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diff --git a/widget/windows/WinWindowOcclusionTracker.h b/widget/windows/WinWindowOcclusionTracker.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 widget_windows_WinWindowOcclusionTracker_h
+#define widget_windows_WinWindowOcclusionTracker_h
+
+#include <string>
+#include <unordered_map>
+#include <unordered_set>
+#include <vector>
+
+#include "nsIWeakReferenceUtils.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/StaticPtr.h"
+#include "mozilla/WindowsVersion.h"
+#include "mozilla/ThreadSafeWeakPtr.h"
+#include "mozilla/widget/WindowOcclusionState.h"
+#include "mozilla/widget/WinEventObserver.h"
+#include "Units.h"
+#include "nsThreadUtils.h"
+
+class nsBaseWidget;
+struct IVirtualDesktopManager;
+class WinWindowOcclusionTrackerTest;
+class WinWindowOcclusionTrackerInteractiveTest;
+
+namespace base {
+class Thread;
+}  // namespace base
+
+namespace mozilla {
+
+namespace widget {
+
+class OcclusionUpdateRunnable;
+class SerializedTaskDispatcher;
+class UpdateOcclusionStateRunnable;
+
+// This class handles window occlusion tracking by using HWND.
+// Implementation is borrowed from chromium's NativeWindowOcclusionTrackerWin.
+class WinWindowOcclusionTracker final : public DisplayStatusListener,
+                                        public SessionChangeListener {
+ public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(WinWindowOcclusionTracker)
+
+  /// Can only be called from the main thread.
+  static WinWindowOcclusionTracker* Get();
+
+  /// Can only be called from the main thread.
+  static void Ensure();
+
+  /// Can only be called from the main thread.
+  static void ShutDown();
+
+  /// Can be called from any thread.
+  static MessageLoop* OcclusionCalculatorLoop();
+
+  /// Can be called from any thread.
+  static bool IsInWinWindowOcclusionThread();
+
+  /// Can only be called from the main thread.
+  void EnsureDisplayStatusObserver();
+
+  /// Can only be called from the main thread.
+  void EnsureSessionChangeObserver();
+
+  // Enables notifying to widget via NotifyOcclusionState() when the occlusion
+  // state has been computed.
+  void Enable(nsBaseWidget* aWindow, HWND aHwnd);
+
+  // Disables notifying to widget via NotifyOcclusionState() when the occlusion
+  // state has been computed.
+  void Disable(nsBaseWidget* aWindow, HWND aHwnd);
+
+  // Called when widget's visibility is changed
+  void OnWindowVisibilityChanged(nsBaseWidget* aWindow, bool aVisible);
+
+  SerializedTaskDispatcher* GetSerializedTaskDispatcher() {
+    return mSerializedTaskDispatcher;
+  }
+
+  void TriggerCalculation();
+
+  void DumpOccludingWindows(HWND aHWnd);
+
+ private:
+  friend class ::WinWindowOcclusionTrackerTest;
+  friend class ::WinWindowOcclusionTrackerInteractiveTest;
+
+  explicit WinWindowOcclusionTracker(UniquePtr<base::Thread> aThread);
+  virtual ~WinWindowOcclusionTracker();
+
+  // This class computes the occlusion state of the tracked windows.
+  // It runs on a separate thread, and notifies the main thread of
+  // the occlusion state of the tracked windows.
+  class WindowOcclusionCalculator {
+    NS_INLINE_DECL_THREADSAFE_REFCOUNTING(WindowOcclusionCalculator)
+   public:
+    // Creates WindowOcclusionCalculator instance.
+    static void CreateInstance();
+
+    // Clear WindowOcclusionCalculator instance.
+    static void ClearInstance();
+
+    // Returns existing WindowOcclusionCalculator instance.
+    static WindowOcclusionCalculator* GetInstance() { return sCalculator; }
+
+    void Initialize();
+    void Shutdown();
+
+    void EnableOcclusionTrackingForWindow(HWND hwnd);
+    void DisableOcclusionTrackingForWindow(HWND hwnd);
+
+    // If a window becomes visible, makes sure event hooks are registered.
+    void HandleVisibilityChanged(bool aVisible);
+
+    void HandleTriggerCalculation();
+
+   private:
+    WindowOcclusionCalculator();
+    ~WindowOcclusionCalculator();
+
+    // Registers event hooks, if not registered.
+    void MaybeRegisterEventHooks();
+
+    // This is the callback registered to get notified of various Windows
+    // events, like window moving/resizing.
+    static void CALLBACK EventHookCallback(HWINEVENTHOOK aWinEventHook,
+                                           DWORD aEvent, HWND aHwnd,
+                                           LONG aIdObject, LONG aIdChild,
+                                           DWORD aEventThread,
+                                           DWORD aMsEventTime);
+
+    // EnumWindows callback used to iterate over all hwnds to determine
+    // occlusion status of all tracked root windows.  Also builds up
+    // |current_pids_with_visible_windows_| and registers event hooks for newly
+    // discovered processes with visible hwnds.
