Implement the algorithm to test merging and overlap in PaintArtifactCompositor.

third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/paint/PaintLayerPainter.h"
 
 #include "core/frame/LocalFrame.h"
 #include "core/layout/LayoutView.h"
 #include "core/paint/ClipPathClipper.h"
 #include "core/paint/FilterPainter.h"
@@ skipping 258 matching lines @@
         context.getPaintController().currentPaintChunkProperties());
     auto& localBorderBoxProperties =
         *objectPaintProperties->localBorderBoxProperties();
     properties.transform =
         localBorderBoxProperties.propertyTreeState.transform();
     properties.scroll = localBorderBoxProperties.propertyTreeState.scroll();
     properties.clip = localBorderBoxProperties.propertyTreeState.clip();
     properties.effect = localBorderBoxProperties.propertyTreeState.effect();
     properties.backfaceHidden =
         m_paintLayer.layoutObject()->hasHiddenBackface();
+    properties.willChangeTransform =
+        m_paintLayer.layoutObject()->styleRef().hasWillChangeCompositingHint();
+
     scopedPaintChunkProperties.emplace(context.getPaintController(),
                                        m_paintLayer, properties);
   }
 
   DCHECK(m_paintLayer.isSelfPaintingLayer() ||
          m_paintLayer.hasSelfPaintingLayerDescendant());
   DCHECK(!(paintFlags & PaintLayerAppliedTransform));
 
   bool isSelfPaintingLayer = m_paintLayer.isSelfPaintingLayer();
   bool isPaintingOverlayScrollbars =
@@ skipping 192 matching lines @@
           context.getPaintController().currentPaintChunkProperties());
       auto& localBorderBoxProperties =
           *objectPaintProperties->localBorderBoxProperties();
       properties.transform =
           localBorderBoxProperties.propertyTreeState.transform();
       properties.scroll = localBorderBoxProperties.propertyTreeState.scroll();
       properties.clip = localBorderBoxProperties.propertyTreeState.clip();
       properties.effect = localBorderBoxProperties.propertyTreeState.effect();
       properties.backfaceHidden =
           m_paintLayer.layoutObject()->hasHiddenBackface();
+      properties.willChangeTransform = m_paintLayer.layoutObject()

[trchen, 2016/12/13 22:46:01]

I think this is not quite right. For example:

<div style="will-change:transform;">
  <div style="position:absolute; z-index:0; overflow:hidden;">Foo</div>
</div>

The "Foo" element should be promoted to a cc layer for the will-change hint, but
this code won't propagate the compositing hint from its parent.

Also simple propagation won't work, for example:

<div style="will-change:transform;">
  <div style="position:absolute; z-index:0; overflow:hidden;">Foo</div>
  <div style="position:absolute; z-index:0; overflow:hidden;">Bar</div>
</div>

Both 'Foo' and 'Bar' want a cc layer other than their grand parent (not shown in
the example), but they certainly can share a layer.

This is why in your doc I said I'm not certain about the concept of "chunk
compositing reason". The only thing I can think of is foreign layer chunk. Other
than that all compositing reasons are from property nodes.

I would model it this way:
A transform node with 3D transform, will-change hint, animation, scrolling, blah
blah blah gets direct compositing reason. A transform node that has direct
compositing reason mark its parent edge. Two chunks can't be merged if the path
between their transform nodes contains marked edge.

+                                           ->styleRef()
+                                           .hasWillChangeCompositingHint();
+
       contentScopedPaintChunkProperties.emplace(context.getPaintController(),
                                                 m_paintLayer, properties);
     }
 
     bool isPaintingRootLayer = (&m_paintLayer) == paintingInfo.rootLayer;
     bool shouldPaintBackground =
         shouldPaintContent && !selectionOnly &&
         (isPaintingCompositedBackground ||
          (isPaintingRootLayer &&
           !(paintFlags & PaintLayerPaintingSkipRootBackground)));
@@ skipping 622 matching lines @@
 
   PaintLayerPaintingInfo paintingInfo(&m_paintLayer,
                                       LayoutRect(enclosingIntRect(damageRect)),
                                       paintFlags, LayoutSize());
   paintLayer(context, paintingInfo, PaintLayerPaintingOverlayScrollbars);
 
   m_paintLayer.setContainsDirtyOverlayScrollbars(false);
 }
 
 }  // namespace blink