Do not block painting for in-body CSS

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

 /*
  * Copyright (C) 2006, 2007 Rob Buis
  * Copyright (C) 2008 Apple, Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 29 matching lines @@
 
 static bool isCSS(Element* element, const AtomicString& type)
 {
     return type.isEmpty() || (element->isHTMLElement() ? equalIgnoringCase(type, "text/css") : (type == "text/css"));
 }
 
 StyleElement::StyleElement(Document* document, bool createdByParser)
     : m_createdByParser(createdByParser)
     , m_loading(false)
     , m_registeredAsCandidate(false)
+    , m_beforeBody(false)
     , m_startPosition(TextPosition::belowRangePosition())
 {
     if (createdByParser && document && document->scriptableDocumentParser() && !document->isInDocumentWrite())
         m_startPosition = document->scriptableDocumentParser()->textPosition();
 }
 
 StyleElement::~StyleElement()
 {
 }
 
@@ skipping 79 matching lines @@
     if (!element || !element->inShadowIncludingDocument())
         return ProcessingSuccessful;
     return createSheet(element, element->textFromChildren());
 }
 
 void StyleElement::clearSheet(Element* ownerElement)
 {
     DCHECK(m_sheet);
 
     if (ownerElement && m_sheet->isLoading())
-        ownerElement->document().styleEngine().removePendingSheet(ownerElement);
+        ownerElement->document().styleEngine().removePendingSheet(ownerElement, m_beforeBody);
 
     m_sheet.release()->clearOwnerNode();
 }
 
 static bool shouldBypassMainWorldCSP(Element* element)
 {
     // Main world CSP is bypassed within an isolated world.
     LocalFrame* frame = element->document().frame();
     if (frame && frame->script().shouldBypassMainWorldCSP())
         return true;
@@ skipping 24 matching lines @@
     // If type is empty or CSS, this is a CSS style sheet.
     const AtomicString& type = this->type();
     if (isCSS(e, type) && passesContentSecurityPolicyChecks) {
         MediaQuerySet* mediaQueries = MediaQuerySet::create(media());
 
         MediaQueryEvaluator screenEval("screen", true);
         MediaQueryEvaluator printEval("print", true);
         if (screenEval.eval(mediaQueries) || printEval.eval(mediaQueries)) {
             m_loading = true;
             TextPosition startPosition = m_startPosition == TextPosition::belowRangePosition() ? TextPosition::minimumPosition() : m_startPosition;
-            newSheet = document.styleEngine().createSheet(e, text, startPosition);
+            m_beforeBody = m_beforeBody || !document.body();

[esprehn, 2016/05/06 21:50:15]

Can we move this into the StyleEngine instead?

[Pat Meenan, 2016/05/09 18:51:05]

The main issue is that we need to keep track of if the sheet was originally
created/requested before the body so when it finishes loading and we remove it
we also decrement the "before body" counter.  The style engine could handle the
part about when it is created but when the sheet is removed it doesn't have the
context to know (at least AFAIK).

It would be possible if the parser didn't keep going after the sheet is created
but as it stands right now, the parser can be before the body when a sheet is
created and after the body when it finishes so the state needed to be attached
to the sheet itself.

+            newSheet = document.styleEngine().createSheet(e, text, startPosition, m_beforeBody);
             newSheet->setMediaQueries(mediaQueries);
             m_loading = false;
         }
     }
 
     if (m_sheet)
         clearSheet(e);
 
     m_sheet = newSheet;
     if (m_sheet)
         m_sheet->contents()->checkLoaded();
 
     return passesContentSecurityPolicyChecks ? ProcessingSuccessful : ProcessingFatalError;
 }
 
 bool StyleElement::isLoading() const
 {
     if (m_loading)
         return true;
     return m_sheet ? m_sheet->isLoading() : false;
 }
 
 bool StyleElement::sheetLoaded(Document& document)
 {
     if (isLoading())
         return false;
 
-    document.styleEngine().removePendingSheet(m_sheet->ownerNode());
+    document.styleEngine().removePendingSheet(m_sheet->ownerNode(), m_beforeBody);
     return true;
 }
 
 void StyleElement::startLoadingDynamicSheet(Document& document)
 {
-    document.styleEngine().addPendingSheet();
+    m_beforeBody = m_beforeBody || !document.body();
+    document.styleEngine().addPendingSheet(m_beforeBody);
 }
 
 DEFINE_TRACE(StyleElement)
 {
     visitor->trace(m_sheet);
 }
 
 } // namespace blink