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diff --git a/layout/painting/DisplayItemClipChain.h b/layout/painting/DisplayItemClipChain.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 DISPLAYITEMCLIPCHAIN_H_
+#define DISPLAYITEMCLIPCHAIN_H_
+
+#include "mozilla/Assertions.h"
+#include "DisplayItemClip.h"
+#include "nsString.h"
+
+class nsIScrollableFrame;
+
+namespace mozilla {
+
+struct ActiveScrolledRoot;
+
+/**
+ * A DisplayItemClipChain is a linked list of DisplayItemClips where each clip
+ * is associated with an active scrolled root that describes what the clip
+ * moves with.
+ * We use a chain instead of just one intersected clip due to async scrolling:
+ * A clip that moves along with a display item can be fused to the item's
+ * contents when drawing the layer contents, but all other clips in the chain
+ * need to be kept separate so that they can be applied at composition time,
+ * after any async scroll offsets have been applied.
+ * The clip chain is created during display list construction by the builder's
+ * DisplayListClipState.
+ * The clip chain order is determined by the active scrolled root order.
+ * For every DisplayItemClipChain object |clipChain|, the following holds:
+ * !clipChain->mParent ||
+ * ActiveScrolledRoot::IsAncestor(clipChain->mParent->mASR, clipChain->mASR).
+ * The clip chain can skip over active scrolled roots. That just means that
+ * there is no clip that moves with the skipped ASR in this chain.
+ */
+struct DisplayItemClipChain {
+  /**
+   * Get the display item clip in this chain that moves with aASR, or nullptr
+   * if no such clip exists. aClipChain can be null.
+   */
+  static const DisplayItemClip* ClipForASR(
+      const DisplayItemClipChain* aClipChain, const ActiveScrolledRoot* aASR);
+
+  static bool Equal(const DisplayItemClipChain* aClip1,
+                    const DisplayItemClipChain* aClip2);
+  /**
+   * Hash function that returns the same value for any two clips A and B
+   * where Equal(A, B) is true.
+   */
+  static uint32_t Hash(const DisplayItemClipChain* aClip);
+
+  static nsCString ToString(const DisplayItemClipChain* aClipChain);
+
+  bool HasRoundedCorners() const;
+
+  void AddRef() { mRefCount++; }
+  void Release() {
+    MOZ_ASSERT(mRefCount > 0);
+    mRefCount--;
+  }
+
+  DisplayItemClipChain(const DisplayItemClip& aClip,
+                       const ActiveScrolledRoot* aASR,
+                       const DisplayItemClipChain* aParent,
+                       DisplayItemClipChain* aNextClipChainToDestroy)
+      : mClip(aClip),
+        mASR(aASR),
+        mParent(aParent),
+        mNextClipChainToDestroy(aNextClipChainToDestroy)
+#ifdef DEBUG
+        ,
+        mOnStack(true)
+#endif
+  {
+  }
+
+  DisplayItemClipChain()
+      : mASR(nullptr),
+        mNextClipChainToDestroy(nullptr)
+#ifdef DEBUG
+        ,
+        mOnStack(true)
+#endif
+  {
+  }
+
+  DisplayItemClip mClip;
+  const ActiveScrolledRoot* mASR;
+  RefPtr<const DisplayItemClipChain> mParent;
+  uint32_t mRefCount = 0;
+  DisplayItemClipChain* mNextClipChainToDestroy;
+#ifdef DEBUG
+  bool mOnStack;
+#endif
+};
+
+struct DisplayItemClipChainHasher {
+  typedef const DisplayItemClipChain* Key;
+
+  std::size_t operator()(const Key& aKey) const {
+    return DisplayItemClipChain::Hash(aKey);
+  }
+};
+
+struct DisplayItemClipChainEqualer {
+  typedef const DisplayItemClipChain* Key;
+
+  bool operator()(const Key& lhs, const Key& rhs) const {
+    return DisplayItemClipChain::Equal(lhs, rhs);
+  }
+};
+
+}  // namespace mozilla
+
+#endif /* DISPLAYITEMCLIPCHAIN_H_ */