title: TabControl: all TabItems collapsed, but content of 1st TabItem still visible
body: <p>I've got a rather strange behavior on a TabControl, whose TabItems are all collapsed: The content of the first TabItem is still visible (but the header is not).</p>
  
  <p>The TabControl and its TabItems are setup like this:</p>
  
  <pre><code>&lt;TabControl&gt;
      &lt;TabItem Header="Data 1"
               Visibility="{Binding Path=DataTable1.HasRows, 
                                    Converter={StaticResource BoolToVisibility}}"&gt;
          &lt;UI:ShowData DataContext="{Binding Path=DataTable1}"/&gt;
      &lt;/TabItem&gt;
      &lt;TabItem Header="Data 2"
               Visibility="{Binding Path=DataTable2.HasRows, 
                                    Converter={StaticResource BoolToVisibility}}"&gt;
          &lt;UI:ShowData DataContext="{Binding Path=DataTable2}"/&gt;
      &lt;/TabItem&gt;
  &lt;/TabControl&gt;
  </code></pre>
  
  <p>If none of the data tables contains any rows, none of the TabItems should be shown. (I known that I could hide the whole TabControl in that case, but that's not the point here.)</p>
  
  <p>Actually the content of the tab item "Header 1" will be displayed despite the TabItem being collapsed! The TabItem's header itself is collapsed, the TabItems border which contains its content and the content itself are displayed.</p>
  
  <p>Edit/Add:
  This can easily be reproduced using this code (note using Collapsed or Hidden does not make any difference:</p>
  
  <pre><code>&lt;TabControl&gt;
      &lt;TabItem Header="Test 1" Visibility="Hidden"&gt;
          &lt;Label&gt;Test1&lt;/Label&gt;
      &lt;/TabItem&gt;
  
      &lt;TabItem Header="Test 2" Visibility="Hidden"&gt;
          &lt;Label&gt;Test2&lt;/Label&gt;
      &lt;/TabItem&gt;
  &lt;/TabControl&gt;
  </code></pre>
  
  <p>So what's wrong here? Any help/hints are appreciated!</p>