Handle nested position:sticky elements correctly (main thread)

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

Index: third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
index d9d504bf573b63b0e753f78cd7ab3c20467d2b50..a241925f30d7005a06a2797d6c20320d986afadc 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositingReasonFinder.cpp
@@ -231,8 +231,21 @@ bool CompositingReasonFinder::requiresCompositingForScrollDependentPosition(
   // container rather than the enclosing frame.
   if (layer->sticksToViewport())
     return m_layoutView.frameView()->isScrollable();
-  return layer->layoutObject().style()->position() == EPosition::kSticky &&
-         layer->ancestorOverflowLayer()->scrollsOverflow();
+
+  if (layer->layoutObject().style()->position() != EPosition::kSticky)
+    return false;
+
+  // Don't promote nested sticky elements; the compositor can't handle them.
+  // TODO(smcgruer): Add cc nested sticky support (http://crbug.com/672710)
+  PaintLayerScrollableArea* scrollableArea =
+      layer->ancestorOverflowLayer()->getScrollableArea();
+  DCHECK(scrollableArea->stickyConstraintsMap().contains(
+      const_cast<PaintLayer*>(layer)));
+
+  return layer->ancestorOverflowLayer()->scrollsOverflow() &&
+         !scrollableArea->stickyConstraintsMap()
+              .at(const_cast<PaintLayer*>(layer))
+              .hasAncestorStickyElement();
 }
 
 }  // namespace blink