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diff --git a/gfx/layers/apz/public/APZInputBridge.h b/gfx/layers/apz/public/APZInputBridge.h
new file mode 100644
index 0000000000..117e4b7bb2
--- /dev/null
+++ b/gfx/layers/apz/public/APZInputBridge.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_APZInputBridge_h
+#define mozilla_layers_APZInputBridge_h
+
+#include "Units.h"                  // for LayoutDeviceIntPoint
+#include "mozilla/EventForwards.h"  // for WidgetInputEvent, nsEventStatus
+#include "mozilla/layers/APZPublicUtils.h"       // for APZWheelAction
+#include "mozilla/layers/LayersTypes.h"          // for ScrollDirections
+#include "mozilla/layers/ScrollableLayerGuid.h"  // for ScrollableLayerGuid
+
+namespace mozilla {
+
+class InputData;
+
+namespace layers {
+
+class APZInputBridgeParent;
+class AsyncPanZoomController;
+class InputBlockState;
+class TouchBlockState;
+struct ScrollableLayerGuid;
+struct TargetConfirmationFlags;
+struct PointerEventsConsumableFlags;
+
+enum class APZHandledPlace : uint8_t {
+  Unhandled = 0,         // we know for sure that the event will not be handled
+                         // by either the root APZC or others
+  HandledByRoot = 1,     // we know for sure that the event will be handled
+                         // by the root content APZC
+  HandledByContent = 2,  // we know for sure it will be handled by a non-root
+                         // APZC or by an event listener using preventDefault()
+                         // in a document
+  Invalid = 3,
+  Last = Invalid
+};
+
+struct APZHandledResult {
+  APZHandledPlace mPlace = APZHandledPlace::Invalid;
+  SideBits mScrollableDirections = SideBits::eNone;
+  ScrollDirections mOverscrollDirections = ScrollDirections();
+
+  APZHandledResult() = default;
+  // A constructor for cases where we have the target of the input block this
+  // event is part of, the target might be adjusted to be the root in the
+  // ScrollingDownWillMoveDynamicToolbar case.
+  //
+  // NOTE: There's a case where |aTarget| is the APZC for the root content but
+  // |aPlace| has to be `HandledByContent`, for example, the root content has
+  // an event handler using preventDefault() in the callback, so call sites of
+  // this function should be responsible to set a proper |aPlace|.
+  APZHandledResult(APZHandledPlace aPlace,
+                   const AsyncPanZoomController* aTarget,
+                   bool aPopulateDirectionsForUnhandled = false);
+  APZHandledResult(APZHandledPlace aPlace, SideBits aScrollableDirections,
+                   ScrollDirections aOverscrollDirections)
+      : mPlace(aPlace),
+        mScrollableDirections(aScrollableDirections),
+        mOverscrollDirections(aOverscrollDirections) {}
+
+  bool IsHandledByContent() const {
+    return mPlace == APZHandledPlace::HandledByContent;
+  }
+  bool IsHandledByRoot() const {
+    return mPlace == APZHandledPlace::HandledByRoot;
+  }
+  bool operator==(const APZHandledResult& aOther) const {
+    return mPlace == aOther.mPlace &&
+           mScrollableDirections == aOther.mScrollableDirections &&
+           mOverscrollDirections == aOther.mOverscrollDirections;
+  }
+};
+
+/**
+ * Represents the outcome of APZ receiving and processing an input event.
+ * This is returned from APZInputBridge::ReceiveInputEvent() and related APIs.
+ */
+struct APZEventResult {
+  /**
+   * Creates a default result with a status of eIgnore, no block ID, and empty
+   * target guid.
+   */
+  APZEventResult();
+
+  /**
+   * Creates a result with a status of eIgnore, no block ID, the guid of the
+   * given initial target, and an APZHandledResult if we are sure the event
+   * is not going to be dispatched to contents.
+   */
+  APZEventResult(const RefPtr<AsyncPanZoomController>& aInitialTarget,
+                 TargetConfirmationFlags aFlags);
+
+  void SetStatusAsConsumeNoDefault() {
+    mStatus = nsEventStatus_eConsumeNoDefault;
+  }
+
+  void SetStatusAsIgnore() { mStatus = nsEventStatus_eIgnore; }
+
+  // Set mStatus to nsEventStatus_eConsumeDoDefault and set mHandledResult
+  // depending on |aTarget|.
