The code I am copying from Visual Studio uses spaces for indentation, in Blend I was using tabs, this means when I paste code from VS, Blend doesn’t see any tabs to ‘unindent’.

Simple solution – make sure your Visual Studio indentation settings match your Blend settings.

Here is my original code that kept losing it’s binding:

<ItemsControl ItemsSource="{Binding Elements}" Margin="0,10,0,10">
  <ItemsControl.ItemTemplate>
    <DataTemplate>
      <RadioButton Content="{Binding Key}"
        GroupName="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}}, Path=DataContext.Title}"
        IsChecked="{Binding Path=IsSelected}"
        Margin="0,5,0,5"/>
    </DataTemplate>
  </ItemsControl.ItemTemplate>
</ItemsControl>

There a couple of ways to solve this,

1. You can subclass the RadioButton
2. You could use OneWay binding on IsChecked to set the RadioButton state from code, then bind the Command property to a  Command when user clicks on the RadioButton.

   <ItemsControl ItemsSource="{Binding Elements}" Margin="0,10,0,10">
     <ItemsControl.ItemTemplate>
       <DataTemplate>
         <RadioButton Content="{Binding Key}"
           GroupName="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}}, Path=DataContext.Title}"
           IsChecked="{Binding Path=IsSelected, Mode=OneWay}"
           Command="{Binding IsSelectedCommand}"
           CommandParameter="{Binding}"
           Margin="0,5,0,5"/>
       </DataTemplate>
     </ItemsControl.ItemTemplate>
   </ItemsControl>

3. My preferred option: (as suggested by karliwatson) use a ListBox for the grouping and bind the ListBoxItem’s IsSelected property to your ViewModel, then bind the RadioButton’s IsChecked property to the list box item’s IsSelected property. Don’t set a group on the RadioButton.

   <ListBox ItemsSource="{Binding Elements}" Margin="0,10,0,10" BorderThickness="0" Background="Transparent">
     <ListBox.ItemContainerStyle>
       <Style TargetType="{x:Type ListBoxItem}">
         <Setter Property="IsSelected" Value="{Binding Path=IsSelected, Mode=TwoWay}" />
         <Setter Property="Template">
           <Setter.Value>
             <ControlTemplate TargetType="ListBoxItem">
               <ContentPresenter/>
             </ControlTemplate>
           </Setter.Value>
         </Setter>
       </Style>
     </ListBox.ItemContainerStyle>
     <ListBox.ItemTemplate>
       <DataTemplate>
           <RadioButton Content="{Binding Key}"
               IsChecked="{Binding Path=IsSelected, Mode=TwoWay, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListBoxItem}}}"
               Margin="0,5,0,5"/>
       </DataTemplate>
     </ListBox.ItemTemplate>
   </ListBox>