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diff --git a/dom/base/IDTracker.h b/dom/base/IDTracker.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_IDTracker_h_
+#define mozilla_dom_IDTracker_h_
+
+#include "mozilla/Attributes.h"
+#include "nsIObserver.h"
+#include "nsThreadUtils.h"
+
+class nsAtom;
+class nsIContent;
+class nsINode;
+class nsIURI;
+class nsIReferrerInfo;
+
+namespace mozilla::dom {
+
+class Document;
+class DocumentOrShadowRoot;
+class Element;
+
+/**
+ * Class to track what element is referenced by a given ID.
+ *
+ * To use it, call one of the Reset methods to set it up to watch a given ID.
+ * Call get() anytime to determine the referenced element (which may be null if
+ * the element isn't found). When the element changes, ElementChanged
+ * will be called, so subclass this class if you want to receive that
+ * notification. ElementChanged runs at safe-for-script time, i.e. outside
+ * of the content update. Call Unlink() if you want to stop watching
+ * for changes (get() will then return null).
+ *
+ * By default this is a single-shot tracker --- i.e., when ElementChanged
+ * fires, we will automatically stop tracking. get() will continue to return
+ * the changed-to element.
+ * Override IsPersistent to return true if you want to keep tracking after
+ * the first change.
+ */
+class IDTracker {
+ public:
+  using Element = mozilla::dom::Element;
+
+  IDTracker();
+
+  ~IDTracker();
+
+  /**
+   * Find which element, if any, is referenced.
+   */
+  Element* get() { return mElement; }
+
+  /**
+   * Set up the reference. This can be called multiple times to
+   * change which reference is being tracked, but these changes
+   * do not trigger ElementChanged.
+   * @param aFrom the source element for context
+   * @param aURI the URI containing a hash-reference to the element
+   * @param aReferrerInfo the referrerInfo for loading external resource
+   * @param aWatch if false, then we do not set up the notifications to track
+   * changes, so ElementChanged won't fire and get() will always return the same
+   * value, the current element for the ID.
+   * @param aReferenceImage whether the ID references image elements which are
+   * subject to the document's mozSetImageElement overriding mechanism.
+   */
+  void ResetToURIFragmentID(nsIContent* aFrom, nsIURI* aURI,
+                            nsIReferrerInfo* aReferrerInfo, bool aWatch = true,
+                            bool aReferenceImage = false);
+
+  /**
+   * A variation on ResetToURIFragmentID() to set up a reference that consists
+   * of the ID of an element in the same document as aFrom.
+   * @param aFrom the source element for context
+   * @param aID the ID of the element
+   * @param aWatch if false, then we do not set up the notifications to track
+   * changes, so ElementChanged won't fire and get() will always return the same
+   * value, the current element for the ID.
+   */
+  void ResetWithID(Element& aFrom, nsAtom* aID, bool aWatch = true);
+
+  /**
+   * Clears the reference. ElementChanged is not triggered. get() will return
+   * null.
+   */
+  void Unlink();
+
+  void Traverse(nsCycleCollectionTraversalCallback* aCB);
+
+ protected:
+  /**
+   * Override this to be notified of element changes. Don't forget
+   * to call this superclass method to change mElement. This is called
+   * at script-runnable time.
+   */
+  virtual void ElementChanged(Element* aFrom, Element* aTo);
+
+  /**
+   * Override this to convert from a single-shot notification to
+   * a persistent notification.
+   */
+  virtual bool IsPersistent() { return false; }
+
+  /**
+   * Set ourselves up with our new document.  Note that aDocument might be
+   * null.  Either aWatch must be false or aRef must be empty.
+   */
+  void HaveNewDocumentOrShadowRoot(DocumentOrShadowRoot*, bool aWatch,
+                                   const nsString& aRef);
+
+ private:
+  static bool Observe(Element* aOldElement, Element* aNewElement, void* aData);
+
+  class Notification : public nsISupports {
+   public:
+    virtual void SetTo(Element* aTo) = 0;
+    virtual void Clear() { mTarget = nullptr; }
+    virtual ~Notification() = default;
+
+   protected:
+    explicit Notification(IDTracker* aTarget) : mTarget(aTarget) {
+      MOZ_ASSERT(aTarget, "Must have a target");
+    }
+    IDTracker* mTarget;
+  };
+
+  class ChangeNotification : public mozilla::Runnable, public Notification {
+   public:
+    ChangeNotification(IDTracker* aTarget, Element* aFrom, Element* aTo);
+
+    // We need to actually declare all of nsISupports, because
+    // Notification inherits from it but doesn't declare it.
+    NS_DECL_ISUPPORTS_INHERITED
+    NS_IMETHOD Run() override {
+      if (mTarget) {
+        mTarget->mPendingNotification = nullptr;
+        mTarget->ElementChanged(mFrom, mTo);
+      }
+      return NS_OK;
+    }
+    void SetTo(Element* aTo) override;
+    void Clear() override;
+
+   protected:
+    virtual ~ChangeNotification();
+
+    RefPtr<Element> mFrom;
+    RefPtr<Element> mTo;
+  };
+  friend class ChangeNotification;
+
+  class DocumentLoadNotification : public Notification, public nsIObserver {
+   public:
+    DocumentLoadNotification(IDTracker* aTarget, const nsString& aRef)
+        : Notification(aTarget) {
+      if (!mTarget->IsPersistent()) {
+        mRef = aRef;
+      }
+    }
+
+    NS_DECL_ISUPPORTS
+    NS_DECL_NSIOBSERVER
+   private:
+    virtual ~DocumentLoadNotification() = default;
+
+    virtual void SetTo(Element* aTo) override {}
+
+    nsString mRef;
+  };
+  friend class DocumentLoadNotification;
+
+  DocumentOrShadowRoot* GetWatchDocOrShadowRoot() const;
+
+  RefPtr<nsAtom> mWatchID;
+  nsCOMPtr<nsINode>
+      mWatchDocumentOrShadowRoot;  // Always a `DocumentOrShadowRoot`.
+  RefPtr<Element> mElement;
+  RefPtr<Notification> mPendingNotification;
+  bool mReferencingImage = false;
+};
+
+inline void ImplCycleCollectionUnlink(IDTracker& aField) { aField.Unlink(); }
+
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback, IDTracker& aField,
+    const char* aName, uint32_t aFlags = 0) {
+  aField.Traverse(&aCallback);
+}
+
+}  // namespace mozilla::dom
+
+#endif /* mozilla_dom_IDTracker_h_ */