+  void SetStatusAsConsumeDoDefault(
+      const RefPtr<AsyncPanZoomController>& aTarget);
+
+  // Set mStatus to nsEventStatus_eConsumeDoDefault, unlike above
+  // SetStatusAsConsumeDoDefault(const RefPtr<AsyncPanZoomController>&) this
+  // function doesn't mutate mHandledResult.
+  void SetStatusAsConsumeDoDefault() {
+    mStatus = nsEventStatus_eConsumeDoDefault;
+  }
+
+  // Set mStatus to nsEventStatus_eConsumeDoDefault and set mHandledResult
+  // depending on |aBlock|'s target APZC.
+  void SetStatusAsConsumeDoDefault(const InputBlockState& aBlock);
+  // Set mStatus and mHandledResult for a touch event which is not dropped
+  // altogether (i.e. the status is not eConsumeNoDefault).
+  void SetStatusForTouchEvent(const InputBlockState& aBlock,
+                              TargetConfirmationFlags aFlags,
+                              PointerEventsConsumableFlags aConsumableFlags,
+                              const AsyncPanZoomController* aTarget);
+
+  // Set mStatus and mHandledResult during in a stat of fast fling.
+  void SetStatusForFastFling(const TouchBlockState& aBlock,
+                             TargetConfirmationFlags aFlags,
+                             PointerEventsConsumableFlags aConsumableFlags,
+                             const AsyncPanZoomController* aTarget);
+
+  // DO NOT USE THIS UpdateStatus DIRECTLY. THIS FUNCTION IS ONLY FOR
+  // SERIALIZATION / DESERIALIZATION OF THIS STRUCT IN IPC.
+  void UpdateStatus(nsEventStatus aStatus) { mStatus = aStatus; }
+  nsEventStatus GetStatus() const { return mStatus; };
+
+  // DO NOT USE THIS UpdateHandledResult DIRECTLY. THIS FUNCTION IS ONLY FOR
+  // SERIALIZATION / DESERIALIZATION OF THIS STRUCT IN IPC.
+  void UpdateHandledResult(const Maybe<APZHandledResult>& aHandledResult) {
+    mHandledResult = aHandledResult;
+  }
+  const Maybe<APZHandledResult>& GetHandledResult() const {
+    return mHandledResult;
+  }
+
+  bool WillHaveDelayedResult() const {
+    return GetStatus() != nsEventStatus_eConsumeNoDefault &&
+           !GetHandledResult();
+  }
+
+ private:
+  void UpdateHandledResult(const InputBlockState& aBlock,
+                           PointerEventsConsumableFlags aConsumableFlags,
+                           const AsyncPanZoomController* aTarget,
+                           bool aDispatchToContent);
+
+  /**
+   * A status flag indicated how APZ handled the event.
+   * The interpretation of each value is as follows:
+   *
+   * nsEventStatus_eConsumeNoDefault is returned to indicate the
+   *   APZ is consuming this event and the caller should discard the event with
+   *   extreme prejudice. The exact scenarios under which this is returned is
+   *   implementation-dependent and may vary.
+   * nsEventStatus_eIgnore is returned to indicate that the APZ code didn't
+   *   use this event. This might be because it was directed at a point on
+   *   the screen where there was no APZ, or because the thing the user was
+   *   trying to do was not allowed. (For example, attempting to pan a
+   *   non-pannable document).
+   * nsEventStatus_eConsumeDoDefault is returned to indicate that the APZ
+   *   code may have used this event to do some user-visible thing. Note that
+   *   in some cases CONSUMED is returned even if the event was NOT used. This
+   *   is because we cannot always know at the time of event delivery whether
+   *   the event will be used or not. So we err on the side of sending
+   *   CONSUMED when we are uncertain.
+   */
+  nsEventStatus mStatus;
+
+  /**
+   * This is:
+   *  - set to HandledByRoot if we know for sure that the event will be handled
+   *    by the root content APZC;
+   *  - set to HandledByContent if we know for sure it will not be;
+   *  - left empty if we are unsure.
+   */
+  Maybe<APZHandledResult> mHandledResult;
+
+ public:
+  /**
+   * The guid of the APZC initially targeted by this event.
+   * This will usually be the APZC that handles the event, but in cases
+   * where the event is dispatched to content, it may end up being
+   * handled by a different APZC.
+   */
+  ScrollableLayerGuid mTargetGuid;
+  /**
+   * If this event started or was added to an input block, the id of that
+   * input block, otherwise InputBlockState::NO_BLOCK_ID.
