActiveSheetsChanged
                                    : ActiveSheetsChanged;
 }
 
 }  // namespace blink