title: Looking for explanation for WPF Grid ColumnSpan behavior
body: <p>I asked a question at <a href="http://social.msdn.microsoft.com/Forums/en-US/wpf/thread/5c7f5cdf-4351-4969-990f-29ce9ec84b87/">http://social.msdn.microsoft.com/Forums/en-US/wpf/thread/5c7f5cdf-4351-4969-990f-29ce9ec84b87/</a> , but still lack a good explanation for a strange behavior.</p>
  
  <p>Running the following XAML shows that the TextBlock in column 0 is width greater than 100 even though the column is set to width 100. I think that the strangeness may have something to do with it being wrapped in a ScrollViewer, but I don't know why. If I set a MaxWidth on the columns, it works fine, but setting Width does not.</p>
  
  <ol>
  <li>Why is the width of column 0 not being honored?</li>
  <li>Why does the column sizing behave differently when you remove the scroll viewer?</li>
  </ol>
  
  <p>I appreciate any explanation! This is a real puzzle to me.</p>
  
  <pre><code>&lt;Window x:Class="WpfApplication2.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
          xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="MainWindow" Width="300"&gt;
      &lt;ScrollViewer HorizontalScrollBarVisibility="Auto" &gt;
          &lt;Grid&gt;
              &lt;Grid.RowDefinitions&gt;
                  &lt;RowDefinition Height="Auto" /&gt;
                  &lt;RowDefinition Height="Auto" /&gt;
              &lt;/Grid.RowDefinitions&gt;
              &lt;Grid.ColumnDefinitions&gt;
                  &lt;ColumnDefinition Width="100" /&gt;
                  &lt;ColumnDefinition Width="100" /&gt;
                  &lt;ColumnDefinition /&gt;
              &lt;/Grid.ColumnDefinitions&gt;
              &lt;TextBlock x:Name="textBlock" Text="{Binding ElementName=textBlock, Path=ActualWidth}" /&gt;
              &lt;TextBlock Text="column 1" Grid.Column="1" /&gt;
              &lt;TextBlock Grid.Row="1" Grid.ColumnSpan="3" Text="text here that is wider than the first two columns combined" /&gt;
          &lt;/Grid&gt;
      &lt;/ScrollViewer&gt;
  &lt;/Window&gt;
  </code></pre>