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diff --git a/dom/base/DocumentOrShadowRoot.h b/dom/base/DocumentOrShadowRoot.h
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+++ b/dom/base/DocumentOrShadowRoot.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_DocumentOrShadowRoot_h__
+#define mozilla_dom_DocumentOrShadowRoot_h__
+
+#include "mozilla/dom/NameSpaceConstants.h"
+#include "mozilla/IdentifierMapEntry.h"
+#include "mozilla/RelativeTo.h"
+#include "mozilla/ReverseIterator.h"
+#include "nsClassHashtable.h"
+#include "nsContentListDeclarations.h"
+#include "nsTArray.h"
+#include "nsTHashSet.h"
+
+class nsContentList;
+class nsCycleCollectionTraversalCallback;
+class nsINode;
+class nsINodeList;
+class nsWindowSizes;
+
+namespace mozilla {
+class ErrorResult;
+class StyleSheet;
+class ErrorResult;
+
+namespace dom {
+
+class Animation;
+class Element;
+class Document;
+class DocumentOrShadowRoot;
+class HTMLInputElement;
+class StyleSheetList;
+class ShadowRoot;
+template <typename T>
+class Sequence;
+
+/**
+ * A class meant to be shared by ShadowRoot and Document, that holds a list of
+ * stylesheets.
+ *
+ * TODO(emilio, bug 1418159): In the future this should hold most of the
+ * relevant style state, this should allow us to fix bug 548397.
+ */
+class DocumentOrShadowRoot {
+  enum class Kind {
+    Document,
+    ShadowRoot,
+  };
+
+ public:
+  // These should always be non-null, but can't use a reference because
+  // dereferencing `this` on initializer lists is UB, apparently, see
+  // bug 1596499.
+  explicit DocumentOrShadowRoot(Document*);
+  explicit DocumentOrShadowRoot(ShadowRoot*);
+
+  // Unusual argument naming is because of cycle collection macros.
+  static void Traverse(DocumentOrShadowRoot* tmp,
+                       nsCycleCollectionTraversalCallback& cb);
+  static void Unlink(DocumentOrShadowRoot* tmp);
+
+  nsINode& AsNode() { return *mAsNode; }
+
+  const nsINode& AsNode() const { return *mAsNode; }
+
+  StyleSheet* SheetAt(size_t aIndex) const {
+    return mStyleSheets.SafeElementAt(aIndex);
+  }
+
+  size_t SheetCount() const { return mStyleSheets.Length(); }
+
+  const nsTArray<RefPtr<StyleSheet>>& AdoptedStyleSheets() const {
+    return mAdoptedStyleSheets;
+  }
+
+  /**
+   * Returns an index for the sheet in relative style order.
+   * If there are non-applicable sheets, then this index may
+   * not match 1:1 with the sheet's actual index in the style set.
+   *
+   * Handles sheets from both mStyleSheets and mAdoptedStyleSheets
+   */
+  int32_t StyleOrderIndexOfSheet(const StyleSheet& aSheet) const;
+
+  StyleSheetList* StyleSheets();
+
+  void RemoveStyleSheet(StyleSheet&);
+
+  Element* GetElementById(const nsAString& aElementId) const;
+  Element* GetElementById(nsAtom* aElementId) const;
+
+  /**
+   * This method returns _all_ the elements in this scope which have id
+   * aElementId, if there are any.  Otherwise it returns null.
+   *
+   * This is useful for stuff like QuerySelector optimization and such.
+   */
+  const nsTArray<Element*>* GetAllElementsForId(
+      const IdentifierMapEntry::DependentAtomOrString& aElementId) const {
+    IdentifierMapEntry* entry = mIdentifierMap.GetEntry(aElementId);
+    return entry ? &entry->GetIdElements() : nullptr;
+  }
+
+  already_AddRefed<nsContentList> GetElementsByTagName(
+      const nsAString& aTagName) {
+    return NS_GetContentList(&AsNode(), kNameSpaceID_Unknown, aTagName);
+  }
+
+  already_AddRefed<nsContentList> GetElementsByTagNameNS(
+      const nsAString& aNamespaceURI, const nsAString& aLocalName);
+
+  already_AddRefed<nsContentList> GetElementsByTagNameNS(
+      const nsAString& aNamespaceURI, const nsAString& aLocalName,
+      mozilla::ErrorResult&);
+
+  already_AddRefed<nsContentList> GetElementsByClassName(
+      const nsAString& aClasses);
+
+  ~DocumentOrShadowRoot();
+
+  Element* GetPointerLockElement();
+  Element* GetFullscreenElement() const;
+
+  Element* ElementFromPoint(float aX, float aY);
+  nsINode* NodeFromPoint(float aX, float aY);
+
+  void ElementsFromPoint(float aX, float aY, nsTArray<RefPtr<Element>>&);
+  void NodesFromPoint(float aX, float aY, nsTArray<RefPtr<nsINode>>&);
+
+  /**
+   * Helper for elementFromPoint implementation that allows
+   * ignoring the scroll frame and/or avoiding layout flushes.
+   *
+   * @see nsIDOMWindowUtils::elementFromPoint
+   */
+  Element* ElementFromPointHelper(float aX, float aY,
+                                  bool aIgnoreRootScrollFrame,
+                                  bool aFlushLayout,
+                                  ViewportType aViewportType);
+
+  void NodesFromRect(float aX, float aY, float aTopSize, float aRightSize,
+                     float aBottomSize, float aLeftSize,
+                     bool aIgnoreRootScrollFrame, bool aFlushLayout,
+                     bool aOnlyVisible, float aVisibleThreshold,
+                     nsTArray<RefPtr<nsINode>>&);
+
+  /**
+   * This gets fired when the element that an id refers to changes.
