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diff --git a/gfx/layers/apz/src/SimpleVelocityTracker.h b/gfx/layers/apz/src/SimpleVelocityTracker.h
new file mode 100644
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+++ b/gfx/layers/apz/src/SimpleVelocityTracker.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_VelocityTracker_h
+#define mozilla_layers_VelocityTracker_h
+
+#include <utility>
+#include <cstdint>
+
+#include "Axis.h"
+#include "mozilla/Attributes.h"
+#include "nsTArray.h"
+
+namespace mozilla {
+namespace layers {
+
+class SimpleVelocityTracker : public VelocityTracker {
+ public:
+  explicit SimpleVelocityTracker(Axis* aAxis);
+  void StartTracking(ParentLayerCoord aPos, TimeStamp aTimestamp) override;
+  Maybe<float> AddPosition(ParentLayerCoord aPos,
+                           TimeStamp aTimestamp) override;
+  Maybe<float> ComputeVelocity(TimeStamp aTimestamp) override;
+  void Clear() override;
+
+ private:
+  void AddVelocityToQueue(TimeStamp aTimestamp, float aVelocity);
+  float ApplyFlingCurveToVelocity(float aVelocity) const;
+
+  // The Axis that uses this velocity tracker.
+  // This is a raw pointer because the Axis owns the velocity tracker
+  // by UniquePtr, so the velocity tracker cannot outlive the Axis.
+  Axis* MOZ_NON_OWNING_REF mAxis;
+
+  // A queue of (timestamp, velocity) pairs; these are the historical
+  // velocities at the given timestamps. Velocities are in screen pixels per ms.
+  // This member can only be accessed on the controller/UI thread.
+  nsTArray<std::pair<TimeStamp, float>> mVelocityQueue;
+
+  // mVelocitySampleTime and mVelocitySamplePos are the time and position
+  // used in the last velocity sampling. They get updated when a new sample is
+  // taken (which may not happen on every input event, if the time delta is too
+  // small).
+  TimeStamp mVelocitySampleTime;
+  ParentLayerCoord mVelocitySamplePos;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif