How to get a WPF DataGrid cell to right align without making the selectable area on a new row tiny?

You could apply ElementStyle on the DataGridTextColumn targetted to TextBlock and right align that, it would work.

      <DataGridTextColumn Binding="{Binding Path=ImpaId}">
          <DataGridTextColumn.Header>
               <TextBlock  Style="{StaticResource
                                  DataGridHeader}">
                    Impa
               </TextBlock>
          </DataGridTextColumn.Header>
          <DataGridTextColumn.ElementStyle>
              <Style TargetType="{x:Type TextBlock}">
                  <Setter Property="HorizontalAlignment" Value="Right" />
              </Style>
          </DataGridTextColumn.ElementStyle>
      </DataGridTextColumn> 

Just wanted to add to more examples for future code searches.

I put this in the top of the xaml file:

<UserControl.Resources>
    <Style TargetType="{x:Type TextBlock}" x:Key="RightCell">
        <Setter Property="Background" Value="{Binding Included, Converter={StaticResource BoolToColorConverter}}"/>
        <Setter Property="HorizontalAlignment" Value="Stretch"/>
        <Setter Property="TextAlignment" Value="Right"/>
    </Style>
</UserControl.Resources>

And then in the datagrid:

<DataGridTextColumn Header="Excluded" Binding="{Binding Excluded}" ElementStyle="{StaticResource RightCell}"/>

This right aligns the text and sorting is still enabled. The textbox fills the cell and in this case is colored using a bool converter.

You can try a work around :

           <DataGridTemplateColumn>
            <DataGridTemplateColumn.CellTemplate>
                <DataTemplate>
                    <Grid Background="White" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
                        <TextBlock HorizontalAlignment="Right" Text="{Binding Path=ImpaId}"/>
                    </Grid>
                </DataTemplate>
            </DataGridTemplateColumn.CellTemplate>
        </DataGridTemplateColumn>