+   * This fires at difficult times. It is generally not safe to do anything
+   * which could modify the DOM in any way. Use
+   * nsContentUtils::AddScriptRunner.
+   * @return true to keep the callback in the callback set, false
+   * to remove it.
+   */
+  typedef bool (*IDTargetObserver)(Element* aOldElement, Element* aNewelement,
+                                   void* aData);
+
+  /**
+   * Add an IDTargetObserver for a specific ID. The IDTargetObserver
+   * will be fired whenever the content associated with the ID changes
+   * in the future. If aForImage is true, mozSetImageElement can override
+   * what content is associated with the ID. In that case the IDTargetObserver
+   * will be notified at those times when the result of LookupImageElement
+   * changes.
+   * At most one (aObserver, aData, aForImage) triple can be
+   * registered for each ID.
+   * @return the content currently associated with the ID.
+   */
+  Element* AddIDTargetObserver(nsAtom* aID, IDTargetObserver aObserver,
+                               void* aData, bool aForImage);
+
+  /**
+   * Remove the (aObserver, aData, aForImage) triple for a specific ID, if
+   * registered.
+   */
+  void RemoveIDTargetObserver(nsAtom* aID, IDTargetObserver aObserver,
+                              void* aData, bool aForImage);
+
+  /**
+   * Lookup an image element using its associated ID, which is usually provided
+   * by |-moz-element()|. Similar to GetElementById, with the difference that
+   * elements set using mozSetImageElement have higher priority.
+   * @param aId the ID associated the element we want to lookup
+   * @return the element associated with |aId|
+   */
+  Element* LookupImageElement(const nsAString& aElementId);
+
+  /**
+   * Check that aId is not empty and log a message to the console
+   * service if it is.
+   * @returns true if aId looks correct, false otherwise.
+   */
+  inline bool CheckGetElementByIdArg(const nsAString& aId) {
+    if (aId.IsEmpty()) {
+      ReportEmptyGetElementByIdArg();
+      return false;
+    }
+    return true;
+  }
+
+  void ReportEmptyGetElementByIdArg() const;
+
+  // Web Animations
+  MOZ_CAN_RUN_SCRIPT
+  void GetAnimations(nsTArray<RefPtr<Animation>>& aAnimations);
+
+  nsINode* Retarget(nsINode*) const;
+
+  void OnSetAdoptedStyleSheets(StyleSheet&, uint32_t aIndex, ErrorResult&);
+  void OnDeleteAdoptedStyleSheets(StyleSheet&, uint32_t aIndex, ErrorResult&);
+
+  // This is needed because ServoStyleSet / ServoAuthorData don't deal with
+  // duplicate stylesheets (and it's unclear we'd want to support that as it'd
+  // be a bunch of duplicate work), while adopted stylesheets do need to deal
+  // with them.
+  template <typename Callback>
+  void EnumerateUniqueAdoptedStyleSheetsBackToFront(Callback aCallback) {
+    StyleSheetSet set(mAdoptedStyleSheets.Length());
+    for (StyleSheet* sheet : Reversed(mAdoptedStyleSheets)) {
+      if (MOZ_UNLIKELY(!set.EnsureInserted(sheet))) {
+        continue;
+      }
+      aCallback(*sheet);
+    }
+  }
+
+ protected:
+  // Cycle collection helper functions
+  void TraverseSheetRefInStylesIfApplicable(
+      StyleSheet&, nsCycleCollectionTraversalCallback&);
+  void TraverseStyleSheets(nsTArray<RefPtr<StyleSheet>>&, const char*,
+                           nsCycleCollectionTraversalCallback&);
+  void UnlinkStyleSheets(nsTArray<RefPtr<StyleSheet>>&);
+
+  using StyleSheetSet = nsTHashSet<const StyleSheet*>;
+  void RemoveSheetFromStylesIfApplicable(StyleSheet&);
+  void ClearAdoptedStyleSheets();
+
+  /**
+   * Clone's the argument's adopted style sheets into this.
+   * This should only be used when cloning a static document for printing.
+   */
+  void CloneAdoptedSheetsFrom(const DocumentOrShadowRoot&);
+
+  void InsertSheetAt(size_t aIndex, StyleSheet& aSheet);
+
+  void AddSizeOfExcludingThis(nsWindowSizes&) const;
+  void AddSizeOfOwnedSheetArrayExcludingThis(
+      nsWindowSizes&, const nsTArray<RefPtr<StyleSheet>>&) const;
+
+  /**
+   * If focused element's subtree root is this document or shadow root, return
+   * focused element, otherwise, get the shadow host recursively until the
+   * shadow host's subtree root is this document or shadow root.
+   */
+  Element* GetRetargetedFocusedElement();
+
+  nsTArray<RefPtr<StyleSheet>> mStyleSheets;
+  RefPtr<StyleSheetList> mDOMStyleSheets;
+
+  /**
+   * Style sheets that are adopted by assinging to the `adoptedStyleSheets`
+   * WebIDL atribute. These can only be constructed stylesheets.
+   */
+  nsTArray<RefPtr<StyleSheet>> mAdoptedStyleSheets;
+
+  /*
+   * mIdentifierMap works as follows for IDs:
+   * 1) Attribute changes affect the table immediately (removing and adding
+   *    entries as needed).
+   * 2) Removals from the DOM affect the table immediately
+   * 3) Additions to the DOM always update existing entries for names, and add
+   *    new ones for IDs.
+   */
+  nsTHashtable<IdentifierMapEntry> mIdentifierMap;
+
+  // Always non-null, see comment in the constructor as to why a pointer instead
+  // of a reference.
+  nsINode* mAsNode;
+  const Kind mKind;
+};
+
+}  // namespace dom
+
+}  // namespace mozilla
+
+#endif