Avoid compositing fixed-position elements if they cannot scroll

Source/WebCore/rendering/RenderLayerCompositor.cpp

Index: Source/WebCore/rendering/RenderLayerCompositor.cpp
diff --git a/Source/WebCore/rendering/RenderLayerCompositor.cpp b/Source/WebCore/rendering/RenderLayerCompositor.cpp
index c9cebfc32df8bbf05334b91d62137c49d51517b6..70d34483c9c08dda1c86ba65cda6c9cb6c8f8b56 100644
--- a/Source/WebCore/rendering/RenderLayerCompositor.cpp
+++ b/Source/WebCore/rendering/RenderLayerCompositor.cpp
@@ -1976,7 +1976,27 @@ bool RenderLayerCompositor::requiresCompositingForPosition(RenderObject* rendere
             *viewportConstrainedNotCompositedReason = RenderLayer::NotCompositedForNonViewContainer;
         return false;
     }
-    
+
+    // If the fixed-position element does not have any scrollable ancestor between it and its container,
+    // then we do not need to spend compositor resources for it.
+    if (container->isRenderView()) {
+
+        RenderLayer* ancestor = layer->parent();
+        bool noScrollableAncestor = true;
+
+        if (m_renderView->frameView() && (m_renderView->frameView()->isScrollable()))
+            noScrollableAncestor = false;

[trchen, 2013/04/10 01:24:03]

I'm concerned about the platforms that use impl-side pinch zoom (Android &
CrOS). FrameView::isScrollable becomes a dynamic property. In that case I think
we should always create composited layers for RenderView-contained
fixed-position elements.

+
+        while (ancestor && noScrollableAncestor) {
+            if (ancestor->hasHorizontalScrollbar() || ancestor->hasVerticalScrollbar())
+                noScrollableAncestor = false;
+            ancestor = ancestor->parent();
+        }
+
+        if (noScrollableAncestor)
+            return false;
+    }
+
     // Subsequent tests depend on layout. If we can't tell now, just keep things the way they are until layout is done.
     if (!m_inPostLayoutUpdate) {
         m_reevaluateCompositingAfterLayout = true;