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diff --git a/dom/svg/DOMSVGPathSegList.h b/dom/svg/DOMSVGPathSegList.h
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+++ b/dom/svg/DOMSVGPathSegList.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 DOM_SVG_DOMSVGPATHSEGLIST_H_
+#define DOM_SVG_DOMSVGPATHSEGLIST_H_
+
+#include "mozAutoDocUpdate.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsDebug.h"
+#include "nsTArray.h"
+#include "SVGPathData.h"  // IWYU pragma: keep
+#include "mozilla/Attributes.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/dom/SVGElement.h"
+
+namespace mozilla {
+
+class ErrorResult;
+class SVGAnimatedPathSegList;
+
+namespace dom {
+
+class DOMSVGPathSeg;
+
+//----------------------------------------------------------------------
+// Helper class: AutoChangePathSegListNotifier
+// Stack-based helper class to pair calls to WillChangePathSegList and
+// DidChangePathSegList. Used by DOMSVGPathSeg and DOMSVGPathSegList.
+template <class T>
+class MOZ_RAII AutoChangePathSegListNotifier : public mozAutoDocUpdate {
+ public:
+  explicit AutoChangePathSegListNotifier(T* aValue)
+      : mozAutoDocUpdate(aValue->Element()->GetComposedDoc(), true),
+        mValue(aValue) {
+    MOZ_ASSERT(mValue, "Expecting non-null value");
+    mEmptyOrOldValue = mValue->Element()->WillChangePathSegList(*this);
+  }
+
+  ~AutoChangePathSegListNotifier() {
+    mValue->Element()->DidChangePathSegList(mEmptyOrOldValue, *this);
+    if (mValue->AttrIsAnimating()) {
+      mValue->Element()->AnimationNeedsResample();
+    }
+  }
+
+ private:
+  T* const mValue;
+  nsAttrValue mEmptyOrOldValue;
+};
+
+/**
+ * Class DOMSVGPathSegList
+ *
+ * This class is used to create the DOM tearoff objects that wrap internal
+ * SVGPathData objects.
+ *
+ * See the architecture comment in DOMSVGAnimatedLengthList.h first (that's
+ * LENGTH list), then continue reading the remainder of this comment.
+ *
+ * The architecture of this class is very similar to that of DOMSVGLengthList
+ * except that, since there is no nsIDOMSVGAnimatedPathSegList interface
+ * in SVG, we have no parent DOMSVGAnimatedPathSegList (unlike DOMSVGLengthList
+ * which has a parent DOMSVGAnimatedLengthList class). (There is an
+ * SVGAnimatedPathData interface, but that is quite different to
+ * DOMSVGAnimatedLengthList, since it is inherited by elements rather than
+ * elements having members of that type.) As a consequence, much of the logic
+ * that would otherwise be in DOMSVGAnimatedPathSegList (and is in
+ * DOMSVGAnimatedLengthList) is contained in this class.
+ *
+ * This class is strongly intertwined with DOMSVGPathSeg. Our DOMSVGPathSeg
+ * items are friends of us and responsible for nulling out our pointers to
+ * them when they die.
+ *
+ * Our DOM items are created lazily on demand as and when script requests them.
+ */
+class DOMSVGPathSegList final : public nsISupports, public nsWrapperCache {
+  template <class T>
+  friend class AutoChangePathSegListNotifier;
+  friend class DOMSVGPathSeg;
+
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(DOMSVGPathSegList)
+
+  JSObject* WrapObject(JSContext* cx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  nsISupports* GetParentObject() { return static_cast<nsIContent*>(mElement); }
+
+  /**
+   * Factory method to create and return a DOMSVGPathSegList wrapper
+   * for a given internal SVGPathData object. The factory takes care
+   * of caching the object that it returns so that the same object can be
+   * returned for the given SVGPathData each time it is requested.
+   * The cached object is only removed from the cache when it is destroyed due
+   * to there being no more references to it or to any of its descendant
+   * objects. If that happens, any subsequent call requesting the DOM wrapper
+   * for the SVGPathData will naturally result in a new
+   * DOMSVGPathSegList being returned.
+   *
+   * It's unfortunate that aList is a void* instead of a typed argument. This
+   * is because the mBaseVal and mAnimVal members of SVGAnimatedPathSegList are
+   * of different types - a plain SVGPathData, and a SVGPathData*. We
+   * use the addresses of these members as the key for the hash table, and
+   * clearly SVGPathData* and a SVGPathData** are not the same type.
+   */
+  static already_AddRefed<DOMSVGPathSegList> GetDOMWrapper(
+      void* aList, dom::SVGElement* aElement, bool aIsAnimValList);
+
+  /**
+   * This method returns the DOMSVGPathSegList wrapper for an internal
+   * SVGPathData object if it currently has a wrapper. If it does
+   * not, then nullptr is returned.
+   */
+  static DOMSVGPathSegList* GetDOMWrapperIfExists(void* aList);
+
+  /**
+   * This will normally be the same as InternalList().CountItems(), except if
+   * we've hit OOM, in which case our length will be zero.
+   */
+  uint32_t LengthNoFlush() const {
+    MOZ_ASSERT(
+        mItems.Length() == 0 || mItems.Length() == InternalList().CountItems(),
+        "DOM wrapper's list length is out of sync");
+    return mItems.Length();
+  }
+
+  /**
+   * WATCH OUT! If you add code to call this on a baseVal wrapper, then you
+   * must also call it on the animVal wrapper too if necessary!! See other
+   * callers!
+   *
+   * Called by internal code to notify us when we need to sync the length of
+   * this DOM list with its internal list. This is called immediately prior to
+   * the length of the internal list being changed so that any DOM list items
+   * that need to be removed from the DOM list can first copy their values from
+   * their internal counterpart.
