Dirty tree scopes are always a subset of active tree scopes.

third_party/WebKit/Source/core/dom/StyleEngine.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2008, 2009, 2011, 2012 Google Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) Research In Motion Limited 2010-2011. All rights reserved.
@@ skipping 75 matching lines @@
 
     if (m_fontSelector) {
         m_fontSelector->clearDocument();
         m_fontSelector->unregisterForInvalidationCallbacks(this);
     }
 
     // Decrement reference counts for things we could be keeping alive.
     m_fontSelector.clear();
     m_resolver.clear();
     m_styleSheetCollectionMap.clear();
+    m_dirtyTreeScopes.clear();
     m_activeTreeScopes.clear();
 }
 #endif
 
 inline Document* StyleEngine::master()
 {
     if (isMaster())
         return m_document;
     HTMLImportsController* import = document().importsController();
     if (!import) // Document::import() can return null while executing its destructor.
@@ skipping 158 matching lines @@
         ShadowTreeStyleSheetCollection* collection = static_cast<ShadowTreeStyleSheetCollection*>(styleSheetCollectionFor(*treeScope));
         ASSERT(collection);
         collection->clearMediaQueryRuleSetStyleSheets();
     }
 }
 
 void StyleEngine::clearMediaQueryRuleSetStyleSheets()
 {
     documentStyleSheetCollection()->clearMediaQueryRuleSetStyleSheets();
     clearMediaQueryRuleSetOnTreeScopeStyleSheets(m_activeTreeScopes);
-    clearMediaQueryRuleSetOnTreeScopeStyleSheets(m_dirtyTreeScopes);
 }
 
 void StyleEngine::updateStyleSheetsInImport(DocumentStyleSheetCollector& parentCollector)
 {
     ASSERT(!isMaster());
     WillBeHeapVector<RefPtrWillBeMember<StyleSheet>> sheetsForList;
     ImportedDocumentStyleSheetCollector subcollector(parentCollector, sheetsForList);
     documentStyleSheetCollection()->collectStyleSheets(*this, subcollector);
     documentStyleSheetCollection()->swapSheetsForSheetList(sheetsForList);
 }
@@ skipping 482 matching lines @@
     visitor->trace(m_dirtyTreeScopes);
     visitor->trace(m_activeTreeScopes);
     visitor->trace(m_fontSelector);
     visitor->trace(m_textToSheetCache);
     visitor->trace(m_sheetToTextCache);
 #endif
     CSSFontSelectorClient::trace(visitor);
 }
 
 } // namespace blink