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diff --git a/gfx/layers/apz/src/SampledAPZCState.h b/gfx/layers/apz/src/SampledAPZCState.h
new file mode 100644
index 0000000000..a521eeaf87
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+++ b/gfx/layers/apz/src/SampledAPZCState.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_layers_SampledAPZCState_h
+#define mozilla_layers_SampledAPZCState_h
+
+#include "FrameMetrics.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/ScrollGeneration.h"
+
+namespace mozilla {
+namespace layers {
+
+class SampledAPZCState {
+ public:
+  SampledAPZCState();
+  explicit SampledAPZCState(const FrameMetrics& aMetrics);
+  SampledAPZCState(const FrameMetrics& aMetrics,
+                   Maybe<CompositionPayload>&& aPayload,
+                   APZScrollGeneration aGeneration);
+
+  bool operator==(const SampledAPZCState& aOther) const;
+  bool operator!=(const SampledAPZCState& aOther) const;
+
+  CSSRect GetLayoutViewport() const { return mLayoutViewport; }
+  CSSPoint GetVisualScrollOffset() const { return mVisualScrollOffset; }
+  CSSToParentLayerScale GetZoom() const { return mZoom; }
+  Maybe<CompositionPayload> TakeScrollPayload();
+  const APZScrollGeneration& Generation() const { return mGeneration; }
+
+  void UpdateScrollProperties(const FrameMetrics& aMetrics);
+  void UpdateScrollPropertiesWithRelativeDelta(const FrameMetrics& aMetrics,
+                                               const CSSPoint& aRelativeDelta);
+
+  void UpdateZoomProperties(const FrameMetrics& aMetrics);
+
+  /**
+   * Re-clamp mVisualScrollOffset to the scroll range specified by the provided
+   * metrics. This only needs to be called if the scroll offset changes
+   * outside of AsyncPanZoomController::SampleCompositedAsyncTransform().
+   * It also recalculates mLayoutViewport so that it continues to enclose
+   * the visual viewport. This only needs to be called if the
+   * layout viewport changes outside of SampleCompositedAsyncTransform().
+   */
+  void ClampVisualScrollOffset(const FrameMetrics& aMetrics);
+
+  void ZoomBy(float aScale);
+
+ private:
+  // These variables cache the layout viewport, scroll offset, and zoom stored
+  // in |Metrics()| at the time this class was constructed.
+  CSSRect mLayoutViewport;
+  CSSPoint mVisualScrollOffset;
+  CSSToParentLayerScale mZoom;
+  // An optional payload that rides along with the sampled state.
+  Maybe<CompositionPayload> mScrollPayload;
+  APZScrollGeneration mGeneration;
+
+  void RemoveFractionalAsyncDelta();
+  // A handy wrapper to call
+  // FrameMetrics::KeepLayoutViewportEnclosingVisualViewport with this
+  // SampledAPZCState and the given |aMetrics|.
+  void KeepLayoutViewportEnclosingVisualViewport(const FrameMetrics& aMetrics);
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_SampledAPZCState_h