title: Where is ComboBox popup in visual tree?
body: <p>Where in the visual tree can I find the ComboBox popup (the list with the ComboBoxItems)?</p>
  
  <p>I have programmatically opened a ComboBox and when watching it in the WPF Tree Visualizer in the debugger I see the following:</p>
  
  <pre class="lang-none prettyprint-override"><code>: ComboBox
    templateRoot : Grid
      PART_Popup : Popup
      toggleButton : ToggleButton
        templateRoot : Border
          splitBorder : Border
            Arrow : Path
      contentPresenter : ContentPresenter
        : TextBlock
  </code></pre>
  
  <p>I expected to see a ScrollViewer with some kind of item host (StackPanel?), perhaps where the PART_Popup is, but nothing.</p>
  
  <p>So where is it?</p>