Ignoring while preserving line breaks in WPF TextBox
<Grid Background="AliceBlue">
  <Grid.RowDefinitions>
     <RowDefinition Height="100"/>
     <RowDefinition Height="100"/>
  </Grid.RowDefinitions>

  <TextBox x:Name="FirstTextBox"
     AcceptsReturn="True"
     TextWrapping="Wrap"
     Text="{Binding MyString}"
     Width="150"/>

  <TextBox x:Name="SecondTextBox"
     Grid.Row="1"
     AcceptsReturn="False"
     TextWrapping="NoWrap"
     Text="{Binding MyString}"
     VerticalAlignment="Top"/>
</Grid>

public class IgnoreLinebreaksConverter : IValueConverter
{
    private const string Sub = " u200B";
    private const string Lb = "rn";

    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var s = (string)value;
        return string.IsNullOrEmpty(s) ? s : Regex.Replace(s, Lb, Sub);
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var s = (string)value;
        return string.IsNullOrEmpty(s) ? s : Regex.Replace(s, Sub, Lb);
    }
}