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diff --git a/layout/generic/ScrollAnimationBezierPhysics.h b/layout/generic/ScrollAnimationBezierPhysics.h
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+++ b/layout/generic/ScrollAnimationBezierPhysics.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_layout_ScrollAnimationBezierPhysics_h_
+#define mozilla_layout_ScrollAnimationBezierPhysics_h_
+
+#include "ScrollAnimationPhysics.h"
+#include "mozilla/SMILKeySpline.h"
+
+namespace mozilla {
+
+struct ScrollAnimationBezierPhysicsSettings {
+  // These values are minimum and maximum animation duration per event,
+  // and a global ratio which defines how longer is the animation's duration
+  // compared to the average recent events intervals (such that for a relatively
+  // consistent events rate, the next event arrives before current animation
+  // ends)
+  int32_t mMinMS;
+  int32_t mMaxMS;
+  double mIntervalRatio;
+};
+
+// This class implements a cubic bezier timing function and automatically
+// adapts the animation duration based on the scrolling rate.
+class ScrollAnimationBezierPhysics final : public ScrollAnimationPhysics {
+ public:
+  explicit ScrollAnimationBezierPhysics(
+      const nsPoint& aStartPos,
+      const ScrollAnimationBezierPhysicsSettings& aSettings);
+
+  void Update(const TimeStamp& aTime, const nsPoint& aDestination,
+              const nsSize& aCurrentVelocity) override;
+
+  void ApplyContentShift(const CSSPoint& aShiftDelta) override;
+
+  // Get the velocity at a point in time in nscoords/sec.
+  nsSize VelocityAt(const TimeStamp& aTime) override;
+
+  // Returns the expected scroll position at a given point in time, in app
+  // units, relative to the scroll frame.
+  nsPoint PositionAt(const TimeStamp& aTime) override;
+
+  bool IsFinished(const TimeStamp& aTime) override {
+    return aTime > mStartTime + mDuration;
+  }
+
+ protected:
+  double ProgressAt(const TimeStamp& aTime) const {
+    return clamped((aTime - mStartTime) / mDuration, 0.0, 1.0);
+  }
+
+  nscoord VelocityComponent(double aTimeProgress,
+                            const SMILKeySpline& aTimingFunction,
+                            nscoord aStart, nscoord aDestination) const;
+
+  // Calculate duration, possibly dynamically according to events rate and
+  // event origin. (also maintain previous timestamps - which are only used
+  // here).
+  TimeDuration ComputeDuration(const TimeStamp& aTime);
+
+  // Initializes the timing function in such a way that the current velocity is
+  // preserved.
+  void InitTimingFunction(SMILKeySpline& aTimingFunction, nscoord aCurrentPos,
+                          nscoord aCurrentVelocity, nscoord aDestination);
+
+  // Initialize event history.
+  void InitializeHistory(const TimeStamp& aTime);
+
+  // Cached Preferences values.
+  ScrollAnimationBezierPhysicsSettings mSettings;
+
+  // mPrevEventTime holds previous 3 timestamps for intervals averaging (to
+  // reduce duration fluctuations). When AsyncScroll is constructed and no
+  // previous timestamps are available (indicated with mIsFirstIteration),
+  // initialize mPrevEventTime using imaginary previous timestamps with maximum
+  // relevant intervals between them.
+  TimeStamp mPrevEventTime[3];
+
+  TimeStamp mStartTime;
+
+  nsPoint mStartPos;
+  nsPoint mDestination;
+  TimeDuration mDuration;
+  SMILKeySpline mTimingFunctionX;
+  SMILKeySpline mTimingFunctionY;
+  bool mIsFirstIteration;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_layout_ScrollAnimationBezierPhysics_h_