title: Combobox select item only by mouse or Enter key
body: <p>I have a WPF <code>ComboBox</code><br>
  I need to change default behaivor in popup list.  </p>
  
  <p>Now, by pressing <code>up</code> and <code>down</code> keys <code>SelectedItem</code> changes automatically.<br>
  I need change <code>SelectedItem</code> only by pressing <code>Enter</code> key, or clicking by mouse.</p>
  
  <p>How can it be done?</p>
  
  <p>I've subclassed <code>ComboBox</code>:</p>
  
  <pre><code>protected override void OnPreviewKeyDown(System.Windows.Input.KeyEventArgs e)
  {
      Debug.Write("Pressed " + e.Key+ " ");
      if (e.Key == System.Windows.Input.Key.Up || e.Key == System.Windows.Input.Key.Down)
      {
          // ???
          e.Handled = true;
          return;
      }
      base.OnPreviewKeyDown(e);
  }
  </code></pre>
  
  <p>This code doesn't work - no popup is shown and user can't select items.
  What I shoud write and where? :)</p>
  
  <p>Thanks.</p>
  
  <p><strong>UPD1:</strong></p>
  
  <p>I need same functionality as <code>ComboBox's</code> popup is open and user can select items by mouse.<br>
  Each item can be hovered by mouse, but not selected. Selection become only by pressing mouse button. I need the same. 'Up' and 'Down' only highlights items in popup, but <code>SelectedItem</code> will be changed only by pressing <code>Enter</code> or mouse clicking.</p>
  
  <p><strong>UPD2:</strong>
  If I press by mouse on button, that opens Popup in ComboCox, I can highlight items in Popup by mouse, but SelectedItem will change only if I click on item.</p>
  
  <p>I need same functionality by keyboard. If I start typing somewhat in ComboBox, Popup opens. And I have to hightlight items by keyboard <code>Up</code> and <code>Down</code>. TextBox in ComboBox must not change during highlighting and SelectedItem must change only if I press <code>Enter</code> (or mouse click)</p>
  
  <p><strong>UPD3:</strong>
  Link to demo solution: <a href="http://files.rsdn.ru/10267/combotest.zip" rel="noreferrer">download</a></p>