Don't force layout for mouse event hit tests

Source/core/page/EventHandler.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2012 Digia Plc. and/or its subsidiary(-ies)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 921 matching lines @@
             FrameView* mainView = mainFrame->view();
             if (frameView && mainView) {
                 IntPoint mainFramePoint = mainView->rootViewToContents(frameView->contentsToRootView(roundedIntPoint(point)));
                 return mainFrame->eventHandler()->hitTestResultAtPoint(mainFramePoint, hitType, padding);
             }
         }
     }
 
     HitTestResult result(point, padding.height(), padding.width(), padding.height(), padding.width());
 
-    if (!m_frame->contentRenderer())
+    // 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 Document::prepareMouseEvent.
+    if (!m_frame->contentRenderer() || !m_frame->view() || !m_frame->view()->didFirstLayout())
         return result;
 
     // hitTestResultAtPoint is specifically used to hitTest into all frames, thus it always allows child frame content.
     HitTestRequest request(hitType | HitTestRequest::AllowChildFrameContent);
     m_frame->contentRenderer()->hitTest(request, result);
     if (!request.readOnly())
         m_frame->document()->updateHoverActiveState(request, result.innerElement());
 
     if (request.disallowsShadowContent())
         result.setToNodesInDocumentTreeScope();
@@ skipping 2967 matching lines @@
 unsigned EventHandler::accessKeyModifiers()
 {
 #if OS(DARWIN)
     return PlatformEvent::CtrlKey | PlatformEvent::AltKey;
 #else
     return PlatformEvent::AltKey;
 #endif
 }
 
 } // namespace WebCore