title: Transparent background goes black in WPF
body: <p>I try to create a window with rounded corner. I set Window background to transparent and set the border background to white. However on the region between the border and the window, I get black background instead of transparent.</p>
  
  <p>I develop on C# WPF, VS2010 on Window 7. Below is my XAML and Screenshot.</p>
  
  <p><strong>XAML:</strong></p>
  
  <pre><code>&lt;Window WindowStyle="None" Background="Transparent"&gt;
      &lt;Border BorderBrush="Black" BorderThickness="1" CornerRadius="25" Background="White"&gt;
          &lt;Grid&gt;
              ... some content ...
          &lt;/Grid&gt;
      &lt;/Border&gt;
  &lt;/Window&gt;
  </code></pre>
  
  <p><strong>Screenshot:</strong>
  <img src="https://i.stack.imgur.com/7XCzV.png" alt="enter image description here"></p>