<TextBox TextWrapping="Wrap" Background="Transparent" Foreground="Transparent" Text="{Binding Word, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
<TextBlock TextWrapping="Wrap" ><Run Text="{Binding WordNormal, Mode=OneWay}" /><Run Text="{Binding WordOver, Mode=OneWay}" Background="Yellow" /></TextBlock>