title: How to remove extra border in WPF TextBox?
body: <p>I have a problem with WPF <code>TextBox</code>. I have an <code>ItemsControl</code> with <code>TextBox</code> controls, which binds to <code>double</code> properties in some ViewModel. For these <code>TextBox</code> controls I have a <code>DataTrigger</code></p>
  
  <pre><code>&lt;DataTrigger Binding="{c:Binding 'IsCorrect'}" Value="False"&gt;
        &lt;Setter Property="BorderThickness" Value="5"/&gt;
        &lt;Setter Property="BorderBrush" Value="GreenYellow"/&gt;
        &lt;Setter Property="Background" Value="#FFD2D2"/&gt;
  &lt;/DataTrigger&gt;
  </code></pre>
  
  <p>It changes <code>BorderThickness</code>, <code>BorderBrush</code> and <code>Background</code> of the <code>TextBox</code> if there was entered a wrong number.
  But, if I remove all text from <code>TextBox</code>, it will not set data to binding property and change <code>BorderThickness</code> and <code>BorderBrush</code> properties, and if <code>DataTrigger</code> styles have been already set, there will be second border, which is extern for <code>DataTrigger's</code> made border.</p>
  
  <p>Result Image:</p>
  
  <p><img src="https://i.stack.imgur.com/kw28q.png" alt="Screenshot of the result"></p>
  
  <p>On the image <code>GreenYellow</code> border is set by <code>DataTrigger</code> and <code>Red</code> extern border set by itself.</p>
  
  <p>So the QUESTION is - what is this extern border? and how to remove it?!</p>