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 cabe639395f150f56f4b3e5a65cd40df4a5f5db7..225c434063713d46f9cda7c32fb5f9a49123bf03 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 +168,10 @@ static WebLayerPositionConstraint computePositionConstraint(const RenderLayer* l
         }
 
         layer = layer->parent();
-    } while (layer && !layer->isComposited());
+        // FIXME: should this walk end at the first layer with any backing? Or should it end at the
+        //        first composited layer that does not paint into its ancestor? And what about for
+        //        squashed layers? For now, keeping this code equivalent to what it has been already.

[Ian Vollick, 2013/10/01 19:46:22]

My interpretation of this function:

If any layer contributing to |layer|'s backing is fixed, return the appropriate
constraint, else no constraint.

If that's right, should the stopping condition be layer &&
layer->compositingState() != NotComposited ?

[shawnsingh, 2013/10/03 03:04:04]

I'm not sure if your phrasing means the same thing - to me, this function says
"return the position constraint, if it exists, of any fixed position layer that
lies between |layer| and the nearest ancestor backing."  That way the composited
layer gets fixed correctly.

So after thinking over it, I think the condition you're proposing is almost
correct, except that we should also exclude squashing layers when walking up to
find a fixed-position layer.  If we don't do that, then we run into issues where
a composited layer is positioned with respect to a squashed layer that it is not
a descendant of in the layer hierarchy.

+    } while (layer && !layer->backing());
     return WebLayerPositionConstraint();
 }
 
@@ -805,12 +808,12 @@ 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;