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diff --git a/gfx/layers/apz/src/AutoDirWheelDeltaAdjuster.h b/gfx/layers/apz/src/AutoDirWheelDeltaAdjuster.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_AutoDirWheelDeltaAdjuster_h__
+#define __mozilla_layers_AutoDirWheelDeltaAdjuster_h__
+
+#include "Axis.h"                         // for AxisX, AxisY, Side
+#include "mozilla/WheelHandlingHelper.h"  // for AutoDirWheelDeltaAdjuster
+
+namespace mozilla {
+namespace layers {
+
+/**
+ * About AutoDirWheelDeltaAdjuster:
+ * For an AutoDir wheel scroll, there's some situations where we should adjust a
+ * wheel event's delta values. AutoDirWheelDeltaAdjuster converts delta values
+ * for AutoDir scrolling. An AutoDir wheel scroll lets the user scroll a frame
+ * with only one scrollbar, using either a vertical or a horzizontal wheel.
+ * For more detail about the concept of AutoDir scrolling, see the comments in
+ * AutoDirWheelDeltaAdjuster.
+ *
+ * This is the APZ implementation of AutoDirWheelDeltaAdjuster.
+ */
+class MOZ_STACK_CLASS APZAutoDirWheelDeltaAdjuster final
+    : public AutoDirWheelDeltaAdjuster {
+ public:
+  /**
+   * @param aDeltaX            DeltaX for a wheel event whose delta values will
+   *                           be adjusted upon calling adjust() when
+   *                           ShouldBeAdjusted() returns true.
+   * @param aDeltaY            DeltaY for a wheel event, like DeltaX.
+   * @param aAxisX             The X axis information provider for the current
+   *                           frame, such as whether the frame can be scrolled
+   *                           horizontally, leftwards or rightwards.
+   * @param aAxisY             The Y axis information provider for the current
+   *                           frame, such as whether the frame can be scrolled
+   *                           vertically, upwards or downwards.
+   * @param aIsHorizontalContentRightToLeft
+   *                           Indicates whether the horizontal content starts
+   *                           at rightside. This value will decide which edge
+   *                           the adjusted scroll goes towards, in other words,
+   *                           it will decide the sign of the adjusted delta
+   *                           values). For detailed information, see
+   *                           IsHorizontalContentRightToLeft() in
+   *                           the base class AutoDirWheelDeltaAdjuster.
+   */
+  APZAutoDirWheelDeltaAdjuster(double& aDeltaX, double& aDeltaY,
+                               const AxisX& aAxisX, const AxisY& aAxisY,
+                               bool aIsHorizontalContentRightToLeft)
+      : AutoDirWheelDeltaAdjuster(aDeltaX, aDeltaY),
+        mAxisX(aAxisX),
+        mAxisY(aAxisY),
+        mIsHorizontalContentRightToLeft(aIsHorizontalContentRightToLeft) {}
+
+ private:
+  virtual bool CanScrollAlongXAxis() const override {
+    return mAxisX.CanScroll();
+  }
+  virtual bool CanScrollAlongYAxis() const override {
+    return mAxisY.CanScroll();
+  }
+  virtual bool CanScrollUpwards() const override {
+    return mAxisY.CanScrollTo(eSideTop);
+  }
+  virtual bool CanScrollDownwards() const override {
+    return mAxisY.CanScrollTo(eSideBottom);
+  }
+  virtual bool CanScrollLeftwards() const override {
+    return mAxisX.CanScrollTo(eSideLeft);
+  }
+  virtual bool CanScrollRightwards() const override {
+    return mAxisX.CanScrollTo(eSideRight);
+  }
+  virtual bool IsHorizontalContentRightToLeft() const override {
+    return mIsHorizontalContentRightToLeft;
+  }
+
+  const AxisX& mAxisX;
+  const AxisY& mAxisY;
+  bool mIsHorizontalContentRightToLeft;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // __mozilla_layers_AutoDirWheelDeltaAdjuster_h__