title: UWP Combobox items scrolls to mid for no reason
body: <p>I noticed that in a combobox which has some itemsource attached to it, when there is no selected item it tends to scroll to middle of the item instead of starting at the top (first item) and when an item is selected it does sometime scroll to the selected item.</p>
  <p>So I want to scroll to first item when no item is selected. For that I tried following fixes.</p>
  <h2>Code</h2>
  <pre><code>    private void ComboBoxKeyboardSelectionBehavior_DropDownOpened(object sender, object e)
      {
          var comboBox = (ComboBox) sender;
          if(comboBox.SelectedIndex == -1)
          {
              //var scrollviewer = comboBox.GetScrollViewer();
              //scrollviewer.ChangeView(0, 0, null);
              //var allItems = comboBox.Items.ToList();
              //var cccc = comboBox.Items.Count;
              //var firstItem = allItems.First();
              var ci = comboBox.ContainerFromIndex(0) as ComboBoxItem;
              if (ci != null)
              {
                  ci.StartBringIntoView();
              }
          }
          else
          {
              var ci = comboBox.ContainerFromIndex(comboBox.SelectedIndex) as ComboBoxItem;
              if (ci != null)
              {
                  ci.StartBringIntoView();
              }
          }
      }
  </code></pre>
  <p><strong>WinRTXamlToolkit.Controls.Extensions</strong> gave me the option to get scrollviewer and then try the <strong>ChangeView</strong> method but that isn't working. I got the First item from the list successfully and used the <strong>ContainerFromItem</strong> method but it returned me null. So I also tried <strong>ContainerFromIndex</strong> method and provided index as <strong>0</strong> because that is supposed to be the first item, but that aint working either.</p>
  <p>In case of selected item (else statement) it is working fine with <code>ContainerFromIndex(comboBox.SelectedIndex)</code> but just to test when I tried it with <strong>ContainerFromItem</strong> it returned null.</p>
  <p>Just FYI, this event is in an attached behavior to the combobox style but that shouldn't matter because behavor works flawlessly for selected item scenario.</p>