title: WPF, Whats with this weird whitespace in Tooltip/Border
body: <p>I have styled a tooltip like</p>
  
  <pre><code>&lt;Setter Property="ToolTip"&gt;
      &lt;Setter.Value&gt;
          &lt;Border Background="Red" CornerRadius="5" Padding="5" Margin="0"&gt;
              &lt;TextBlock Text="Hello" Foreground="White" /&gt;
          &lt;/Border&gt;
      &lt;/Setter.Value&gt;
  &lt;/Setter&gt;
  </code></pre>
  
  <p>And it gets rendered like </p>
  
  <p><img src="https://i.stack.imgur.com/yempv.jpg" alt="alt text"></p>
  
  <p>Whats that white "rectangle" about? How can I get rid of it</p>