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diff --git a/layout/generic/ScrollbarActivity.h b/layout/generic/ScrollbarActivity.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 ScrollbarActivity_h___
+#define ScrollbarActivity_h___
+
+#include "mozilla/Attributes.h"
+#include "nsCOMPtr.h"
+#include "nsIDOMEventListener.h"
+#include "mozilla/TimeStamp.h"
+#include "nsRefreshObservers.h"
+
+class nsIContent;
+class nsIScrollbarMediator;
+class nsITimer;
+class nsRefreshDriver;
+
+namespace mozilla {
+
+namespace dom {
+class Element;
+class EventTarget;
+}  // namespace dom
+
+namespace layout {
+
+/**
+ * ScrollbarActivity
+ *
+ * This class manages scrollbar behavior that imitates the native Mac OS X
+ * Lion overlay scrollbar behavior: Scrollbars are only shown while "scrollbar
+ * activity" occurs, and they're hidden with a fade animation after a short
+ * delay.
+ *
+ * Scrollbar activity has these states:
+ *  - inactive:
+ *      Scrollbars are hidden.
+ *  - ongoing activity:
+ *      Scrollbars are visible and being operated on in some way, for example
+ *      because they're hovered or pressed.
+ *  - active, but waiting for fade out
+ *      Scrollbars are still completely visible but are about to fade away.
+ *  - fading out
+ *      Scrollbars are subject to a fade-out animation.
+ *
+ * Initial scrollbar activity needs to be reported by the scrollbar holder that
+ * owns the ScrollbarActivity instance. This needs to happen via a call to
+ * ActivityOccurred(), for example when the current scroll position or the size
+ * of the scroll area changes.
+ *
+ * As soon as scrollbars are visible, the ScrollbarActivity class manages the
+ * rest of the activity behavior: It ensures that mouse motions inside the
+ * scroll area keep the scrollbars visible, and that scrollbars don't fade away
+ * while they're being hovered / dragged. It also sets a sticky hover attribute
+ * on the most recently hovered scrollbar.
+ *
+ * ScrollbarActivity falls into hibernation after the scrollbars have faded
+ * out. It only starts acting after the next call to ActivityOccurred() /
+ * ActivityStarted().
+ */
+
+class ScrollbarActivity final : public nsIDOMEventListener,
+                                public nsARefreshObserver {
+ public:
+  explicit ScrollbarActivity(nsIScrollbarMediator* aScrollableFrame)
+      : mScrollableFrame(aScrollableFrame),
+        mNestedActivityCounter(0),
+        mIsActive(false),
+        mIsFading(false),
+        mListeningForScrollbarEvents(false),
+        mListeningForScrollAreaEvents(false),
+        mHScrollbarHovered(false),
+        mVScrollbarHovered(false),
+        mDisplayOnMouseMove(false),
+        mScrollbarFadeBeginDelay(0),
+        mScrollbarFadeDuration(0) {
+    QueryLookAndFeelVals();
+  }
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIDOMEVENTLISTENER
+
+  void Destroy();
+
+  void ActivityOccurred();
+  void ActivityStarted();
+  void ActivityStopped();
+
+  virtual void WillRefresh(TimeStamp aTime) override;
+
+  static void FadeBeginTimerFired(nsITimer* aTimer, void* aSelf) {
+    RefPtr<ScrollbarActivity> scrollbarActivity(
+        reinterpret_cast<ScrollbarActivity*>(aSelf));
+    scrollbarActivity->BeginFade();
+  }
+
+ protected:
+  virtual ~ScrollbarActivity() = default;
+
+  bool IsActivityOngoing() { return mNestedActivityCounter > 0; }
+  bool IsStillFading(TimeStamp aTime);
+  void QueryLookAndFeelVals();
+
+  void HandleEventForScrollbar(const nsAString& aType, nsIContent* aTarget,
+                               dom::Element* aScrollbar,
+                               bool* aStoredHoverState);
+
+  void SetIsActive(bool aNewActive);
+  bool SetIsFading(bool aNewFading);  // returns false if 'this' was destroyed
+
+  void BeginFade();
+  void EndFade();
+
+  void StartFadeBeginTimer();
+  void CancelFadeBeginTimer();
+
+  void StartListeningForScrollbarEvents();
+  void StartListeningForScrollAreaEvents();
+  void StopListeningForScrollbarEvents();
+  void StopListeningForScrollAreaEvents();
+  void AddScrollbarEventListeners(dom::EventTarget* aScrollbar);
+  void RemoveScrollbarEventListeners(dom::EventTarget* aScrollbar);
+
+  void RegisterWithRefreshDriver();
+  void UnregisterFromRefreshDriver();
+
+  bool UpdateOpacity(TimeStamp aTime);  // returns false if 'this' was destroyed
+  void HoveredScrollbar(dom::Element* aScrollbar);
+
+  nsRefreshDriver* GetRefreshDriver();
+  dom::Element* GetScrollbarContent(bool aVertical);
+  dom::Element* GetHorizontalScrollbar() { return GetScrollbarContent(false); }
+  dom::Element* GetVerticalScrollbar() { return GetScrollbarContent(true); }
+
+  const TimeDuration FadeDuration() {
+    return TimeDuration::FromMilliseconds(mScrollbarFadeDuration);
+  }
+
+  nsIScrollbarMediator* mScrollableFrame;
+  TimeStamp mFadeBeginTime;
+  nsCOMPtr<nsITimer> mFadeBeginTimer;
+  nsCOMPtr<dom::EventTarget> mHorizontalScrollbar;  // null while inactive
+  nsCOMPtr<dom::EventTarget> mVerticalScrollbar;    // null while inactive
+  int mNestedActivityCounter;
+  bool mIsActive;
+  bool mIsFading;
+  bool mListeningForScrollbarEvents;
+  bool mListeningForScrollAreaEvents;
+  bool mHScrollbarHovered;
+  bool mVScrollbarHovered;
+
+  // LookAndFeel values we load on creation
+  bool mDisplayOnMouseMove;
+  int mScrollbarFadeBeginDelay;
+  int mScrollbarFadeDuration;
+};
+
+}  // namespace layout
+}  // namespace mozilla
+
+#endif /* ScrollbarActivity_h___ */