title: Prevent ComboBox to expand acording to selected item
body: <p>I want to prevent a ComboBox to resize acording to the size of the selected item. Consider this simplified example:</p>
  
  <pre><code>&lt;Window x:Class="MainWindow"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      Title="MainWindow" Height="150" Width="300"&gt;
      &lt;GroupBox Header="Group Header" Margin="5"&gt;
          &lt;ScrollViewer VerticalScrollBarVisibility="Auto" HorizontalScrollBarVisibility="Auto"&gt;
              &lt;Grid&gt;
                  &lt;Grid.ColumnDefinitions&gt;
                      &lt;ColumnDefinition Width="Auto"/&gt;
                      &lt;ColumnDefinition/&gt;
                  &lt;/Grid.ColumnDefinitions&gt;
                  &lt;Grid.RowDefinitions&gt;
                      &lt;RowDefinition Height="Auto"/&gt;
                  &lt;/Grid.RowDefinitions&gt;
                  &lt;ComboBox MinWidth="100" Grid.Column="0" Margin="5" MaxWidth="150"/&gt;
                  &lt;ComboBox Grid.Column="1" Margin="5" MinWidth="110"&gt;
                      &lt;ComboBoxItem Content="Very loooooooooooooooooooooooooooooooooooooooong text"/&gt;
                      &lt;ComboBoxItem Content="Normal text"/&gt;
                  &lt;/ComboBox&gt;
              &lt;/Grid&gt;
          &lt;/ScrollViewer&gt;
      &lt;/GroupBox&gt;
  &lt;/Window&gt;
  </code></pre>
  
  <p>When you select the first item in the second ComboBox, that ComboBox is expanded so the whole content of the selected item fits, hiding the ComboBox from the left</p>
  
  <p>I would like to achieve three things:</p>
  
  <ul>
  <li>Have the scrollviewer so the controls fit in their minimum size</li>
  <li>If the Window is very large, I want the second ComboBox to fit the remaining size</li>
  <li>If I select a very long item in the second ComboBox, I want it to preserve the size it had and <strong>not</strong> adjust to the contents of the selected item</li>
  </ul>
  
  <p>Is it possible?</p>