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diff --git a/gfx/layers/ScrollableLayerGuid.h b/gfx/layers/ScrollableLayerGuid.h
new file mode 100644
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+++ b/gfx/layers/ScrollableLayerGuid.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 GFX_SCROLLABLELAYERGUID_H
+#define GFX_SCROLLABLELAYERGUID_H
+
+#include <iosfwd>                        // for ostream
+#include <stdint.h>                      // for uint8_t, uint32_t, uint64_t
+#include "mozilla/layers/LayersTypes.h"  // for LayersId
+#include "nsHashKeys.h"                  // for nsUint64HashKey
+
+namespace mozilla {
+namespace layers {
+
+/**
+ * This class allows us to uniquely identify a scrollable layer. The
+ * mLayersId identifies the layer tree (corresponding to a child process
+ * and/or tab) that the scrollable layer belongs to. The mPresShellId
+ * is a temporal identifier (corresponding to the document loaded that
+ * contains the scrollable layer, which may change over time). The
+ * mScrollId corresponds to the actual frame that is scrollable.
+ */
+struct ScrollableLayerGuid {
+  // We use IDs to identify frames across processes.
+  typedef uint64_t ViewID;
+  typedef nsUint64HashKey ViewIDHashKey;
+  static const ViewID NULL_SCROLL_ID;  // This container layer does not scroll.
+  static const ViewID START_SCROLL_ID = 2;  // This is the ID that scrolling
+                                            // subframes will begin at.
+
+  LayersId mLayersId;
+  uint32_t mPresShellId;
+  ViewID mScrollId;
+
+  ScrollableLayerGuid();
+
+  ScrollableLayerGuid(LayersId aLayersId, uint32_t aPresShellId,
+                      ViewID aScrollId);
+
+  ScrollableLayerGuid(const ScrollableLayerGuid& other) = default;
+
+  ~ScrollableLayerGuid() = default;
+
+  bool operator==(const ScrollableLayerGuid& other) const;
+  bool operator!=(const ScrollableLayerGuid& other) const;
+  bool operator<(const ScrollableLayerGuid& other) const;
+
+  friend std::ostream& operator<<(std::ostream& aOut,
+                                  const ScrollableLayerGuid& aGuid);
+
+  using Comparator = bool (*)(const ScrollableLayerGuid&,
+                              const ScrollableLayerGuid&);
+
+  static bool EqualsIgnoringPresShell(const ScrollableLayerGuid& aA,
+                                      const ScrollableLayerGuid& aB);
+
+  // Helper structs to use as hash/equality functions in std::unordered_map.
+  // e.g. std::unordered_map<ScrollableLayerGuid,
+  //                    ValueType,
+  //                    ScrollableLayerGuid::HashFn> myMap;
+  // std::unordered_map<ScrollableLayerGuid,
+  //                    ValueType,
+  //                    ScrollableLayerGuid::HashIgnoringPresShellFn,
+  //                    ScrollableLayerGuid::EqualIgnoringPresShellFn> myMap;
+
+  struct HashFn {
+    std::size_t operator()(const ScrollableLayerGuid& aGuid) const;
+  };
+
+  struct HashIgnoringPresShellFn {
+    std::size_t operator()(const ScrollableLayerGuid& aGuid) const;
+  };
+
+  struct EqualIgnoringPresShellFn {
+    bool operator()(const ScrollableLayerGuid& lhs,
+                    const ScrollableLayerGuid& rhs) const;
+  };
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif /* GFX_SCROLLABLELAYERGUID_H */