Grid.OpacityMask>
    LinearGradientBrush StartPoint="0.5,1" EndPoint="0.5,0">
        GradientStop Color="#00000000" Offset="0"/>
        GradientStop Color="#00000000" Offset="1"/>
        GradientStop Color="#00000000" Offset="1"/>
    /LinearGradientBrush>
/Grid.OpacityMask>

Grid.Triggers>
    EventTrigger RoutedEvent="Loaded">
        EventTrigger.Actions>
            BeginStoryboard>
                Storyboard>
                    DoubleAnimation From="1" To="0" Duration="0:0:1.5" Storyboard.TargetProperty="OpacityMask.(GradientBrush.GradientStops)[1].Offset"/>
                    DoubleAnimation From="1" To="0" Duration="0:0:1" BeginTime="0:0:0.5" Storyboard.TargetProperty="OpacityMask.(GradientBrush.GradientStops)[2].Offset"/>
                    ColorAnimation To="#FF000000" Duration="0" Storyboard.TargetProperty="OpacityMask.(GradientBrush.GradientStops)[2].Color"/>
                /Storyboard>
            /BeginStoryboard>
        /EventTrigger.Actions>
    /EventTrigger>
/Grid.Triggers>

Window.Resources>
    Storyboard x:Key="ClosedStoryboard" Storyboard.TargetName="LoginGrid">
        DoubleAnimation From="1" To="0" Duration="0:0:2" Storyboard.TargetProperty="OpacityMask.(GradientBrush.GradientStops)[1].Offset"/>
        DoubleAnimation From="1" To="0" Duration="0:0:1.5" BeginTime="0:0:0.5"Storyboard.TargetProperty="OpacityMask.(GradientBrush.GradientStops)[2].Offset"/>
        ColorAnimation To="#00000000" Duration="0" Storyboard.TargetProperty="OpacityMask.(GradientBrush.GradientStops)[2].Color"/>
    /Storyboard>
    LinearGradientBrush x:Key="ClosedBrush" StartPoint="0.5,0" EndPoint="0.5,1">
        GradientStop Color="#FF000000" Offset="0"/>
        GradientStop Color="#FF000000" Offset="1"/>
        GradientStop Color="#FF000000" Offset="1"/>
    /LinearGradientBrush>
/Window.Resources>

private void btnCancel_Click(object sender, RoutedEventArgs e)
{
    this.IsEnabled = false;

    LoginGrid.OpacityMask = this.Resources["ClosedBrush"] as LinearGradientBrush;
    Storyboard std = this.Resources["ClosedStoryboard"] as Storyboard;
    std.Completed += delegate { this.Close(); };

    std.Begin();
}