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diff --git a/gfx/layers/apz/src/OverscrollHandoffState.h b/gfx/layers/apz/src/OverscrollHandoffState.h
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+++ b/gfx/layers/apz/src/OverscrollHandoffState.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_OverscrollHandoffChain_h
+#define mozilla_layers_OverscrollHandoffChain_h
+
+#include <vector>
+#include "mozilla/RefPtr.h"   // for RefPtr
+#include "nsISupportsImpl.h"  // for NS_INLINE_DECL_THREADSAFE_REFCOUNTING
+#include "APZUtils.h"         // for CancelAnimationFlags
+#include "mozilla/layers/LayersTypes.h"  // for Layer::ScrollDirection
+#include "Units.h"                       // for ScreenPoint
+
+namespace mozilla {
+
+class InputData;
+
+namespace layers {
+
+class AsyncPanZoomController;
+
+/**
+ * This class represents the chain of APZCs along which overscroll is handed
+ * off. It is created by APZCTreeManager by starting from an initial APZC which
+ * is the target for input events, and following the scroll parent ID links
+ * (often but not always corresponding to parent pointers in the APZC tree),
+ * then adjusting for scrollgrab.
+ */
+class OverscrollHandoffChain {
+ protected:
+  // Reference-counted classes cannot have public destructors.
+  ~OverscrollHandoffChain();
+
+ public:
+  // Threadsafe so that the controller and sampler threads can both maintain
+  // nsRefPtrs to the same handoff chain.
+  // Mutable so that we can pass around the class by
+  // RefPtr<const OverscrollHandoffChain> and thus enforce that, once built,
+  // the chain is not modified.
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(OverscrollHandoffChain)
+
+  /*
+   * Methods for building the handoff chain.
+   * These should be used only by
+   * AsyncPanZoomController::BuildOverscrollHandoffChain().
+   */
+  void Add(AsyncPanZoomController* aApzc);
+  void SortByScrollPriority();
+
+  /*
+   * Methods for accessing the handoff chain.
+   */
+  uint32_t Length() const { return mChain.size(); }
+  const RefPtr<AsyncPanZoomController>& GetApzcAtIndex(uint32_t aIndex) const;
+  // Returns Length() if |aApzc| is not on this chain.
+  uint32_t IndexOf(const AsyncPanZoomController* aApzc) const;
+
+  /*
+   * Convenience methods for performing operations on APZCs in the chain.
+   */
+
+  // Flush repaints all the way up the chain.
+  void FlushRepaints() const;
+
+  // Cancel animations all the way up the chain.
+  void CancelAnimations(CancelAnimationFlags aFlags = Default) const;
+
+  // Clear overscroll all the way up the chain.
+  void ClearOverscroll() const;
+
+  // Snap back the APZC that is overscrolled on the subset of the chain from
+  // |aStart| onwards, if any.
+  void SnapBackOverscrolledApzc(const AsyncPanZoomController* aStart) const;
+
+  // Similar to above SnapbackOverscrolledApzc but for pan gestures with
+  // momentum events, this function doesn't end up calling each APZC's
+  // ScrollSnap.
+  // |aVelocity| is the initial velocity of |aStart|.
+  void SnapBackOverscrolledApzcForMomentum(
+      const AsyncPanZoomController* aStart,
+      const ParentLayerPoint& aVelocity) const;
+
+  // Determine whether the given APZC, or any APZC further in the chain,
+  // has room to be panned.
+  bool CanBePanned(const AsyncPanZoomController* aApzc) const;
+
+  // Determine whether the given APZC, or any APZC further in the chain,
+  // can scroll in the given direction.
+  bool CanScrollInDirection(const AsyncPanZoomController* aApzc,
+                            ScrollDirection aDirection) const;
+
+  // Determine whether any APZC along this handoff chain is overscrolled.
+  bool HasOverscrolledApzc() const;
+
+  // Determine whether any APZC along this handoff chain has been flung fast.
+  bool HasFastFlungApzc() const;
+
+  // Determine whether any APZC along this handoff chain is autoscroll.
+  bool HasAutoscrollApzc() const;
+
+  // Find the first APZC in this handoff chain that can be scrolled by |aInput|.
