How to automatically scroll ScrollViewer - only if the user did not change scroll position

You can use ScrollChangedEventArgs.ExtentHeightChange to know if a ScrollChanged is due to a change in the content or to a user action... When the content is unchanged, the ScrollBar position sets or unsets the auto-scroll mode. When the content has changed you can apply auto-scrolling.

Code behind:

    private Boolean AutoScroll = true;

    private void ScrollViewer_ScrollChanged(Object sender, ScrollChangedEventArgs e)
    {
        // User scroll event : set or unset auto-scroll mode
        if (e.ExtentHeightChange == 0)
        {   // Content unchanged : user scroll event
            if (ScrollViewer.VerticalOffset == ScrollViewer.ScrollableHeight)
            {   // Scroll bar is in bottom
                // Set auto-scroll mode
                AutoScroll = true;
            }
            else
            {   // Scroll bar isn't in bottom
                // Unset auto-scroll mode
                AutoScroll = false;
            }
        }

        // Content scroll event : auto-scroll eventually
        if (AutoScroll && e.ExtentHeightChange != 0)
        {   // Content changed and auto-scroll mode set
            // Autoscroll
            ScrollViewer.ScrollToVerticalOffset(ScrollViewer.ExtentHeight);
        }
    }

Here is an adaptation from several sources.

public class ScrollViewerExtensions
    {
        public static readonly DependencyProperty AlwaysScrollToEndProperty = DependencyProperty.RegisterAttached("AlwaysScrollToEnd", typeof(bool), typeof(ScrollViewerExtensions), new PropertyMetadata(false, AlwaysScrollToEndChanged));
        private static bool _autoScroll;

        private static void AlwaysScrollToEndChanged(object sender, DependencyPropertyChangedEventArgs e)
        {
            ScrollViewer scroll = sender as ScrollViewer;
            if (scroll != null)
            {
                bool alwaysScrollToEnd = (e.NewValue != null) && (bool)e.NewValue;
                if (alwaysScrollToEnd)
                {
                    scroll.ScrollToEnd();
                    scroll.ScrollChanged += ScrollChanged;
                }
                else { scroll.ScrollChanged -= ScrollChanged; }
            }
            else { throw new InvalidOperationException("The attached AlwaysScrollToEnd property can only be applied to ScrollViewer instances."); }
        }

        public static bool GetAlwaysScrollToEnd(ScrollViewer scroll)
        {
            if (scroll == null) { throw new ArgumentNullException("scroll"); }
            return (bool)scroll.GetValue(AlwaysScrollToEndProperty);
        }

        public static void SetAlwaysScrollToEnd(ScrollViewer scroll, bool alwaysScrollToEnd)
        {
            if (scroll == null) { throw new ArgumentNullException("scroll"); }
            scroll.SetValue(AlwaysScrollToEndProperty, alwaysScrollToEnd);
        }

        private static void ScrollChanged(object sender, ScrollChangedEventArgs e)
        {
            ScrollViewer scroll = sender as ScrollViewer;
            if (scroll == null) { throw new InvalidOperationException("The attached AlwaysScrollToEnd property can only be applied to ScrollViewer instances."); }

            // User scroll event : set or unset autoscroll mode
            if (e.ExtentHeightChange == 0) { _autoScroll = scroll.VerticalOffset == scroll.ScrollableHeight; }

            // Content scroll event : autoscroll eventually
            if (_autoScroll && e.ExtentHeightChange != 0) { scroll.ScrollToVerticalOffset(scroll.ExtentHeight); }
        }
    }

Use it in your XAML like so:

<ScrollViewer Height="230" HorizontalScrollBarVisibility="Auto" extensionProperties:ScrollViewerExtension.AlwaysScrollToEnd="True">
    <TextBlock x:Name="Trace"/>
</ScrollViewer>

This code will automatically scroll to end when the content grows if it was previously scrolled all the way down.

XAML:

<Window x:Class="AutoScrollTest.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Height="300" Width="300">
    <ScrollViewer Name="_scrollViewer">
        <Border BorderBrush="Red" BorderThickness="5" Name="_contentCtrl" Height="200" VerticalAlignment="Top">
        </Border>
    </ScrollViewer>
</Window>

Code behind:

using System;
using System.Windows;
using System.Windows.Threading;

namespace AutoScrollTest
{
    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();

            DispatcherTimer timer = new DispatcherTimer();
            timer.Interval = new TimeSpan(0, 0, 2);
            timer.Tick += ((sender, e) =>
                {
                    _contentCtrl.Height += 10;

                    if (_scrollViewer.VerticalOffset == _scrollViewer.ScrollableHeight)
                    {
                        _scrollViewer.ScrollToEnd();
                    }
                });
            timer.Start();
        }
    }
}