Handle nested position:sticky elements

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 7a0159a8b36d08f32e2bcdd9fb541b1edf20374b..a1bf0fe936ed90d9d4dc102e31f15a56e4a8c049 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -169,6 +169,27 @@ static ScrollingCoordinator* scrollingCoordinatorFromLayer(PaintLayer& layer) {
   return (!page) ? nullptr : page->scrollingCoordinator();
 }
 
+static WebLayer* nearestCompositedStickyLayer(
+    const StickyConstraintsMap& constraintsMap,
+    LayoutBoxModelObject* obj) {
+  while (obj && obj->compositingState() == NotComposited) {
+    DCHECK(constraintsMap.contains(obj->layer()));
+    const StickyPositionScrollingConstraints constraints =
+        constraintsMap.get(obj->layer());
+    obj = constraints.nearestStickyElementShiftingStickyBox()
+              ? constraints.nearestStickyElementShiftingStickyBox()
+              : constraints.nearestStickyElementShiftingContainingBlock();
+  }
+
+  if (!obj)
+    return nullptr;
+
+  return obj->layer()
+      ->compositedLayerMapping()
+      ->mainGraphicsLayer()
+      ->platformLayer();
+}
+
 CompositedLayerMapping::CompositedLayerMapping(PaintLayer& layer)
     : m_owningLayer(layer),
       m_contentOffsetInCompositingLayerDirty(false),
@@ -302,9 +323,10 @@ void CompositedLayerMapping::updateStickyConstraints(
 
   WebLayerStickyPositionConstraint webConstraint;
   if (sticky) {
+    const StickyConstraintsMap& constraintsMap =
+        ancestorOverflowLayer->getScrollableArea()->stickyConstraintsMap();
     const StickyPositionScrollingConstraints& constraints =
-        ancestorOverflowLayer->getScrollableArea()->stickyConstraintsMap().get(
-            &m_owningLayer);
+        constraintsMap.get(&m_owningLayer);
 
     // Find the layout offset of the unshifted sticky box within its enclosing
     // layer.
@@ -312,6 +334,7 @@ void CompositedLayerMapping::updateStickyConstraints(
     m_owningLayer.enclosingLayerWithCompositedLayerMapping(ExcludeSelf)
         ->convertToLayerCoords(m_owningLayer.ancestorOverflowLayer(),
                                enclosingLayerOffset);
+
     FloatPoint stickyBoxOffset =
         constraints.scrollContainerRelativeStickyBoxRect().location();
     DCHECK(!m_contentOffsetInCompositingLayerDirty);
@@ -341,6 +364,14 @@ void CompositedLayerMapping::updateStickyConstraints(
         enclosingIntRect(constraints.scrollContainerRelativeStickyBoxRect());
     webConstraint.scrollContainerRelativeContainingBlockRect = enclosingIntRect(
         constraints.scrollContainerRelativeContainingBlockRect());
+    webConstraint.nearestLayerShiftingStickyBox = nearestCompositedStickyLayer(
+        constraintsMap, constraints.nearestStickyElementShiftingStickyBox());
+    webConstraint.nearestLayerShiftingContainingBlock =
+        nearestCompositedStickyLayer(
+            constraintsMap,
+            constraints.nearestStickyElementShiftingContainingBlock());
+    webConstraint.cachedComputedStickyOffset =
+        constraints.getCachedComputedStickyOffset();
   }
 
   m_graphicsLayer->setStickyPositionConstraint(webConstraint);