Don't force layout for mouse event hit tests

Source/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 2782 matching lines @@
     else if (equalIgnoringCase(policy, "always"))
         m_referrerPolicy = ReferrerPolicyAlways;
     else if (equalIgnoringCase(policy, "origin"))
         m_referrerPolicy = ReferrerPolicyOrigin;
 }
 
 MouseEventWithHitTestResults Document::prepareMouseEvent(const HitTestRequest& request, const LayoutPoint& documentPoint, const PlatformMouseEvent& event)
 {
     ASSERT(!renderer() || renderer()->isRenderView());
 
-    if (!renderer())
+    // RenderView::hitTest causes a layout, and we don't want to hit that until the first
+    // layout because until then, there is nothing shown on the screen - the user can't
+    // have intentionally clicked on something belonging to this page. Furthermore,
+    // mousemove events before the first layout should not lead to a premature layout()
+    // happening, which could show a flash of white.
+    // See also the similar code in EventHandler::hitTestResultAtPoint.
+    if (!renderer() || !view() || !view()->didFirstLayout())
         return MouseEventWithHitTestResults(event, HitTestResult(LayoutPoint()));
 
     HitTestResult result(documentPoint);
     renderView()->hitTest(request, result);
 
     if (!request.readOnly())
         updateHoverActiveState(request, result.innerElement());
 
     return MouseEventWithHitTestResults(event, result);
 }
@@ skipping 2786 matching lines @@
         return;
 
     Vector<RefPtr<Element> > associatedFormControls;
     copyToVector(m_associatedFormControls, associatedFormControls);
 
     frame()->page()->chrome()->client()->didAssociateFormControls(associatedFormControls);
     m_associatedFormControls.clear();
 }
 
 } // namespace WebCore