The following example demonstrates how to prevent horizontal scrolling of the group-by control in the fixed header section.

XAML
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<Grid xmlns:xcdg="http://schemas.xceed.com/wpf/xaml/datagrid">
  <Grid.Resources>
    <xcdg:DataGridCollectionViewSource x:Key="cvs_orders"
                                       Source="{Binding Source={x:Static Application.Current},
                                                        Path=Orders}">
      <xcdg:DataGridCollectionViewSource.GroupDescriptions>
        <xcdg:DataGridGroupDescription PropertyName="ShipCountry"/>
      </xcdg:DataGridCollectionViewSource.GroupDescriptions>
    </xcdg:DataGridCollectionViewSource>
  </Grid.Resources>

  <xcdg:DataGridControl x:Name="OrdersGrid"
                        ItemsSource="{Binding Source={StaticResource cvs_orders}}">
    <xcdg:DataGridControl.View>
      <xcdg:TableView UseDefaultHeadersFooters="False">
        <xcdg:TableView.FixedHeaders>
          <DataTemplate>
            <xcdg:GroupByControl xcdg:TableView.CanScrollHorizontally="True"/>
          </DataTemplate>
          <DataTemplate>
            <xcdg:ColumnManagerRow/>
          </DataTemplate>
        </xcdg:TableView.FixedHeaders>
      </xcdg:TableView>
    </xcdg:DataGridControl.View>
  </xcdg:DataGridControl>
</Grid>