+  // Since overscroll-behavior can restrict handoff in some directions,
+  // |aOutAllowedScrollDirections| is populated with the scroll directions
+  // in which scrolling of the returned APZC is allowed.
+  // |aIncludeOverscroll| is an optional flag whether to consider overscrollable
+  // as scrollable or not.
+  enum class IncludeOverscroll : bool { No, Yes };
+  RefPtr<AsyncPanZoomController> FindFirstScrollable(
+      const InputData& aInput, ScrollDirections* aOutAllowedScrollDirections,
+      IncludeOverscroll aIncludeOverscroll = IncludeOverscroll::Yes) const;
+
+  // Return a pair of true and the root content APZC if all non-root APZCs in
+  // this handoff chain starting from |aApzc| are not able to scroll downwards
+  // (i.e. there is no room to scroll downwards in each APZC respectively) and
+  // there is any contents covered by the dynamic toolbar, otherwise return a
+  // pair of false and nullptr.
+  std::tuple<bool, const AsyncPanZoomController*>
+  ScrollingDownWillMoveDynamicToolbar(
+      const AsyncPanZoomController* aApzc) const;
+
+ private:
+  std::vector<RefPtr<AsyncPanZoomController>> mChain;
+
+  typedef void (AsyncPanZoomController::*APZCMethod)();
+  typedef bool (AsyncPanZoomController::*APZCPredicate)() const;
+  void ForEachApzc(APZCMethod aMethod) const;
+  bool AnyApzc(APZCPredicate aPredicate) const;
+};
+
+/**
+ * This class groups the state maintained during overscroll handoff.
+ */
+struct OverscrollHandoffState {
+  OverscrollHandoffState(const OverscrollHandoffChain& aChain,
+                         const ScreenPoint& aPanDistance,
+                         ScrollSource aScrollSource)
+      : mChain(aChain),
+        mChainIndex(0),
+        mPanDistance(aPanDistance),
+        mScrollSource(aScrollSource) {}
+
+  // The chain of APZCs along which we hand off scroll.
+  // This is const to indicate that the chain does not change over the
+  // course of handoff.
+  const OverscrollHandoffChain& mChain;
+
+  // The index of the APZC in the chain that we are currently giving scroll to.
+  // This is non-const to indicate that this changes over the course of handoff.
+  uint32_t mChainIndex;
+
+  // The total distance since touch-start of the pan that triggered the
+  // handoff. This is const to indicate that it does not change over the
+  // course of handoff.
+  // The x/y components of this are non-negative.
+  const ScreenPoint mPanDistance;
+
+  ScrollSource mScrollSource;
+
+  // The total amount of actual movement that this scroll caused, including
+  // scrolling and changes to overscroll. This starts at zero and is accumulated
+  // over the course of the handoff.
+  ScreenPoint mTotalMovement;
+};
+
+/*
+ * This class groups the state maintained during fling handoff.
+ */
+struct FlingHandoffState {
+  // The velocity of the fling being handed off.
+  ParentLayerPoint mVelocity;
+
+  // The chain of APZCs along which we hand off the fling.
+  // Unlike in OverscrollHandoffState, this is stored by RefPtr because
+  // otherwise it may not stay alive for the entire handoff.
+  RefPtr<const OverscrollHandoffChain> mChain;
+
+  // The time duration between the touch start and the touch move that started
+  // the pan gesture which triggered this fling. In other words, the time it
+  // took for the finger to move enough to cross the touch slop threshold.
+  // Nothing if this fling was not immediately caused by a touch pan.
+  Maybe<TimeDuration> mTouchStartRestingTime;
+
+  // The slowest panning velocity encountered during the pan that triggered this
+  // fling.
+  ParentLayerCoord mMinPanVelocity;
+
+  // Whether handoff has happened by this point, or we're still process
+  // the original fling.
+  bool mIsHandoff;
+
+  // The single APZC that was scrolled by the pan that started this fling.
+  // The fling is only allowed to scroll this APZC, too.
+  // Used only if immediate scroll handoff is disallowed.
+  RefPtr<const AsyncPanZoomController> mScrolledApzc;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif /* mozilla_layers_OverscrollHandoffChain_h */