+   *
+   * The only time this method could fail is on OOM when trying to increase the
+   * length of the DOM list. If that happens then this method simply clears the
+   * list and returns. Callers just proceed as normal, and we simply accept
+   * that the DOM list will be empty (until successfully set to a new value).
+   */
+  void InternalListWillChangeTo(const SVGPathData& aNewValue);
+
+  /**
+   * Returns true if our attribute is animating (in which case our animVal is
+   * not simply a mirror of our baseVal).
+   */
+  bool AttrIsAnimating() const;
+  /**
+   * Returns true if there is an animated list mirroring the base list.
+   */
+  bool AnimListMirrorsBaseList() const;
+
+  uint32_t NumberOfItems() const {
+    if (IsAnimValList()) {
+      Element()->FlushAnimations();
+    }
+    return LengthNoFlush();
+  }
+  void Clear(ErrorResult& aError);
+  already_AddRefed<DOMSVGPathSeg> Initialize(DOMSVGPathSeg& aNewItem,
+                                             ErrorResult& aError);
+  already_AddRefed<DOMSVGPathSeg> GetItem(uint32_t index, ErrorResult& error);
+  already_AddRefed<DOMSVGPathSeg> IndexedGetter(uint32_t index, bool& found,
+                                                ErrorResult& error);
+  already_AddRefed<DOMSVGPathSeg> InsertItemBefore(DOMSVGPathSeg& aNewItem,
+                                                   uint32_t aIndex,
+                                                   ErrorResult& aError);
+  already_AddRefed<DOMSVGPathSeg> ReplaceItem(DOMSVGPathSeg& aNewItem,
+                                              uint32_t aIndex,
+                                              ErrorResult& aError);
+  already_AddRefed<DOMSVGPathSeg> RemoveItem(uint32_t aIndex,
+                                             ErrorResult& aError);
+  already_AddRefed<DOMSVGPathSeg> AppendItem(DOMSVGPathSeg& aNewItem,
+                                             ErrorResult& aError) {
+    return InsertItemBefore(aNewItem, LengthNoFlush(), aError);
+  }
+  uint32_t Length() const { return NumberOfItems(); }
+
+ private:
+  /**
+   * Only our static GetDOMWrapper() factory method may create objects of our
+   * type.
+   */
+  DOMSVGPathSegList(dom::SVGElement* aElement, bool aIsAnimValList)
+      : mElement(aElement), mIsAnimValList(aIsAnimValList) {
+    InternalListWillChangeTo(InternalList());  // Sync mItems
+  }
+
+  ~DOMSVGPathSegList();
+
+  dom::SVGElement* Element() const { return mElement.get(); }
+
+  /// Used to determine if this list is the baseVal or animVal list.
+  bool IsAnimValList() const { return mIsAnimValList; }
+
+  /**
+   * Get a reference to this object's corresponding internal SVGPathData.
+   *
+   * To simplify the code we just have this one method for obtaining both
+   * base val and anim val internal lists. This means that anim val lists don't
+   * get const protection, but our setter methods guard against changing
+   * anim val lists.
+   */
+  SVGPathData& InternalList() const;
+
+  SVGAnimatedPathSegList& InternalAList() const;
+
+  /// Creates an instance of the appropriate DOMSVGPathSeg sub-class for
+  // aIndex, if it doesn't already exist, and then returns it.
+  already_AddRefed<DOMSVGPathSeg> GetItemAt(uint32_t aIndex);
+
+  void MaybeInsertNullInAnimValListAt(uint32_t aIndex, uint32_t aInternalIndex,
+                                      uint32_t aArgCountForItem);
+  void MaybeRemoveItemFromAnimValListAt(uint32_t aIndex,
+                                        int32_t aArgCountForItem);
+
+  // Calls UpdateListIndex on all elements in |mItems| that satisfy ItemAt(),
+  // from |aStartingIndex| to the end of |mItems|.  Also adjusts
+  // |mItems.mInternalDataIndex| by the requested amount.
+  void UpdateListIndicesFromIndex(uint32_t aStartingIndex,
+                                  int32_t aInternalDataIndexDelta);
+
+  DOMSVGPathSeg*& ItemAt(uint32_t aIndex) { return mItems[aIndex].mItem; }
+
+  void RemoveFromTearoffTable();
+
+  /**
+   * This struct is used in our array of mItems to provide us with somewhere to
+   * store the indexes into the internal SVGPathData of the internal seg data
+   * that our DOMSVGPathSeg items wrap (the internal segment data is or varying
+   * length, so we can't just use the index of our DOMSVGPathSeg items
+   * themselves). The reason that we have this separate struct rather than
+   * just storing the internal indexes in the DOMSVGPathSeg items is because we
+   * want to create the DOMSVGPathSeg items lazily on demand.
+   */
+  struct ItemProxy {
+    ItemProxy() : mItem(nullptr), mInternalDataIndex(0) {}
+    ItemProxy(DOMSVGPathSeg* aItem, uint32_t aInternalDataIndex)
+        : mItem(aItem), mInternalDataIndex(aInternalDataIndex) {}
+
+    DOMSVGPathSeg* mItem;
+    uint32_t mInternalDataIndex;
+  };
+
+  // Weak refs to our DOMSVGPathSeg items. The items are friends and take care
+  // of clearing our pointer to them when they die.
+  FallibleTArray<ItemProxy> mItems;
+
+  // Strong ref to our element to keep it alive. We hold this not only for
+  // ourself, but also for our DOMSVGPathSeg items too.
+  RefPtr<dom::SVGElement> mElement;
+
+  bool mIsAnimValList;
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // DOM_SVG_DOMSVGPATHSEGLIST_H_