StringFormat or using Converter break the Undo / Redo of a textbox

[alraseensaad]

Description

Undo / Redo functionality of System.Windows.Controls.TextBox stops working correctly when the Text property is data-bound using either StringFormat or a Converter.

This issue occurs even when using UpdateSourceTrigger=LostFocus, not only PropertyChanged.

Reproduction Steps

Just create a Window like that and run tests

<Window
    x:Class="UIX.Tests.ControlTests.DecimalTextBoxTests.DecimalWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    xmlns:local="clr-namespace:UIX.Tests.ControlTests.DecimalTextBoxTests"
    xmlns:control="clr-namespace:AppControl;assembly=AppControl"
    d:DataContext="{d:DesignInstance Type=local:TestViewModel}">
	<Window.Resources>
		<local:StringFormatConverter x:Key="DecimalConverter" />
	</Window.Resources>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition
                Height="auto" />
            <RowDefinition />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition />
            <ColumnDefinition />
            <ColumnDefinition />
        </Grid.ColumnDefinitions>
        <StackPanel
            Grid.Row="1"
            Grid.Column="1">
            
            <Button
                Grid.Row="2"
                Margin="0 10"
                Grid.Column="1"
                Content="save" />
            <TextBox
                x:Name="DefaultTextBox"
					 Text="{Binding DefaultTextBox, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, ValidatesOnNotifyDataErrors=True}" />
			<TextBox x:Name="StringFormatTextBox"
					 Text="{Binding DefaultTextBox, Mode=TwoWay, UpdateSourceTrigger=LostFocus, ValidatesOnNotifyDataErrors=True,StringFormat=N2}" />
			<TextBox x:Name="ConverterTextBox"
					 Text="{Binding DefaultTextBox, Mode=TwoWay, UpdateSourceTrigger=LostFocus, ValidatesOnNotifyDataErrors=True,Converter={StaticResource DecimalConverter}}" />
		</StackPanel>
    </Grid>
</Window>

and that's the converter

internal class StringFormatConverter : IValueConverter
{
	public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
	{
		if (value is null) return string.Empty;
		var amount = value.ToString();

		if (decimal.TryParse(amount, NumberStyles.Any, CultureInfo.InvariantCulture, out decimal result))
		{
			return result.ToString("N2");
		}

		return value!;
	}
	public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
	{
		return value ?? Binding.DoNothing;
	}

} 

and that the tests

 	public class DefaultTextBoxTests
	{
		[WpfFact]
		public async Task DefaultTextBox_Should_UndoAndRedoFullValueChanges()
		{
			var window = new DecimalWindow();
			window.Show();

			var box = window.DefaultTextBox;
			var vm = (TestViewModel)window.DataContext;

			box.Focus();
			box.ApplyTemplate();
			box.UndoLimit = 100; // ensure undo stack works

			// Step 1: Initial value
			box.Text = "7000";
			box.MoveFocus(new TraversalRequest(FocusNavigationDirection.Next));
			await Task.Delay(200);
			Assert.Equal("7000", box.Text);
			Assert.Equal(7000m, vm.DefaultTextBox);

			// Step 2: Change value (simulate typing so undo is recorded)
			box.Focus();
			box.SelectAll();
			box.SelectedText = "5000";
			box.MoveFocus(new TraversalRequest(FocusNavigationDirection.Next));
			await Task.Delay(200);
			Assert.Equal("5000", box.Text);
			Assert.Equal(5000m, vm.DefaultTextBox);

			// Step 3: Undo → back to 7000
			box.Focus();
			Assert.True(box.CanUndo);
			box.Undo();
			box.MoveFocus(new TraversalRequest(FocusNavigationDirection.Next));
			await Task.Delay(200);
			Assert.Equal("7000", box.Text);
			Assert.Equal(7000m, vm.DefaultTextBox);

