simple .NET WPF BackgroundWorker test where long running task reports text back and it scrolls into a textbox that won't get messed up when you click/select it. the textbox scrolls to the end as lines come in.

MainWindow.xaml

<Window x:Class="databinding.MainWindow"
        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"
        xmlns:local="clr-namespace:databinding"
        mc:Ignorable="d"
        Title="MainWindow"
        Height="450"
        Width="400">
    <Grid>
        <StackPanel Orientation="Vertical"
                    Margin="10">
            <Button Name="BackgroundWorkerButton"
                    Height="25"
                    Content="Do Async"
                    Margin="0 5 0 5"
                    Click="BackgroundWorkerButton_Click" />
            <Button Name="BackgroundWorkerButtonCancel"
                    Height="25"
                    Content="Cancel"
                    Margin="0 5 0 5"
                    Click="BackgroundWorkerButtonCancel_Click" />
            
            <TextBox Height="100"
                     IsReadOnly="True"
                     TextWrapping="Wrap"
                     AcceptsReturn="True"
                     SelectionChanged="progressBox_SelectionChanged"
                     ScrollViewer.VerticalScrollBarVisibility="Auto"
                     Text="Output will go here"
                     Name="progressBox" 
                     />
            <ProgressBar Name="BackgroundWorkerProgressBar"
                         Height="25"
                         Maximum="1000"
                         Margin="0 5 0 5" />
            <Button Name="UIResponsiveButton"
                    Height="25"
                    Content="UI Still Works"
                    Margin="0 5 0 5"
                    Click="UIResponsiveButton_Click" />
        </StackPanel>
    </Grid>
</Window>

MainWindow.xaml.cs

using System.ComponentModel;
using System.Text;
using System.Threading;
using System.Windows;

namespace databinding
{
    public partial class MainWindow : Window
    {
        private double progressBarMaximum;
        private StringBuilder stringBuilder;

        private BackgroundWorker backgroundWorker = new BackgroundWorker()
        {
            WorkerReportsProgress = true,
            WorkerSupportsCancellation = true
        };
        
        public MainWindow()
        {
            InitializeComponent();

            stringBuilder = new StringBuilder(1000);

            backgroundWorker.DoWork += (s, args) =>
            {
                backgroundWorker.ReportProgress(0);
                for (int i = 1; i <= progressBarMaximum; i++)
                {
                    Thread.Sleep(50);
                    backgroundWorker.ReportProgress(i, $"Computed {i}");
                    if (backgroundWorker.CancellationPending)
                    {
                        backgroundWorker.ReportProgress(0);
                        break;
                    }
                }
            };

            backgroundWorker.ProgressChanged += (s, args) =>
            {
                stringBuilder.AppendLine((string)args.UserState);
                progressBox.Text = stringBuilder.ToString();
                progressBox.ScrollToEnd();
                BackgroundWorkerProgressBar.Value = args.ProgressPercentage;
            };

            backgroundWorker.RunWorkerCompleted += (s, args) =>
            {
                BackgroundWorkerButton.IsEnabled = true;
                BackgroundWorkerButtonCancel.IsEnabled = false;
                
                stringBuilder = new StringBuilder(1000);
            };

            BackgroundWorkerButtonCancel.IsEnabled = false;
        }
     

        private void BackgroundWorkerButton_Click(object sender, RoutedEventArgs e)
        {
            BackgroundWorkerButton.IsEnabled = false;
            BackgroundWorkerButtonCancel.IsEnabled = true;

            progressBox.Text = "";
            progressBarMaximum = BackgroundWorkerProgressBar.Maximum;
            backgroundWorker.RunWorkerAsync();
        }

        private void UIResponsiveButton_Click(object sender, RoutedEventArgs e)
        {
            MessageBox.Show("UI still works");
        }

        private void BackgroundWorkerButtonCancel_Click(object sender, RoutedEventArgs e)
        {
            backgroundWorker.CancelAsync();
            BackgroundWorkerButton.IsEnabled = true;
        }

        private void progressBox_SelectionChanged(object sender, RoutedEventArgs e)
        {
            progressBox.ScrollToEnd();
            e.Handled = true;
        }
    }
}