Handle nested position:sticky elements correctly (compositor)

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp

Index: third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
index b7ed656d74a80e1f7942872ad72f305004e7f4b1..3a45d32f557d11809bd9ac9b1efce44f08174e3f 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -173,6 +173,27 @@ static ScrollingCoordinator* scrollingCoordinatorFromLayer(PaintLayer& layer) {
   return (!page) ? nullptr : page->scrollingCoordinator();
 }
 
+static WebLayer* nearestCompositedStickyLayer(
+    const StickyConstraintsMap& constraintsMap,
+    LayoutBoxModelObject* obj) {
+  while (obj && !obj->layer()->compositedLayerMapping()) {
+    DCHECK(constraintsMap.contains(obj->layer()));
+    const StickyPositionScrollingConstraints constraints =
+        constraintsMap.at(obj->layer());
+    obj = constraints.nearestStickyBoxShiftingStickyBox()
+              ? constraints.nearestStickyBoxShiftingStickyBox()
+              : constraints.nearestStickyBoxShiftingContainingBlock();
+  }
+
+  if (!obj)
+    return nullptr;
+
+  return obj->layer()
+      ->compositedLayerMapping()
+      ->mainGraphicsLayer()
+      ->platformLayer();
+}
+
 CompositedLayerMapping::CompositedLayerMapping(PaintLayer& layer)
     : m_owningLayer(layer),
       m_contentOffsetInCompositingLayerDirty(false),
@@ -308,9 +329,10 @@ void CompositedLayerMapping::updateStickyConstraints(
 
   WebLayerStickyPositionConstraint webConstraint;
   if (sticky) {
+    const StickyConstraintsMap& constraintsMap =
+        ancestorOverflowLayer->getScrollableArea()->stickyConstraintsMap();
     const StickyPositionScrollingConstraints& constraints =
-        ancestorOverflowLayer->getScrollableArea()->stickyConstraintsMap().at(
-            &m_owningLayer);
+        constraintsMap.at(&m_owningLayer);
 
     // Find the layout offset of the unshifted sticky box within its
     // compositingContainer. If the enclosing layer is not the scroller, then
@@ -353,7 +375,13 @@ void CompositedLayerMapping::updateStickyConstraints(
         enclosingIntRect(constraints.scrollContainerRelativeStickyBoxRect());
     webConstraint.scrollContainerRelativeContainingBlockRect = enclosingIntRect(
         constraints.scrollContainerRelativeContainingBlockRect());
-    // TODO(smcgruer): Copy fields for nested sticky in cc (crbug.com/672710)
+    webConstraint.nearestLayerShiftingStickyBox = nearestCompositedStickyLayer(
+        constraintsMap, constraints.nearestStickyBoxShiftingStickyBox());
+    webConstraint.nearestLayerShiftingContainingBlock =
+        nearestCompositedStickyLayer(
+            constraintsMap,
+            constraints.nearestStickyBoxShiftingContainingBlock());
+    webConstraint.localStickyOffset = constraints.getLocalStickyOffset();
   }
 
   m_graphicsLayer->setStickyPositionConstraint(webConstraint);