Defer the real work in updateCompositingLayers until it's really needed.

Source/core/rendering/RenderGeometryMap.cpp

 /*
  * Copyright (C) 2012 Apple Inc. All rights reserved.
  *
  * 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.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 100 matching lines @@
 
     if (!hasFixedPositionStep() && !hasTransformStep() && !hasNonUniformStep() && (!container || (m_mapping.size() && container == m_mapping[0].m_renderer)))
         result = p + roundedIntSize(m_accumulatedOffset);
     else {
         TransformState transformState(TransformState::ApplyTransformDirection, p);
         mapToContainer(transformState, container);
         result = transformState.lastPlanarPoint();
     }
 
 #if !ASSERT_DISABLED
-    FloatPoint rendererMappedResult = m_mapping.last().m_renderer->localToAbsolute(p, m_mapCoordinatesFlags);
-    ASSERT(roundedIntPoint(rendererMappedResult) == roundedIntPoint(result));
-//    if (roundedIntPoint(rendererMappedResult) != roundedIntPoint(result))
-//        fprintf(stderr, "Mismatched point\n");
+    if (m_mapping.size() > 0) {
+        FloatPoint rendererMappedResult = m_mapping.last().m_renderer->localToAbsolute(p, m_mapCoordinatesFlags);
+        ASSERT(roundedIntPoint(rendererMappedResult) == roundedIntPoint(result));
+        //    if (roundedIntPoint(rendererMappedResult) != roundedIntPoint(result))

[jamesr, 2013/10/18 15:56:42]

please remove before landing

+        //        fprintf(stderr, "Mismatched point\n");
+    }
 #endif
 
     return result;
 }
 
 #ifndef NDEBUG
 // Handy function to call from gdb while debugging mismatched point/rect errors.
 void RenderGeometryMap::dumpSteps()
 {
     fprintf(stderr, "RenderGeometryMap::dumpSteps accumulatedOffset=%d,%d\n", m_accumulatedOffset.width().toInt(), m_accumulatedOffset.height().toInt());
@@ skipping 13 matching lines @@
     if (!hasFixedPositionStep() && !hasTransformStep() && !hasNonUniformStep() && (!container || (m_mapping.size() && container == m_mapping[0].m_renderer))) {
         result = rect;
         result.move(m_accumulatedOffset);
     } else {
         TransformState transformState(TransformState::ApplyTransformDirection, rect.center(), rect);
         mapToContainer(transformState, container);
         result = transformState.lastPlanarQuad().boundingBox();
     }
 
 #if !ASSERT_DISABLED
-    const RenderObject* lastRenderer = m_mapping.last().m_renderer;
-    const RenderLayer* layer = lastRenderer->enclosingLayer();
+    if (m_mapping.size() > 0) {
+        const RenderObject* lastRenderer = m_mapping.last().m_renderer;
+        const RenderLayer* layer = lastRenderer->enclosingLayer();
 
-    // Bounds for invisible layers are intentionally not calculated, and are
-    // therefore not necessarily expected to be correct here. This is ok,
-    // because they will be recomputed if the layer becomes visible.
-    if (!layer || !layer->subtreeIsInvisible()) {
-        FloatRect rendererMappedResult = lastRenderer->localToContainerQuad(rect, container, m_mapCoordinatesFlags).boundingBox();
+        // Bounds for invisible layers are intentionally not calculated, and are
+        // therefore not necessarily expected to be correct here. This is ok,
+        // because they will be recomputed if the layer becomes visible.
+        if (!layer || !layer->subtreeIsInvisible()) {
+            FloatRect rendererMappedResult = lastRenderer->localToContainerQuad(rect, container, m_mapCoordinatesFlags).boundingBox();
 
-        // Inspector creates renderers with negative width <https://bugs.webkit.org/show_bug.cgi?id=87194>.
-        // Taking FloatQuad bounds avoids spurious assertions because of that.
-        ASSERT(enclosingIntRect(rendererMappedResult) == enclosingIntRect(FloatQuad(result).boundingBox()));
-//        if (enclosingIntRect(rendererMappedResult) != enclosingIntRect(FloatQuad(result).boundingBox()))
-//            fprintf(stderr, "Mismatched rects\n");
+            // Inspector creates renderers with negative width <https://bugs.webkit.org/show_bug.cgi?id=87194>.
+            // Taking FloatQuad bounds avoids spurious assertions because of that.
+            ASSERT(enclosingIntRect(rendererMappedResult) == enclosingIntRect(FloatQuad(result).boundingBox()));
+            //        if (enclosingIntRect(rendererMappedResult) != enclosingIntRect(FloatQuad(result).boundingBox()))

[jamesr, 2013/10/18 15:56:42]

ditto

+            //            fprintf(stderr, "Mismatched rects\n");
+        }
     }
 #endif
 
     return result;
 }
 
 void RenderGeometryMap::pushMappingsToAncestor(const RenderObject* renderer, const RenderLayerModelObject* ancestorRenderer)
 {
     // We need to push mappings in reverse order here, so do insertions rather than appends.
     TemporaryChange<size_t> positionChange(m_insertionPosition, m_mapping.size());
@@ skipping 148 matching lines @@
     // "topmost" (to preserve original behavior).
     if (!(m_mapCoordinatesFlags & TraverseDocumentBoundaries))
         return true;
 
     Frame* thisFrame = renderer->frame();
     return thisFrame == thisFrame->page()->mainFrame();
 }
 #endif
 
 } // namespace WebCore