WINAPI/DWMAPI Blur-behind window with irregular shape

Question

NB: THIS IS NOT A QUESTION ABOUT A BORDERLESS WINDOW.

So, I stumbled upon this program while I was exploring my Start menu the other day on Windows 7:

Answer 1

So, unbeknownst to me, hRgn can take an irregular shape (and DwmEnableBlurBehindWindow takes an hRgn, but I knew that). So, here's my solution that's (more or less) compatible with WPF:

...and source code:

MainWindow.xaml:

MainWindow.xaml.cs:

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

    this.SourceInitialized += MainWindow_SourceInitialized;
    this.KeyDown += MainWindow_KeyDown;
  }

  void MainWindow_KeyDown(object sender, KeyEventArgs e) {
    if (e.Key == Key.Escape) this.Close();
  }

  void MainWindow_SourceInitialized(object sender, EventArgs e) {
    var helper = new WindowInteropHelper(this);
    var hwnd = helper.Handle;
    var src = HwndSource.FromHwnd(hwnd);

    src.CompositionTarget.BackgroundColor = Colors.Transparent;

    WindowChrome.SetWindowChrome(this, new WindowChrome {
      CaptionHeight = 500,
      CornerRadius = new CornerRadius(0),
      GlassFrameThickness = new Thickness(0),
      NonClientFrameEdges = NonClientFrameEdges.None,
      ResizeBorderThickness = new Thickness(0),
      UseAeroCaptionButtons = false
    });

    GraphicsPath path = new GraphicsPath(FillMode.Alternate);
    path.StartFigure();
    path.AddArc(new RectangleF(0, 0, 500, 500), 0, 360);
    path.CloseFigure();

    var dbb = new DwmBlurBehind(true);
    dbb.SetRegion(Graphics.FromHwnd(hwnd), new Region(path));
    DwmApi.DwmEnableBlurBehindWindow(hwnd, ref dbb);
  }
}

I think somebody else beat me to it, but here's how my solution works:

When the window's SourceInitialized event is fired, that means that we have a handle for our window. So in the handler of this function, I get the window handle. Then I make a call to a function I imported from dwmapi.dll called DwmEnableBlurBehindWindow. This basically turns transparent areas of the window into glass for a certain region. The DwmBlurBehind struct I got from pinvoke.net, and it converts a GDI+ System.Drawing.Region into an hRgn. The hRgn is passed to DwmEnableBlurBehindWindow, and it clips the transparent parts to the Region. In this case, I used a circle. Then the XAML is just the accent borders. It's worth noting that, for some reason, setting Window.Background to Transparent doesn't enable hit-testing when AllowsTransparency is true here. No idea why, but it probably has something to do with the code-behind.