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..9d2a3f9de0df62a9bd1ed4ab50fe1693356e3b98 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();
+  PaintLayer* nonConstLayer = const_cast<PaintLayer*>(layer);
+  DCHECK(scrollableArea->stickyConstraintsMap().contains(nonConstLayer));
+
+  return layer->ancestorOverflowLayer()->scrollsOverflow() &&
+         !scrollableArea->stickyConstraintsMap()
+              .get(nonConstLayer)

[flackr, 2017/02/24 01:33:33]

nit: I think it'd be better to do the const conversion inline so as to emphasize
that it is only used for the map lookup.

[smcgruer, 2017/02/24 14:17:43]

Done.

+              .hasAncestorStickyElement();
 }
 
 }  // namespace blink