C# – Hooking into Windows message loop in WPF window adds white border on the inside

I am trying to create a WPF window with WindowStyle="None" (for custom buttons and no title) that cannot be resized. Setting ResizeMode to NoResize removes the aero border, which I want to keep.

I could set the min/max size properties and be done with it, except that:

1. The resize cursors are still visible, and
2. The window is displayed in response to a user action and fits to its contents. It displays an image, so the size changes.

So, I have a simple scheme that gets me 99% of the way there:

public class BorderedWindowNoResize : Window
{
    [DllImport( "DwmApi.dll" )]
    public static extern int DwmExtendFrameIntoClientArea(
        IntPtr hwnd,
        ref MARGINS pMarInset );

    [DllImport( "user32.dll", CharSet = CharSet.Auto )]
    public static extern IntPtr DefWindowProc(
        IntPtr hWnd,
        int msg,
        IntPtr wParam,
        IntPtr lParam );

    public BorderedWindowNoResize()
    {           
        Loaded += BorderedWindowNoResize_Loaded;
    }

    private void BorderedWindowNoResize_Loaded( object sender, RoutedEventArgs e )
    {           
        IntPtr mainWindowPtr = new WindowInteropHelper( this ).Handle;
        HwndSource mainWindowSrc = HwndSource.FromHwnd( mainWindowPtr );            
        mainWindowSrc.AddHook( WndProc );
    }

    private IntPtr WndProc( IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled )
    {           
        var htLocation = DefWindowProc( hwnd, msg, wParam, lParam ).ToInt32();

        if( msg == (uint)WM.NCHITTEST )
        {
            handled = true;
            switch( htLocation )
            {
                case (int)HitTestResult.HTBOTTOM:
                case (int)HitTestResult.HTBOTTOMLEFT:
                case (int)HitTestResult.HTBOTTOMRIGHT:
                case (int)HitTestResult.HTLEFT:
                case (int)HitTestResult.HTRIGHT:
                case (int)HitTestResult.HTTOP:
                case (int)HitTestResult.HTTOPLEFT:
                case (int)HitTestResult.HTTOPRIGHT:
                    htLocation = (int)HitTestResult.HTBORDER;
                    break;
            }               
        }

        return new IntPtr( htLocation );
    }
}

Basically;

1. Override the window procedure.
2. Call the default window procedure.
3. If the message it is WM_NCHITTEST, check for the border results.
4. If it is a border, return the regular HTBORDER.

This works as far as allowing me to keep the aero window border and hiding the resize cursor(s), but it adds a ~5 pixel white border to the inside of my window.

In fact, even if I return the default windows procedure result at the top of WndPrc and do nothing else the border is still there. I need a different background color on my window, so this won't work for me.

Any ideas? Thanks in advance as always.