+    static BOOL CALLBACK
+    ComputeNativeWindowOcclusionStatusCallback(HWND hwnd, LPARAM lParam);
+
+    // EnumWindows callback used to update the list of process ids with
+    // visible hwnds, |pids_for_location_change_hook_|.
+    static BOOL CALLBACK UpdateVisibleWindowProcessIdsCallback(HWND aHwnd,
+                                                               LPARAM aLParam);
+
+    // Determines which processes owning visible application windows to set the
+    // EVENT_OBJECT_LOCATIONCHANGE event hook for and stores the pids in
+    // |pids_for_location_change_hook_|.
+    void UpdateVisibleWindowProcessIds();
+
+    // Computes the native window occlusion status for all tracked root gecko
+    // windows in |root_window_hwnds_occlusion_state_| and notifies them if
+    // their occlusion status has changed.
+    void ComputeNativeWindowOcclusionStatus();
+
+    // Schedules an occlusion calculation , if one isn't already scheduled.
+    void ScheduleOcclusionCalculationIfNeeded();
+
+    // Registers a global event hook (not per process) for the events in the
+    // range from |event_min| to |event_max|, inclusive.
+    void RegisterGlobalEventHook(DWORD aEventMin, DWORD aEventMax);
+
+    // Registers the EVENT_OBJECT_LOCATIONCHANGE event hook for the process with
+    // passed id. The process has one or more visible, opaque windows.
+    void RegisterEventHookForProcess(DWORD aPid);
+
+    // Registers/Unregisters the event hooks necessary for occlusion tracking
+    // via calls to RegisterEventHook. These event hooks are disabled when all
+    // tracked windows are minimized.
+    void RegisterEventHooks();
+    void UnregisterEventHooks();
+
+    // EnumWindows callback for occlusion calculation. Returns true to
+    // continue enumeration, false otherwise. Currently, always returns
+    // true because this function also updates currentPidsWithVisibleWindows,
+    // and needs to see all HWNDs.
+    bool ProcessComputeNativeWindowOcclusionStatusCallback(
+        HWND aHwnd, std::unordered_set<DWORD>* aCurrentPidsWithVisibleWindows);
+
+    // Processes events sent to OcclusionEventHookCallback.
+    // It generally triggers scheduling of the occlusion calculation, but
+    // ignores certain events in order to not calculate occlusion more than
+    // necessary.
+    void ProcessEventHookCallback(HWINEVENTHOOK aWinEventHook, DWORD aEvent,
+                                  HWND aHwnd, LONG aIdObject, LONG aIdChild);
+
+    // EnumWindows callback for determining which processes to set the
+    // EVENT_OBJECT_LOCATIONCHANGE event hook for. We set that event hook for
+    // processes hosting fully visible, opaque windows.
+    void ProcessUpdateVisibleWindowProcessIdsCallback(HWND aHwnd);
+
+    // Returns true if the window is visible, fully opaque, and on the current
+    // virtual desktop, false otherwise.
+    bool WindowCanOccludeOtherWindowsOnCurrentVirtualDesktop(
+        HWND aHwnd, LayoutDeviceIntRect* aWindowRect);
+
+    // Returns true if aHwnd is definitely on the current virtual desktop,
+    // false if it's definitely not on the current virtual desktop, and Nothing
+    // if we we can't tell for sure.
+    Maybe<bool> IsWindowOnCurrentVirtualDesktop(HWND aHwnd);
+
+    static StaticRefPtr<WindowOcclusionCalculator> sCalculator;
+
+    // Map of root app window hwnds and their occlusion state. This contains
+    // both visible and hidden windows.
+    // It is accessed from WinWindowOcclusionTracker::UpdateOcclusionState()
+    // without using mutex. The access is safe by using
+    // SerializedTaskDispatcher.
+    std::unordered_map<HWND, OcclusionState> mRootWindowHwndsOcclusionState;
+
+    // Values returned by SetWinEventHook are stored so that hooks can be
+    // unregistered when necessary.
+    std::vector<HWINEVENTHOOK> mGlobalEventHooks;
+
+    // Map from process id to EVENT_OBJECT_LOCATIONCHANGE event hook.
+    std::unordered_map<DWORD, HWINEVENTHOOK> mProcessEventHooks;
+
+    // Pids of processes for which the EVENT_OBJECT_LOCATIONCHANGE event hook is
+    // set.
+    std::unordered_set<DWORD> mPidsForLocationChangeHook;
+
+    // Used as a timer to delay occlusion update.
