Make compositingState explicit

Source/core/page/scrolling/ScrollingCoordinator.cpp

Index: Source/core/page/scrolling/ScrollingCoordinator.cpp
diff --git a/Source/core/page/scrolling/ScrollingCoordinator.cpp b/Source/core/page/scrolling/ScrollingCoordinator.cpp
index 5df38d44cb0b49ccb0765c54baab7613979707fd..a214e26622c85ddd9a548f5fa5edc00ec9f6b13c 100644
--- a/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -158,7 +158,7 @@ static void clearPositionConstraintExceptForLayer(GraphicsLayer* layer, Graphics
 
 static WebLayerPositionConstraint computePositionConstraint(const RenderLayer* layer)
 {
-    ASSERT(layer->isComposited());
+    ASSERT(layer->backing());
     do {
         if (layer->renderer()->style()->position() == FixedPosition) {
             const RenderObject* fixedPositionObject = layer->renderer();
@@ -168,7 +168,9 @@ static WebLayerPositionConstraint computePositionConstraint(const RenderLayer* l
         }
 
         layer = layer->parent();
-    } while (layer && !layer->isComposited());
+
+        // This walk ends once we found the first layer that has its own backing.
+    } while (layer && (layer->compositingState() == PaintsIntoOwnBacking || layer->compositingState() == HasOwnBackingButPaintsIntoAncestor));

[Julien - ping for review, 2013/10/03 20:08:36]

Couldn't we use backing() here instead to be consistent with the ASSERT above?

[shawnsingh, 2013/10/07 11:45:04]

Actually yes, I am reverting this back to using backing()  (actually
compositedLayerMapping(), the new accessor).  The reason, however, is not just
to be consistent with the ASSERT.  I've actually convinced myself that, for now,
in cases where compositingState is queried for the sake of computing
positioning, usually they care whether the compositedLayerMapping() exists or
not.  They don't care whether the layer paints into ancestor or paints into own
backing.

I'm looking forward to fixing this over time.  As you've suggested, eventually
no code should use the compositedLayerBacking() accessor as a boolean condition
unless they really are dealing with allocating/deallocating it.  For now, a lot
of usage still exists for that.

     return WebLayerPositionConstraint();
 }
 
@@ -805,12 +807,12 @@ bool ScrollingCoordinator::hasVisibleSlowRepaintViewportConstrainedObjects(Frame
             return true;
         RenderLayer* layer = toRenderBoxModelObject(viewportConstrainedObject)->layer();
         // Any explicit reason that a fixed position element is not composited shouldn't cause slow scrolling.
-        if (!layer->isComposited() && layer->viewportConstrainedNotCompositedReason() == RenderLayer::NoNotCompositedReason)
+        if (layer->compositingState() != PaintsIntoOwnBacking && layer->viewportConstrainedNotCompositedReason() == RenderLayer::NoNotCompositedReason)
             return true;
 
         // Composited layers that actually paint into their enclosing ancestor
         // must also force main thread scrolling.
-        if (layer->isComposited() && layer->backing()->paintsIntoCompositedAncestor())
+        if (layer->compositingState() == HasOwnBackingButPaintsIntoAncestor)
             return true;
     }
     return false;