title: Combobox doesn't display selected item
body: <p>in code:</p>
  
  <pre><code>private ObservableCollection&lt;State&gt; allStates = new ObservableCollection&lt;State&gt;();
  </code></pre>
  
  <p>//State is a class with many properties, one of them is int 'Index' and int 'OneArrow'</p>
  
  <pre><code>public MainWindow()
  {
  ...
  
  this.MyComboBox.ItemsSource = allStates;
  this.MyComboBox.DisplayMemberPath = "Index";
  this.MyComboBox.SelectedValuePath = "Index";
  
  this.DataContext = MyState;
  }
  </code></pre>
  
  <p>in xaml:</p>
  
  <pre><code>&lt;ComboBox Name="MyComboBox" 
  Width="60" Height="20" 
  IsEnabled="False" 
  SelectedValue="{Binding Path=OneArrow, UpdateSourceTrigger=PropertyChanged}"/&gt;
  </code></pre>
  
  <p>binding is working, it's fine, but I have another problem.
  Combobox doesn't dsiplay selected item. I mean in a drop-down list a right item is highlighted, but when drop-down list is hidden, nothing is diplayed. </p>
  
  <p><img src="https://i.stack.imgur.com/VPvUo.png" alt="enter image description here"></p>