Implement PaintLayer::scrollsWithViewport for SPv2.

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

Index: third_party/WebKit/Source/core/paint/PaintLayer.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayer.cpp b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
index 13ad90d19de26da9a0d2cb6928d177f0892fa477..dc1d92cc4b4080470ab9199fa82a76b4c95fed4a 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
@@ -339,11 +339,26 @@ void PaintLayer::dirtyAncestorChainHasSelfPaintingLayerDescendantStatus() {
 }
 
 bool PaintLayer::scrollsWithViewport() const {
-  return (layoutObject()->style()->position() == FixedPosition &&
-          layoutObject()->containerForFixedPosition() ==
-              layoutObject()->view()) ||
-         (layoutObject()->style()->position() == StickyPosition &&
-          !ancestorScrollingLayer());
+  if (layoutObject()->style()->position() == FixedPosition &&

[pdr., 2016/12/06 04:38:51]

Can you move the spv2 codepath outside like so?
if (RuntimeEnabledFeatures::slimmingPaintInvalidationEnabled()) {
  DCHECK(layoutObject()->paintProperties());
  return layoutObject()
              ->paintProperties()
              ->localBorderBoxProperties()
              ->propertyTreeState.scroll() ==
          layoutObject()
              ->view()
              ->paintProperties()
              ->contentsProperties()
              .propertyTreeState.scroll();
} else {
  return (layoutObject()->style()->position() == FixedPosition &&
          layoutObject()->containerForFixedPosition() ==
              layoutObject()->view()) ||
          (layoutObject()->style()->position() == StickyPosition &&
          !ancestorScrollingLayer());
}

[chrishtr, 2016/12/06 23:56:06]

This isn't quite right, because the method should return whether the PaintLayer
sticks to the viewport of its LayoutView.

+      layoutObject()->containerForFixedPosition() == layoutObject()->view())
+    return true;
+  if (layoutObject()->style()->position() == StickyPosition) {
+    if (RuntimeEnabledFeatures::slimmingPaintInvalidationEnabled()) {
+      DCHECK(layoutObject()->paintProperties());
+      return layoutObject()
+                 ->paintProperties()
+                 ->localBorderBoxProperties()
+                 ->propertyTreeState.scroll() ==
+             layoutObject()
+                 ->view()
+                 ->paintProperties()
+                 ->contentsProperties()
+                 .propertyTreeState.scroll();
+    } else {
+      return !ancestorScrollingLayer();
+    }
+  }
+  return false;
 }
 
 bool PaintLayer::scrollsWithRespectTo(const PaintLayer* other) const {