title: TextBlock Text property can't be set via style trigger if non-empty - why?
body: <p>The XAML below does not work (the text does not change when mousing over):</p>
  
  <pre><code>&lt;Window.Resources&gt;
      &lt;Style TargetType="TextBlock"&gt;
          &lt;Style.Triggers&gt;
              &lt;Trigger Property="IsMouseOver" Value="true"&gt;
                  &lt;Setter Property="Text" Value="hover"/&gt;
              &lt;/Trigger&gt;
          &lt;/Style.Triggers&gt;
      &lt;/Style&gt;
  &lt;/Window.Resources&gt;
  &lt;Grid&gt;
      &lt;TextBlock Text="original"/&gt;
  &lt;/Grid&gt;
  </code></pre>
  
  <p>But, if the Text attribute is missing:</p>
  
  <pre><code> &lt;Grid&gt;
      &lt;TextBlock/&gt;
  &lt;/Grid&gt;
  </code></pre>
  
  <p>The text does change on mouse over. Anybody knows the theory behind this?</p>