Make compositingState explicit

Source/core/page/scrolling/ScrollingCoordinator.cpp

 /*
  * Copyright (C) 2011 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 140 matching lines @@
 }
 
 static void clearPositionConstraintExceptForLayer(GraphicsLayer* layer, GraphicsLayer* except)
 {
     if (layer && layer != except && scrollingWebLayerForGraphicsLayer(layer))
         scrollingWebLayerForGraphicsLayer(layer)->setPositionConstraint(WebLayerPositionConstraint());
 }
 
 static WebLayerPositionConstraint computePositionConstraint(const RenderLayer* layer)
 {
-    ASSERT(layer->isComposited());
+    ASSERT(layer->backing());
     do {
         if (layer->renderer()->style()->position() == FixedPosition) {
             const RenderObject* fixedPositionObject = layer->renderer();
             bool fixedToRight = !fixedPositionObject->style()->right().isAuto();
             bool fixedToBottom = !fixedPositionObject->style()->bottom().isAuto();
             return WebLayerPositionConstraint::fixedPosition(fixedToRight, fixedToBottom);
         }
 
         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();
 }
 
 void ScrollingCoordinator::updateLayerPositionConstraint(RenderLayer* layer)
 {
     ASSERT(layer->backing());
     RenderLayerBacking* backing = layer->backing();
     GraphicsLayer* mainLayer = backing->childForSuperlayers();
 
     // Avoid unnecessary commits
@@ skipping 616 matching lines @@
     const FrameView::ViewportConstrainedObjectSet* viewportConstrainedObjects = frameView->viewportConstrainedObjects();
     if (!viewportConstrainedObjects)
         return false;
 
     for (FrameView::ViewportConstrainedObjectSet::const_iterator it = viewportConstrainedObjects->begin(), end = viewportConstrainedObjects->end(); it != end; ++it) {
         RenderObject* viewportConstrainedObject = *it;
         if (!viewportConstrainedObject->isBoxModelObject() || !viewportConstrainedObject->hasLayer())
             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;
 }
 
 MainThreadScrollingReasons ScrollingCoordinator::mainThreadScrollingReasons() const
 {
     // The main thread scrolling reasons are applicable to scrolls of the main
     // frame. If it does not exist or if it is not scrollable, there is no
     // reason to force main thread scrolling.
@@ skipping 31 matching lines @@
         stringBuilder.resize(stringBuilder.length() - 2);
     return stringBuilder.toString();
 }
 
 String ScrollingCoordinator::mainThreadScrollingReasonsAsText() const
 {
     return mainThreadScrollingReasonsAsText(mainThreadScrollingReasons());
 }
 
 } // namespace WebCore