MainWindow.xaml

<Window x:Class="FocusMask.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="FocusMask" Height="450" Width="800"
        WindowStyle="None" AllowsTransparency="True" Background="Transparent"
        Topmost="True" ShowInTaskbar="False" ResizeMode="NoResize"
        Loaded="Window_Loaded" Closing="Window_Closing">

    <Grid>
        <Path x:Name="OverlayPath" IsHitTestVisible="False">
            <Path.Fill>
                <SolidColorBrush x:Name="MaskBrush" Color="Black" Opacity="0.75"/>
            </Path.Fill>
            <Path.Data>
                <CombinedGeometry GeometryCombineMode="Exclude">
                    <CombinedGeometry.Geometry1>
                        <RectangleGeometry Rect="-10000,-10000,30000,30000" />
                    </CombinedGeometry.Geometry1>
                    <CombinedGeometry.Geometry2>
                        <RectangleGeometry x:Name="HoleGeometry" Rect="0,0,0,0" RadiusX="12" RadiusY="12" />
                    </CombinedGeometry.Geometry2>
                </CombinedGeometry>
            </Path.Data>
        </Path>
    </Grid>
</Window>