title: items are not displayed after grouping in ItemsControl
body: <p>I've added grouping to ItemsControl:</p>
  
  <pre><code>        &lt;ItemsControl Style="{StaticResource SellingDashboardToDosList}" Grid.Row="2" BorderThickness="1" Background="#C7E8F8" HorizontalAlignment="Stretch" ItemsSource="{Binding ToDoList}" &gt;
              &lt;ItemsControl.GroupStyle&gt;
                  &lt;GroupStyle&gt;
                      &lt;GroupStyle.ContainerStyle&gt;
                          &lt;Style TargetType="GroupItem"&gt;
                              &lt;Setter Property="Template"&gt;
                                  &lt;Setter.Value&gt;
                                      &lt;ControlTemplate TargetType="GroupItem"&gt;
                                          &lt;GroupBox Header="{Binding Name}"&gt;
                                              &lt;ItemsPresenter /&gt;
                                          &lt;/GroupBox&gt;
                                      &lt;/ControlTemplate&gt;
                                  &lt;/Setter.Value&gt;
                              &lt;/Setter&gt;
                          &lt;/Style&gt;
                      &lt;/GroupStyle.ContainerStyle&gt;
                  &lt;/GroupStyle&gt;
              &lt;/ItemsControl.GroupStyle&gt;
          &lt;/ItemsControl&gt;
  </code></pre>
  
  <p>Now I see only empty GroupBoxes. I've used Snoop tool to explore the application and I found out that GroupBox ItemPresenters are empty! What could be the reason of it? </p>
  
  <p>If I remove the grouping from the ItemsControl (the ItemsControl.GroupStyle element), then everything works fine, and I see all items again. I don't need to make any changes to the underlying data context to see all items. The data context (the ItemsSource binging) is of type <strong>CollectionViewSource</strong>.</p>
  
  <p>The binding tracing is turned on, but I don't see any binding errors.</p>