title: WPF control stretching issue
body: <p>I have a <code>DataGrid</code> in a <code>Grid</code> in a <code>TabItem</code>:</p>
  <pre><code>&lt;TabItem Header=&quot;SomeHeader&quot;&gt;
      &lt;Grid HorizontalAlignment=&quot;Stretch&quot; VerticalAlignment=&quot;Stretch&quot;&gt;
          &lt;Grid.RowDefinitions&gt;
              &lt;RowDefinition SharedSizeGroup=&quot;TabHeight&quot; /&gt;
          &lt;/Grid.RowDefinitions&gt;
          &lt;Grid.ColumnDefinitions&gt;
              &lt;ColumnDefinition SharedSizeGroup=&quot;TabWidth&quot;/&gt;
          &lt;/Grid.ColumnDefinitions&gt;
          &lt;DataGrid Margin=&quot;10&quot; Grid.Column=&quot;0&quot; Grid.Row=&quot;0&quot;
                    HorizontalAlignment=&quot;Stretch&quot;
                    VerticalContentAlignment=&quot;Stretch&quot;
                    IsReadOnly=&quot;True&quot; 
                    AutoGenerateColumns=&quot;False&quot;&gt;
          &lt;/DataGrid&gt;
      &lt;/Grid&gt;
  &lt;/TabItem&gt;
  </code></pre>
  <p>Problem is that <code>DataGrid</code> refuses to size to the containing <code>Grid</code>. What prevents it from doing so?</p>