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diff --git a/Source/WebCore/dom/CustomElementReactionQueue.cpp b/Source/WebCore/dom/CustomElementReactionQueue.cpp
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+++ b/Source/WebCore/dom/CustomElementReactionQueue.cpp
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+/*
+ * Copyright (C) 2015, 2016 Apple 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:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
+ * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "config.h"
+#include "CustomElementReactionQueue.h"
+
+#include "CustomElementRegistry.h"
+#include "DOMWindow.h"
+#include "Document.h"
+#include "Element.h"
+#include "HTMLNames.h"
+#include "JSCustomElementInterface.h"
+#include "JSDOMBinding.h"
+#include "Microtasks.h"
+#include <heap/Heap.h>
+#include <wtf/Optional.h>
+#include <wtf/Ref.h>
+#include <wtf/SetForScope.h>
+
+namespace WebCore {
+
+class CustomElementReactionQueueItem {
+public:
+    enum class Type {
+        ElementUpgrade,
+        Connected,
+        Disconnected,
+        Adopted,
+        AttributeChanged,
+    };
+
+    CustomElementReactionQueueItem(Type type)
+        : m_type(type)
+    { }
+
+    CustomElementReactionQueueItem(Document& oldDocument, Document& newDocument)
+        : m_type(Type::Adopted)
+        , m_oldDocument(&oldDocument)
+        , m_newDocument(&newDocument)
+    { }
+
+    CustomElementReactionQueueItem(const QualifiedName& attributeName, const AtomicString& oldValue, const AtomicString& newValue)
+        : m_type(Type::AttributeChanged)
+        , m_attributeName(attributeName)
+        , m_oldValue(oldValue)
+        , m_newValue(newValue)
+    { }
+
+    void invoke(Element& element, JSCustomElementInterface& elementInterface)
+    {
+        switch (m_type) {
+        case Type::ElementUpgrade:
+            elementInterface.upgradeElement(element);
+            break;
+        case Type::Connected:
+            elementInterface.invokeConnectedCallback(element);
+            break;
+        case Type::Disconnected:
+            elementInterface.invokeDisconnectedCallback(element);
+            break;
+        case Type::Adopted:
+            elementInterface.invokeAdoptedCallback(element, *m_oldDocument, *m_newDocument);
+            break;
+        case Type::AttributeChanged:
+            ASSERT(m_attributeName);
+            elementInterface.invokeAttributeChangedCallback(element, m_attributeName.value(), m_oldValue, m_newValue);
+            break;
+        }
+    }
+
+private:
+    Type m_type;
+    RefPtr<Document> m_oldDocument;
+    RefPtr<Document> m_newDocument;
+    std::optional<QualifiedName> m_attributeName;
+    AtomicString m_oldValue;
+    AtomicString m_newValue;
+};
+
+CustomElementReactionQueue::CustomElementReactionQueue(JSCustomElementInterface& elementInterface)
+    : m_interface(elementInterface)
+{ }
+
+CustomElementReactionQueue::~CustomElementReactionQueue()
+{
+    ASSERT(m_items.isEmpty());
+}
+
+void CustomElementReactionQueue::clear()
+{
+    m_items.clear();
+}
+
+void CustomElementReactionQueue::enqueueElementUpgrade(Element& element)
+{
+    auto& queue = CustomElementReactionStack::ensureCurrentQueue(element);
+    queue.m_items.append({CustomElementReactionQueueItem::Type::ElementUpgrade});
+}
+
+void CustomElementReactionQueue::enqueueElementUpgradeIfDefined(Element& element)
+{
+    ASSERT(element.isConnected());
+    ASSERT(element.isCustomElementUpgradeCandidate());
+    auto* window = element.document().domWindow();
+    if (!window)
+        return;
+
+    auto* registry = window->customElementRegistry();
+    if (!registry)
+        return;
+
+    auto* elementInterface = registry->findInterface(element);
+    if (!elementInterface)
+        return;
+
+    element.enqueueToUpgrade(*elementInterface);
+}
+
+void CustomElementReactionQueue::enqueueConnectedCallbackIfNeeded(Element& element)
+{
+    ASSERT(element.isDefinedCustomElement());
+    ASSERT(element.document().refCount() > 0);
+    auto& queue = CustomElementReactionStack::ensureCurrentQueue(element);
+    if (queue.m_interface->hasConnectedCallback())
+        queue.m_items.append({CustomElementReactionQueueItem::Type::Connected});
+}
+
+void CustomElementReactionQueue::enqueueDisconnectedCallbackIfNeeded(Element& element)
+{
+    ASSERT(element.isDefinedCustomElement());
+    if (element.document().refCount() <= 0)
+        return; // Don't enqueue disconnectedCallback if the entire document is getting destructed.
