Check slot's distribution state more strictly

third_party/WebKit/Source/core/html/HTMLSlotElement.h

 /*
  * Copyright (C) 2015 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 38 matching lines @@
 
     Node* firstDistributedNode() const { return m_distributedNodes.isEmpty() ? nullptr : m_distributedNodes.first().get(); }
     Node* lastDistributedNode() const { return m_distributedNodes.isEmpty() ? nullptr : m_distributedNodes.last().get(); }
 
     Node* distributedNodeNextTo(const Node&) const;
     Node* distributedNodePreviousTo(const Node&) const;
 
     void appendAssignedNode(Node&);
     void appendDistributedNode(Node&);
     void appendDistributedNodesFrom(const HTMLSlotElement& other);
-    void clearDistribution();
+    void willUpdateDistribution();

[kochi, 2016/03/04 03:49:07]

nit: Looking at usages, I'd think renaming
willUpdateDistribution/didUpdateDistribution to
startUpdateDistribution()/finishUpdateDistribution()) is better, but it's up to
you.

 
     bool hasSlotChangeEventListener();
 
     void updateDistributedNodesWithFallback();
     void didUpdateDistribution();
 
     void attach(const AttachContext& = AttachContext()) final;
     void detach(const AttachContext& = AttachContext()) final;
 
     void attributeChanged(const QualifiedName&, const AtomicString& oldValue, const AtomicString& newValue, AttributeModificationReason = ModifiedDirectly) final;
 
     DECLARE_VIRTUAL_TRACE();
 
 private:
     HTMLSlotElement(Document&);
 
     enum DistributionState {
-        DistributionReset,
+        DistributionOnGoing,
+        DistributionDone,
         DistributionChanged,
         DistributionUnchanged
     };
 
+    void clearDistribution();
     void childrenChanged(const ChildrenChange&) final;
     InsertionNotificationRequest insertedInto(ContainerNode*) final;
     void removedFrom(ContainerNode*) final;
     void willRecalcStyle(StyleRecalcChange) final;
 
     void dispatchSlotChangeEvent();
     bool distributionChanged();
 
     WillBeHeapVector<RefPtrWillBeMember<Node>> m_assignedNodes;
     WillBeHeapVector<RefPtrWillBeMember<Node>> m_distributedNodes;
     WillBeHeapHashMap<RawPtrWillBeMember<const Node>, size_t> m_distributedIndices;
     // TODO(hayato): Remove m_oldDistibutedNodes and make SlotAssignment check the diffirence between old and new distributed nodes for each slot to save the memories.
     WillBeHeapVector<RefPtrWillBeMember<Node>> m_oldDistributedNodes;
     DistributionState m_distributionState;
 };
 
 } // namespace blink
 
 #endif // HTMLSlotElement_h