dotnet 读 WPF 源代码 Popup 的 StaysOpen 为 false 将会吃掉其他窗口的首次激活

    <Grid>
        <Button x:Name="OpenPopupButton" HorizontalAlignment="Center" VerticalAlignment="Center"
                Content="Open Popup" Click="OpenPopupButton_OnClick"></Button>
        <Popup x:Name="Popup" StaysOpen="False" PlacementTarget="{x:Reference OpenPopupButton}">
            <Grid Background="Gray" Width="100" Height="100">
                <TextBlock HorizontalAlignment="Center" VerticalAlignment="Center">The popup</TextBlock>
            </Grid>
        </Popup>
    </Grid>  

        private void OpenPopupButton_OnClick(object sender, RoutedEventArgs e)
        {
            Popup.IsOpen = true;
        }

        public MainWindow()
        {
            InitializeComponent();

            Loaded += MainWindow_Loaded;
        }

        private void MainWindow_Loaded(object sender, RoutedEventArgs e)
        {
            var window1 = new Window1();
            window1.Show();
        }

    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            Loaded += MainWindow_Loaded;
            MouseDown += MainWindow_MouseDown;
            MouseUp += MainWindow_MouseUp;
            Activated += MainWindow_Activated;
            Deactivated += MainWindow_Deactivated;
            LostFocus += MainWindow_LostFocus;
        }

        private void MainWindow_Loaded(object sender, RoutedEventArgs e)
        {
            var window1 = new Window1();
            window1.Show();
        }

        private void MainWindow_MouseUp(object sender, MouseButtonEventArgs e)
        {
            Debug.WriteLine($"MainWindow_MouseUp");
        }

        private void MainWindow_MouseDown(object sender, MouseButtonEventArgs e)
        {
            Debug.WriteLine($"MainWindow_MouseDown");
        }

        private void MainWindow_Activated(object sender, EventArgs e)
        {
            Debug.WriteLine($"MainWindow_Activated");
        }

        private void MainWindow_Deactivated(object sender, EventArgs e)
        {
            Debug.WriteLine($"MainWindow_Deactivated");
        }

        private void MainWindow_LostFocus(object sender, RoutedEventArgs e)
        {
            Debug.WriteLine($"MainWindow_LostFocus");
        }
    }

MainWindow_Activated
MainWindow_Deactivated

MainWindow_Activated
MainWindow_MouseDown
MainWindow_MouseUp

MainWindow_Activated
MainWindow_Deactivated

MainWindow_MouseDown
MainWindow_MouseUp

        static Popup()
        {
            EventManager.RegisterClassHandler(typeof(Popup), Mouse.LostMouseCaptureEvent, new MouseEventHandler(OnLostMouseCapture));

            // 忽略其他代码
        }

        private static void OnLostMouseCapture(object sender, MouseEventArgs e)
        {
            Popup popup = sender as Popup;

            if (!popup.StaysOpen)
            {
                PopupRoot root = popup._popupRoot.Value;

                // Reestablish capture if an element within us lost capture
                // (hence we receive the LostCapture routed event) and capture
                // is not being acquired anywhere else.
                //
                // Note we do not reestablish capture if we are losing capture
                // ourselves.
                bool reestablishCapture = e.OriginalSource != root && Mouse.Captured == null && MS.Win32.SafeNativeMethods.GetCapture() == IntPtr.Zero;

                if(reestablishCapture)
                {
                    popup.EstablishPopupCapture();
                    e.Handled = true;
                }
                else
                {
                    // 忽略其他代码
                }
            }
        }

        private void EstablishPopupCapture(bool isRestoringCapture=false)
        {
            if (!_cacheValid[(int)CacheBits.CaptureEngaged] && (_popupRoot.Value != null) &&
                (!StaysOpen))
            {
                IInputElement capturedElement = Mouse.Captured;
                PopupRoot parentPopupRoot = capturedElement as PopupRoot;
                if (parentPopupRoot != null)
                {
                    if (isRestoringCapture)
                    {
                        // if the other PopupRoot is restoring capture back to this
                        // popup, ignore mouse button events until both buttons have been
                        // released.  Otherwise a mouse click outside a chain of
                        // "nested" popups would dismiss two of them - one on MouseDown
                        // and another on MouseUp.
                        if (Mouse.LeftButton != MouseButtonState.Released ||
                            Mouse.RightButton != MouseButtonState.Released)
                        {
                            _cacheValid[(int)CacheBits.IsIgnoringMouseEvents] = true;
                        }
                    }
                    else
                    {
                        // this is a "nested" popup, invoked while another popup is open.
                        // We need to restore capture to the previous popup root when
                        // we're done
                        ParentPopupRootField.SetValue(this, parentPopupRoot);
                    }

                    // in either case, taking capture away from the other PopupRoot is OK.
                    capturedElement = null;
                }

                if (capturedElement == null)
                {
                    // When the mouse is not already captured, we will consider the following:
                    // In all cases but Modeless, we want the popup and subtree to receive
                    // mouse events and prevent other elements from receiving those messages.
                    Mouse.Capture(_popupRoot.Value, CaptureMode.SubTree);
                    _cacheValid[(int)CacheBits.CaptureEngaged] = true;
                }
            }
        }