+    RefPtr<CancelableRunnable> mOcclusionUpdateRunnable;
+
+    // Used to determine if a window is occluded. As we iterate through the
+    // hwnds in z-order, we subtract each opaque window's rect from
+    // mUnoccludedDesktopRegion. When we get to a root window, we subtract
+    // it from mUnoccludedDesktopRegion, and if mUnoccludedDesktopRegion
+    // doesn't change, the root window was already occluded.
+    LayoutDeviceIntRegion mUnoccludedDesktopRegion;
+
+    // Keeps track of how many root windows we need to compute the occlusion
+    // state of in a call to ComputeNativeWindowOcclusionStatus. Once we've
+    // determined the state of all root windows, we can stop subtracting
+    // windows from mUnoccludedDesktopRegion;.
+    int mNumRootWindowsWithUnknownOcclusionState;
+
+    // This is true if the task bar thumbnails or the alt tab thumbnails are
+    // showing.
+    bool mShowingThumbnails = false;
+
+    // Used to keep track of the window that's currently moving. That window
+    // is ignored for calculation occlusion so that tab dragging won't
+    // ignore windows occluded by the dragged window.
+    HWND mMovingWindow = 0;
+
+    // Only used on Win10+.
+    RefPtr<IVirtualDesktopManager> mVirtualDesktopManager;
+
+    // Used to serialize tasks related to mRootWindowHwndsOcclusionState.
+    RefPtr<SerializedTaskDispatcher> mSerializedTaskDispatcher;
+
+    // This is an alias to the singleton WinWindowOcclusionTracker mMonitor,
+    // and is used in ShutDown().
+    Monitor& mMonitor;
+
+    friend class OcclusionUpdateRunnable;
+  };
+
+  static BOOL CALLBACK DumpOccludingWindowsCallback(HWND aHWnd, LPARAM aLParam);
+
+  // Returns true if we are interested in |hwnd| for purposes of occlusion
+  // calculation. We are interested in |hwnd| if it is a window that is
+  // visible, opaque, bounded, and not a popup or floating window. If we are
+  // interested in |hwnd|, stores the window rectangle in |window_rect|.
+  static bool IsWindowVisibleAndFullyOpaque(HWND aHwnd,
+                                            LayoutDeviceIntRect* aWindowRect);
+
+  void Destroy();
+
+  static void CallUpdateOcclusionState(
+      std::unordered_map<HWND, OcclusionState>* aMap, bool aShowAllWindows);
+
+  // Updates root windows occclusion state. If aShowAllWindows is true,
+  // all non-hidden windows will be marked visible.  This is used to force
+  // rendering of thumbnails.
+  void UpdateOcclusionState(std::unordered_map<HWND, OcclusionState>* aMap,
+                            bool aShowAllWindows);
+
+  // This is called with session changed notifications. If the screen is locked
+  // by the current session, it marks app windows as occluded.
+  void OnSessionChange(WPARAM aStatusCode,
+                       Maybe<bool> aIsCurrentSession) override;
+
+  // This is called when the display is put to sleep. If the display is sleeping
+  // it marks app windows as occluded.
+  void OnDisplayStateChanged(bool aDisplayOn) override;
+
+  // Marks all root windows as either occluded, or if hwnd IsIconic, hidden.
+  void MarkNonIconicWindowsOccluded();
+
+  static StaticRefPtr<WinWindowOcclusionTracker> sTracker;
+
+  // "WinWindowOcclusionCalc" thread.
+  UniquePtr<base::Thread> mThread;
+  Monitor mMonitor;
+
+  // Has ShutDown been called on us? We might have survived if our thread join
+  // timed out.
+  bool mHasAttemptedShutdown = false;
+
+  // Map of HWND to widget. Maintained on main thread, and used to send
+  // occlusion state notifications to Windows from
+  // mRootWindowHwndsOcclusionState.
+  std::unordered_map<HWND, nsWeakPtr> mHwndRootWindowMap;
+
+  // This is set by UpdateOcclusionState(). It is currently only used by tests.
+  int mNumVisibleRootWindows = 0;
+
+  // If the screen is locked, windows are considered occluded.
+  bool mScreenLocked = false;
+
+  // If the display is off, windows are considered occluded.
+  bool mDisplayOn = true;
+
+  RefPtr<DisplayStatusObserver> mDisplayStatusObserver;
+
+  RefPtr<SessionChangeObserver> mSessionChangeObserver;
+
+  // Used to serialize tasks related to mRootWindowHwndsOcclusionState.
+  RefPtr<SerializedTaskDispatcher> mSerializedTaskDispatcher;
+
+  friend class OcclusionUpdateRunnable;
+  friend class UpdateOcclusionStateRunnable;
+};
+
+}  // namespace widget
+}  // namespace mozilla
+
+#endif  // widget_windows_WinWindowOcclusionTracker_h