WPF showing control with DataTrigger skips it in TabOrder

Question

I would like to show 'additional details' Textbox when the previous TextBox's value is not 0.00 (zero). This is easily accomplished with DataTriggers:

<TextBox Name="FirstTB" Text="{Binding Amount, StringFormat=F2}"/><!--when this TB is not 0-->
<TextBox Name="SecondTB"> <!--show this TB-->
    <TextBox.Style>
        <Style TargetType="{x:Type TextBox}">
            <Style.Triggers>
                <DataTrigger Binding="{Binding Amount}" Value="0">
                    <Setter Property="Visibility" Value="Hidden"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </TextBox.Style>
</TextBox>
<TextBox Name="ThirdTB"/>

The issue is, however, when changing value of FirstTB to <> 0 and pressing Tab the focus jumps to the ThirdTB instead of SecondTB (even though SecondTB now is Visible due to the DataTrigger). How can I fix this issue?

---

Sadly, UpdateSourceTrigger=PropertyChanged does not appear to be an option due to its interference with StringFormats - it gives terrible UX when you are editing the value, carret jumps around like crazy due to constant StringFormat evaluation. Viewmodel used in example above:

public class MyVM : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    private double _amount;
    public double Amount
    {
        get { return _amount; }
        set
        {
            _amount = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Amount)));
        }
    }

}

Answer 1

Maybe a stupid work around. But this will do the work:

View:

<TextBox Name="SecondTB" IsVisibleChanged="SecondTB_OnIsVisibleChanged">
     <!--show this TB-->
     <TextBox.Style>
         <Style TargetType="{x:Type TextBox}">
              <Style.Triggers>
                    <DataTrigger Binding="{Binding Amount, Mode=OneWay}" Value="0">
                         <Setter Property="Visibility" Value="Collapsed"/>
                    </DataTrigger>
              </Style.Triggers>
         </Style>
    </TextBox.Style>
</TextBox>

C#:

private void SecondTB_OnIsVisibleChanged(object sender, DependencyPropertyChangedEventArgs e)
{
    if (((bool) e.NewValue))
    {
        if(!(sender is TextBox txbx)) return;
        ThirdTB.GotFocus += ThirdTbOnGotFocus;
    }
}

private void ThirdTbOnGotFocus(object sender, RoutedEventArgs e)
{
    SecondTB.Focus();
    ThirdTB.GotFocus -= ThirdTbOnGotFocus;
}

Answer 2

If you use an IValueConverter you get a very elegant way for your problem. Everytime the value is changed, the logic is called.

public class IsZeroToHiddenConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value is double d && d == 0)
            return Visibility.Hidden;
        return Visibility.Visible;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

You calso play around and add ConverterParameter to decide if you want Collapsed or Hidden behavior.

Usage

<Grid>
    <Grid.Resources>
        <local:IsZeroToHiddenConverter x:Key="IsZeroToHiddenConverter"/>
    </Grid.Resources>
    <Grid VerticalAlignment="Center" HorizontalAlignment="Center">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <TextBox x:Name="Tb1" Grid.Row="0" Width="100" Margin="5" Text="{Binding Path=Amount, StringFormat=F2, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
        <TextBox x:Name="Tb2" Grid.Row="1" Width="100" Margin="5" Visibility="{Binding Path=Amount, Converter={StaticResource IsZeroToHiddenConverter}}"/>
        <TextBox x:Name="Tb3" Grid.Row="2" Width="100" Margin="5"/>
    </Grid>
</Grid>

Preview