+    auto& queue = CustomElementReactionStack::ensureCurrentQueue(element);
+    if (queue.m_interface->hasDisconnectedCallback())
+        queue.m_items.append({CustomElementReactionQueueItem::Type::Disconnected});
+}
+
+void CustomElementReactionQueue::enqueueAdoptedCallbackIfNeeded(Element& element, Document& oldDocument, Document& newDocument)
+{
+    ASSERT(element.isDefinedCustomElement());
+    ASSERT(element.document().refCount() > 0);
+    auto& queue = CustomElementReactionStack::ensureCurrentQueue(element);
+    if (queue.m_interface->hasAdoptedCallback())
+        queue.m_items.append({oldDocument, newDocument});
+}
+
+void CustomElementReactionQueue::enqueueAttributeChangedCallbackIfNeeded(Element& element, const QualifiedName& attributeName, const AtomicString& oldValue, const AtomicString& newValue)
+{
+    ASSERT(element.isDefinedCustomElement());
+    ASSERT(element.document().refCount() > 0);
+    auto& queue = CustomElementReactionStack::ensureCurrentQueue(element);
+    if (queue.m_interface->observesAttribute(attributeName.localName()))
+        queue.m_items.append({attributeName, oldValue, newValue});
+}
+
+void CustomElementReactionQueue::enqueuePostUpgradeReactions(Element& element)
+{
+    ASSERT(element.isCustomElementUpgradeCandidate());
+    if (!element.hasAttributes() && !element.isConnected())
+        return;
+
+    auto* queue = element.reactionQueue();
+    ASSERT(queue);
+
+    if (element.hasAttributes()) {
+        for (auto& attribute : element.attributesIterator()) {
+            if (queue->m_interface->observesAttribute(attribute.localName()))
+                queue->m_items.append({attribute.name(), nullAtom, attribute.value()});
+        }
+    }
+
+    if (element.isConnected() && queue->m_interface->hasConnectedCallback())
+        queue->m_items.append({CustomElementReactionQueueItem::Type::Connected});
+}
+
+bool CustomElementReactionQueue::observesStyleAttribute() const
+{
+    return m_interface->observesAttribute(HTMLNames::styleAttr.localName());
+}
+
+void CustomElementReactionQueue::invokeAll(Element& element)
+{
+    while (!m_items.isEmpty()) {
+        Vector<CustomElementReactionQueueItem> items = WTFMove(m_items);
+        for (auto& item : items)
+            item.invoke(element, m_interface.get());
+    }
+}
+
+inline void CustomElementReactionStack::ElementQueue::add(Element& element)
+{
+    RELEASE_ASSERT(!m_invoking);
+    // FIXME: Avoid inserting the same element multiple times.
+    m_elements.append(element);
+}
+
+inline void CustomElementReactionStack::ElementQueue::invokeAll()
+{
+    RELEASE_ASSERT(!m_invoking);
+    SetForScope<bool> invoking(m_invoking, true);
+    Vector<Ref<Element>> elements;
+    elements.swap(m_elements);
+    RELEASE_ASSERT(m_elements.isEmpty());
+    for (auto& element : elements) {
+        auto* queue = element->reactionQueue();
+        ASSERT(queue);
+        queue->invokeAll(element.get());
+    }
+    RELEASE_ASSERT(m_elements.isEmpty());
+}
+
+CustomElementReactionQueue& CustomElementReactionStack::ensureCurrentQueue(Element& element)
+{
+    ASSERT(element.reactionQueue());
+    if (!s_currentProcessingStack) {
+        auto& queue = CustomElementReactionStack::ensureBackupQueue();
+        queue.add(element);
+        return *element.reactionQueue();
+    }
+
+    auto*& queue = s_currentProcessingStack->m_queue;
+    if (!queue) // We use a raw pointer to avoid genearing code to delete it in ~CustomElementReactionStack.
+        queue = new ElementQueue;
+    queue->add(element);
+    return *element.reactionQueue();
+}
+
+CustomElementReactionStack* CustomElementReactionStack::s_currentProcessingStack = nullptr;
+
+void CustomElementReactionStack::processQueue()
+{
+    ASSERT(m_queue);
+    m_queue->invokeAll();
+    delete m_queue;
+    m_queue = nullptr;
+}
+
+class BackupElementQueueMicrotask final : public Microtask {
+    WTF_MAKE_FAST_ALLOCATED;
+private:
+    Result run() final
+    {
+        CustomElementReactionStack::processBackupQueue();
+        return Result::Done;
+    }
+};
+
+static bool s_processingBackupElementQueue = false;
+
+CustomElementReactionStack::ElementQueue& CustomElementReactionStack::ensureBackupQueue()
+{
+    if (!s_processingBackupElementQueue) {
+        s_processingBackupElementQueue = true;
+        MicrotaskQueue::mainThreadQueue().append(std::make_unique<BackupElementQueueMicrotask>());
+    }
+    return backupElementQueue();
+}
+
+void CustomElementReactionStack::processBackupQueue()
+{
+    backupElementQueue().invokeAll();
+    s_processingBackupElementQueue = false;
+}
+
+CustomElementReactionStack::ElementQueue& CustomElementReactionStack::backupElementQueue()
+{
+    static NeverDestroyed<ElementQueue> queue;
+    return queue.get();
+}
+
+}