Only the main document should have a StyleEngine.

sky/engine/core/dom/Document.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 274 matching lines @@
     // See fast/dom/early-frame-url.html
     // and fast/dom/location-new-window-no-crash.html, respectively.
     // FIXME: Can/should we unify this behavior?
     if (initializer.shouldSetURL())
         setURL(initializer.url());
 
     InspectorCounters::incrementCounter(InspectorCounters::DocumentCounter);
 
     m_lifecycle.advanceTo(DocumentLifecycle::Inactive);
 
-    // Since CSSFontSelector requires Document::m_fetcher and StyleEngine owns
-    // CSSFontSelector, need to initialize m_styleEngine after initializing
-    // m_fetcher.
-    m_styleEngine = StyleEngine::create(*this);
-
 #ifndef NDEBUG
     liveDocumentSet().add(this);
 #endif
 }
 
 Document::~Document()
 {
     ASSERT(!renderView());
     ASSERT(!parentTreeScope());
 #if !ENABLE(OILPAN)
@@ skipping 913 matching lines @@
 
 PassRefPtr<RenderStyle> Document::styleForElementIgnoringPendingStylesheets(Element* element)
 {
     ASSERT_ARG(element, element->document() == this);
     StyleEngine::IgnoringPendingStylesheet ignoring(m_styleEngine.get());
     return ensureStyleResolver().styleForElement(element, element->parentNode() ? element->parentNode()->computedStyle() : 0);
 }
 
 StyleResolver* Document::styleResolver() const
 {
+    if (!isActive())
+        return 0;
     return m_styleEngine->resolver();
 }
 
 StyleResolver& Document::ensureStyleResolver() const
 {
+    ASSERT(isActive());
     return m_styleEngine->ensureResolver();
 }
 
 void Document::clearStyleResolver()
 {
     m_styleEngine->clearResolver();
 }
 
 void Document::attach(const AttachContext& context)
 {
     ASSERT(m_lifecycle.state() == DocumentLifecycle::Inactive);
 
+    m_styleEngine = StyleEngine::create(*this);
+
     m_renderView = new RenderView(this);
     setRenderer(m_renderView);
 
     m_renderView->setStyle(StyleResolver::styleForDocument(*this));
 
     ContainerNode::attach(context);
 
     m_lifecycle.advanceTo(DocumentLifecycle::StyleClean);
 }
 
@@ skipping 17 matching lines @@
         m_domWindow->clearEventQueue();
 
     m_hoverNode = nullptr;
     m_focusedElement = nullptr;
     m_activeHoverElement = nullptr;
 
     m_renderView = 0;
     ContainerNode::detach(context);
 
     m_styleEngine->didDetach();
+    m_styleEngine = nullptr;
 
     // This is required, as our LocalFrame might delete itself as soon as it detaches
     // us. However, this violates Node::detach() semantics, as it's never
     // possible to re-attach. Eventually Document::detach() should be renamed,
     // or this setting of the frame to 0 could be made explicit in each of the
     // callers of Document::detach().
     m_frame = 0;
 
     if (m_mediaQueryMatcher)
         m_mediaQueryMatcher->documentDetached();
@@ skipping 1469 matching lines @@
 using namespace blink;
 void showLiveDocumentInstances()
 {
     WeakDocumentSet& set = liveDocumentSet();
     fprintf(stderr, "There are %u documents currently alive:\n", set.size());
     for (WeakDocumentSet::const_iterator it = set.begin(); it != set.end(); ++it) {
         fprintf(stderr, "- Document %p URL: %s\n", *it, (*it)->url().string().utf8().data());
     }
 }
 #endif