title: Fix DataGrid Column Headings When Scroll Viewer is Applied
body: <p>I have a DataGrid wrapped in a ScrollViewer like this...</p>
  
  <pre><code>&lt;ScrollViewer MaxHeight="600" VerticalScrollBarVisibility="Auto"
                BorderThickness="0" Padding="0"&gt;
      &lt;sdk:DataGrid ItemsSource="{Binding BatchItems}" IsReadOnly="True"&gt;
          &lt;sdk:DataGrid.Columns&gt;
              &lt;!-- Yada, yada, yada --&gt;
          &lt;/sdk:DataGrid.Columns&gt;
      &lt;/sdk:DataGrid&gt;
  &lt;/ScrollViewer&gt;
  </code></pre>
  
  <p>It works fine and the scrollbar appears when it should.  However, when I scroll it down, the DataGrid column headings move up and are no longer visible.  I want to freeze the column headings so that when I scroll down I still know what I am looking at.  How can this be done?</p>