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

Source/core/rendering/RenderGeometryMap.cpp

Index: Source/core/rendering/RenderGeometryMap.cpp
diff --git a/Source/core/rendering/RenderGeometryMap.cpp b/Source/core/rendering/RenderGeometryMap.cpp
index 6ff1b2404786f91daefa842e5770ebe9a672fff1..f2625d2d8782a1447e6be3838f03c9011794d929 100644
--- a/Source/core/rendering/RenderGeometryMap.cpp
+++ b/Source/core/rendering/RenderGeometryMap.cpp
@@ -118,10 +118,12 @@ FloatPoint RenderGeometryMap::mapToContainer(const FloatPoint& p, const RenderLa
     }
 
 #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;
@@ -155,20 +157,22 @@ FloatQuad RenderGeometryMap::mapToContainer(const FloatRect& rect, const RenderL
     }
 
 #if !ASSERT_DISABLED
-    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();
-
-        // 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");
+    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();
+
+            // 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