Scrolling while dragging and dropping (WPF)

I know this question really old, but here is the MVVM way as attached property:

    using System.Windows;
    using System.Windows.Controls;

    namespace AndroidCtrlUI.XTools.Behaviors
    {
        ///<summary>
        /// TreeItemAttach 
        ///<para/> TreeViewItem
        ///</summary>
        public sealed class TreeItemAttach
        {
            #region BringIntoView

            ///<summary>
            /// DependencyProperty
            ///</summary>
            public static readonly DependencyProperty BringIntoViewProperty = DependencyProperty.RegisterAttached("BringIntoView", typeof(bool), typeof(TreeItemAttach), new UIPropertyMetadata(false, (s, e) =>
            {
                if ((bool)e.NewValue != (bool)e.OldValue && s is TreeViewItem t)
                {
                    if ((bool)e.NewValue)
                    {
                        t.Selected += BringIntoView;
                    }
                    else
                    {
                        t.Selected -= BringIntoView;
                    }
                }
            }));

            ///<summary>
            /// Get
            ///</summary>
            ///<param name="target">DependencyObject</param>
            ///<returns>ICommand</returns>
            public static bool GetBringIntoView(DependencyObject target)
            {
                return (bool)target.GetValue(BringIntoViewProperty);
            }

            ///<summary>
            /// Set
            ///</summary>
            ///<param name="target">DependencyObject</param>
            ///<param name="value">ICommand</param>
            public static void SetBringIntoView(DependencyObject target, bool value)
            {
                target.SetValue(BringIntoViewProperty, value);
            }

            private static void BringIntoView(object sender, RoutedEventArgs e)
            {
                if (e.Source is TreeViewItem s)
                {
                    double h = s.ActualHeight;
                    if (s.IsExpanded && s.Items.Count > 0)
                    {
                        h = s.ActualHeight / TreeWalker(s);
                    }
                    s.BringIntoView(new Rect(0, h * -1, s.ActualWidth, h * 2.5));
                }
            }

            private static long TreeWalker(TreeViewItem item)
            {
                long c = item.Items.Count;
                foreach (object i in item.Items)
                {
                    if (i != null && item.ItemContainerGenerator.ContainerFromItem(i) is TreeViewItem t && t.IsExpanded && t.Items.Count > 0)
                    {
                        c += TreeWalker(t);
                    }
                }
                return c;
            }
            #endregion
        }
    }

And it can be used like:

<Style x:Key="TreeViewItemStyle" TargetType="{x:Type TreeViewItem}">
    <Setter Property="tool:TreeItemAttach.BringIntoView" Value="True"/>
</Style>

I have created an attached property for achieving this behavior, have a look at my post here -

Attached Behavior for auto scrolling containers while doing Drag & Drop

Main logic is something like this -

private static void OnContainerPreviewDragOver(object sender, DragEventArgs e)
{
    FrameworkElement container = sender as FrameworkElement;

    if (container == null) { return; }

    ScrollViewer scrollViewer = GetFirstVisualChild<ScrollViewer>(container);

    if (scrollViewer == null) { return; }

    double tolerance = 60;
    double verticalPos = e.GetPosition(container).Y;
    double offset = 20;

    if (verticalPos < tolerance) // Top of visible list? 
    {
        //Scroll up
        scrollViewer.ScrollToVerticalOffset(scrollViewer.VerticalOffset - offset);
    }
    else if (verticalPos > container.ActualHeight - tolerance) //Bottom of visible list? 
    {
        //Scroll down
        scrollViewer.ScrollToVerticalOffset(scrollViewer.VerticalOffset + offset);     
    }
}

Similar questions on SO (although they are mostly for ListBox/ListView but should work for TreeView too) -

WPF Listbox auto scroll while dragging

WPF ListView Databound Drag/Drop Auto Scroll

WPF Drag-to-scroll doesn't work correctly