+   */
+  uint64_t mInputBlockId;
+};
+
+/**
+ * This class lives in the main process, and is accessed via the controller
+ * thread (which is the process main thread for desktop, and the Java UI
+ * thread for Android). This class exposes a synchronous API to deliver
+ * incoming input events to APZ and modify them in-place to unapply the APZ
+ * async transform. If there is a GPU process, then this class does sync IPC
+ * calls over to the GPU process in order to accomplish this. Otherwise,
+ * APZCTreeManager overrides and implements these methods directly.
+ */
+class APZInputBridge {
+ public:
+  using InputBlockCallback = std::function<void(
+      uint64_t aInputBlockId, const APZHandledResult& aHandledResult)>;
+
+  /**
+   * General handler for incoming input events. Manipulates the frame metrics
+   * based on what type of input it is. For example, a PinchGestureEvent will
+   * cause scaling. This should only be called externally to this class, and
+   * must be called on the controller thread.
+   *
+   * This function transforms |aEvent| to have its coordinates in DOM space.
+   * This is so that the event can be passed through the DOM and content can
+   * handle them. The event may need to be converted to a WidgetInputEvent
+   * by the caller if it wants to do this.
+   *
+   * @param aEvent input event object; is modified in-place
+   * @param aCallback an optional callback to be invoked when the input block is
+   * ready for handling,
+   * @return The result of processing the event. Refer to the documentation of
+   * APZEventResult and its field.
+   */
+  virtual APZEventResult ReceiveInputEvent(
+      InputData& aEvent,
+      InputBlockCallback&& aCallback = InputBlockCallback()) = 0;
+
+  /**
+   * WidgetInputEvent handler. Transforms |aEvent| (which is assumed to be an
+   * already-existing instance of an WidgetInputEvent which may be an
+   * WidgetTouchEvent) to have its coordinates in DOM space. This is so that the
+   * event can be passed through the DOM and content can handle them.
+   *
+   * NOTE: Be careful of invoking the WidgetInputEvent variant. This can only be
+   * called on the main thread. See widget/InputData.h for more information on
+   * why we have InputData and WidgetInputEvent separated. If this function is
+   * used, the controller thread must be the main thread, or undefined behaviour
+   * may occur.
+   * NOTE: On unix, mouse events are treated as touch and are forwarded
+   * to the appropriate apz as such.
+   *
+   * See documentation for other ReceiveInputEvent above.
+   */
+  APZEventResult ReceiveInputEvent(
+      WidgetInputEvent& aEvent,
+      InputBlockCallback&& aCallback = InputBlockCallback());
+
+  // Returns the kind of wheel event action, if any, that will be (or was)
+  // performed by APZ. If this returns true, the event must not perform a
+  // synchronous scroll.
+  //
+  // Even if this returns Nothing(), all wheel events in APZ-aware widgets must
+  // be sent through APZ so they are transformed correctly for BrowserParent.
+  static Maybe<APZWheelAction> ActionForWheelEvent(WidgetWheelEvent* aEvent);
+
+ protected:
+  friend class APZInputBridgeParent;
+
+  // Methods to help process WidgetInputEvents (or manage conversion to/from
+  // InputData)
+
+  virtual void ProcessUnhandledEvent(LayoutDeviceIntPoint* aRefPoint,
+                                     ScrollableLayerGuid* aOutTargetGuid,
+                                     uint64_t* aOutFocusSequenceNumber,
+                                     LayersId* aOutLayersId) = 0;
+
+  virtual void UpdateWheelTransaction(
+      LayoutDeviceIntPoint aRefPoint, EventMessage aEventMessage,
+      const Maybe<ScrollableLayerGuid>& aTargetGuid) = 0;
+
+  virtual ~APZInputBridge() = default;
+};
+
+std::ostream& operator<<(std::ostream& aOut,
+                         const APZHandledResult& aHandledResult);
+
+// This enum class is used for communicating between APZ and the browser gesture
+// support code. APZ needs to wait for the browser to send this response just
+// like APZ waits for the content's response if there's an APZ ware event
+// listener in the content process.
+enum class BrowserGestureResponse : bool {
+  NotConsumed = 0,  // Representing the browser doesn't consume the gesture
+  Consumed = 1,  // Representing the browser has started consuming the gesture.
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_APZInputBridge_h