Mark elements as viewport constrained when they become sticky positioned

third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp

Index: third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
index 2b869f79db28311e900a4ba41ad5071f9a8bebc6..d3d8fa84f720773a5fa467e22b063d8ef5711f54 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
@@ -399,6 +399,8 @@ void LayoutBoxModelObject::styleDidChange(StyleDifference diff,
         if (layer())
           layer()->setNeedsCompositingInputsUpdate();
 
+        frameView->addViewportConstrainedObject(this);

[flackr, 2017/02/21 00:38:14]

There's a call to add sticky position items to the set of viewport constrained
objects which happens during the compositing inputs update. Can you check why
the sticky element isn't getting added then? Doing it here is incorrect as we
don't know whether it is contained by an overflow scroller yet.

[smcgruer, 2017/02/21 15:50:05]

Done.

+
         // TODO(pdr): When slimming paint v2 is enabled, we will need to
         // invalidate the scroll paint property subtree for this so main thread
         // scroll reasons are recomputed.