			// Step 4: Redo → back to 5000
			box.Focus();
			Assert.True(box.CanRedo);
			box.Redo();
			box.MoveFocus(new TraversalRequest(FocusNavigationDirection.Next));
			await Task.Delay(200);
			Assert.Equal("5000", box.Text);
			Assert.Equal(5000m, vm.DefaultTextBox);

			window.Close();
		}
		[WpfFact]
		public async Task StringFormatTextBox_Should_UndoAndRedoFullValueChanges()
		{
			var window = new DecimalWindow();
			window.Show();

			var box = window.StringFormatTextBox;
			var vm = (TestViewModel)window.DataContext;

			box.Focus();
			box.ApplyTemplate();
			box.UndoLimit = 100; // ensure undo stack works

			// Step 1: Initial value
			box.Text = "7000";
			box.MoveFocus(new TraversalRequest(FocusNavigationDirection.Next));
			await Task.Delay(200);
			Assert.Equal("7,000.00", box.Text);
			Assert.Equal(7000m, vm.DefaultTextBox);

			// Step 2: Change value (simulate typing so undo is recorded)
			box.Focus();
			box.SelectAll();
			box.SelectedText = "5000";
			box.MoveFocus(new TraversalRequest(FocusNavigationDirection.Next));
			await Task.Delay(200);
			Assert.Equal("5,000.00", box.Text);
			Assert.Equal(5000m, vm.DefaultTextBox);

			// Step 3: Undo → back to 7000
			box.Focus();
			Assert.True(box.CanUndo);
			box.Undo();
			box.MoveFocus(new TraversalRequest(FocusNavigationDirection.Next));
			await Task.Delay(200);
			Assert.Equal("7,000.00", box.Text);
			Assert.Equal(7000m, vm.DefaultTextBox);

			// Step 4: Redo → back to 5000
			box.Focus();
			Assert.True(box.CanRedo);
			box.Redo();
			box.MoveFocus(new TraversalRequest(FocusNavigationDirection.Next));
			await Task.Delay(200);
			Assert.Equal("5,000.00", box.Text);
			Assert.Equal(5000m, vm.DefaultTextBox);

			window.Close();
		}
		[WpfFact]
		public async Task ConverterTextBox_Should_UndoAndRedoFullValueChanges()
		{
			var window = new DecimalWindow();
			window.Show();

			var box = window.ConverterTextBox;
			var vm = (TestViewModel)window.DataContext;

			box.Focus();
			box.ApplyTemplate();
			box.UndoLimit = 100; // ensure undo stack works

			// Step 1: Initial value
			box.Text = "7000";
			box.MoveFocus(new TraversalRequest(FocusNavigationDirection.Next));
			await Task.Delay(200);
			Assert.Equal("7,000.00", box.Text);
			Assert.Equal(7000m, vm.DefaultTextBox);

			// Step 2: Change value (simulate typing so undo is recorded)
			box.Focus();
			box.SelectAll();
			box.SelectedText = "5000";
			box.MoveFocus(new TraversalRequest(FocusNavigationDirection.Next));
			await Task.Delay(200);
			Assert.Equal("5,000.00", box.Text);
			Assert.Equal(5000m, vm.DefaultTextBox);

			// Step 3: Undo → back to 7000
			box.Focus();
			Assert.True(box.CanUndo);
			box.Undo();
			box.MoveFocus(new TraversalRequest(FocusNavigationDirection.Next));
			await Task.Delay(200);
			Assert.Equal("7,000.00", box.Text);
			Assert.Equal(7000m, vm.DefaultTextBox);

			// Step 4: Redo → back to 5000
			box.Focus();
			Assert.True(box.CanRedo);
			box.Redo();
			box.MoveFocus(new TraversalRequest(FocusNavigationDirection.Next));
			await Task.Delay(200);
			Assert.Equal("5,000.00", box.Text);
			Assert.Equal(5000m, vm.DefaultTextBox);

			window.Close();
		}

	}

Expected behavior

User typing should be recorded in the undo stack

Ctrl + Z / Ctrl + Y should undo/redo user input

Formatting should not invalidate Undo/Redo history

Actual behavior

Undo stack is cleared after binding updates

Ctrl + Z either does nothing

Happens consistently when:

StringFormat is used

OR a Converter is used

Even with UpdateSourceTrigger=LostFocus

Regression?

No response

Known Workarounds

No response

Impact

DefaultTextBox_Should_UndoAndRedoFullValueChanges is pass while the others not

Configuration

dotnet 10

Other information

No response