Implement lazy block layout prototype behind a runtime flag.

Source/core/rendering/RenderLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 81 matching lines @@
 #include "core/rendering/HitTestRequest.h"
 #include "core/rendering/HitTestResult.h"
 #include "core/rendering/HitTestingTransformState.h"
 #include "core/rendering/OverlapTestRequestClient.h"
 #include "core/rendering/RenderArena.h"
 #include "core/rendering/RenderFlowThread.h"
 #include "core/rendering/RenderGeometryMap.h"
 #include "core/rendering/RenderInline.h"
 #include "core/rendering/RenderLayerBacking.h"
 #include "core/rendering/RenderLayerCompositor.h"
+#include "core/rendering/RenderLazyBlock.h"
 #include "core/rendering/RenderMarquee.h"
 #include "core/rendering/RenderReplica.h"
 #include "core/rendering/RenderScrollbar.h"
 #include "core/rendering/RenderScrollbarPart.h"
 #include "core/rendering/RenderTheme.h"
 #include "core/rendering/RenderTreeAsText.h"
 #include "core/rendering/RenderView.h"
 #include "core/rendering/svg/RenderSVGResourceClipper.h"
 #include <wtf/MemoryInstrumentationVector.h>
 #include <wtf/StdLibExtras.h>
@@ skipping 2221 matching lines @@
     // We don't update compositing layers, because we need to do a deep update from the compositing ancestor.
     bool inLayout = view ? view->frameView()->isInLayout() : false;
     if (!inLayout) {
         // If we're in the middle of layout, we'll just update layers once layout has finished.
         updateLayerPositionsAfterOverflowScroll();
         if (view) {
             // Update regions, scrolling may change the clip of a particular region.
             view->frameView()->updateAnnotatedRegions();
 
             view->updateWidgetPositions();
+
+            view->markLazyBlocksForLayout();

[ojan, 2013/04/26 00:41:16]

Doesn't not doing the layout here mean that paint might get stale output?

[esprehn, 2013/04/26 00:45:44]

I don't think it would be any worse than if you got ahead of the scroll. We'll
never be asked to paint() without being asked to layout() first from what I can
tell.

jamesr might know better.

[jamesr, 2013/04/26 00:58:03]

layout() is always called on before painting in the software and compositing
paths (via WebViewImpl::layout()). this does a top-level layout, but it doesn't
have any sort of geometry information currently. we just assume after calling
layout() it's safe to paint() anything anywhere in the render tree. we'll
obviously have to refine that down the road

         }
 
         if (!m_updatingMarqueePosition) {
             // Avoid updating compositing layers if, higher on the stack, we're already updating layer
             // positions. Updating layer positions requires a full walk of up-to-date RenderLayers, and
             // in this case we're still updating their positions; we'll update compositing layers later
             // when that completes.
             updateCompositingLayersAfterScroll();
         }
     }
@@ skipping 4204 matching lines @@
     }
 }
 
 void showLayerTree(const WebCore::RenderObject* renderer)
 {
     if (!renderer)
         return;
     showLayerTree(renderer->enclosingLayer());
 }
 #endif