R – Listbox Scrollbar does not resize properly when delete item from the bottom of list

I'm unable to reproduce your problem (for me, the ListBox is scrolled to the last item in the new collection when changing ItemsSource). Anyway, to scroll the ListBox to the top every time its ItemsSource changes you can use some code behind. First listen to changes in the ItemsSourceProperty and then scroll the ListBox to the top once its items has been generated

Update

Made an attached behavior that does this instead to avoid code behind. It can be used like this

<ListBox ...
         behaviors:ScrollToTopBehavior.ScrollToTop="True"/>

ScrollToTopBehavior

public static class ScrollToTopBehavior 
{
    public static readonly DependencyProperty ScrollToTopProperty = 
        DependencyProperty.RegisterAttached 
        (
            "ScrollToTop", 
            typeof(bool),
            typeof(ScrollToTopBehavior),
            new UIPropertyMetadata(false, OnScrollToTopPropertyChanged) 
        );
    public static bool GetScrollToTop(DependencyObject obj) 
    {
        return (bool)obj.GetValue(ScrollToTopProperty); 
    }
    public static void SetScrollToTop(DependencyObject obj, bool value) 
    {
        obj.SetValue(ScrollToTopProperty, value); 
    }
    private static void OnScrollToTopPropertyChanged(DependencyObject dpo, 
                                                     DependencyPropertyChangedEventArgs e) 
    {
        ItemsControl itemsControl = dpo as ItemsControl;
        if (itemsControl != null) 
        {
            DependencyPropertyDescriptor dependencyPropertyDescriptor =
                    DependencyPropertyDescriptor.FromProperty(ItemsControl.ItemsSourceProperty, typeof(ItemsControl));
            if (dependencyPropertyDescriptor != null)
            {
                if ((bool)e.NewValue == true) 
                {
                    dependencyPropertyDescriptor.AddValueChanged(itemsControl, ItemsSourceChanged);
                }
                else 
                {
                    dependencyPropertyDescriptor.RemoveValueChanged(itemsControl, ItemsSourceChanged);
                }
            } 
        } 
    }
    static void ItemsSourceChanged(object sender, EventArgs e)
    {
        ItemsControl itemsControl = sender as ItemsControl;
        EventHandler eventHandler = null;
        eventHandler = new EventHandler(delegate
        {
            if (itemsControl.ItemContainerGenerator.Status == GeneratorStatus.ContainersGenerated)
            {
                ScrollViewer scrollViewer = GetVisualChild<ScrollViewer>(itemsControl) as ScrollViewer;
                scrollViewer.ScrollToTop();
                itemsControl.ItemContainerGenerator.StatusChanged -= eventHandler;
            }
        });
        itemsControl.ItemContainerGenerator.StatusChanged += eventHandler;
    }
}

And an implementation of GetVisualChild

private T GetVisualChild<T>(DependencyObject parent) where T : Visual
{
    T child = default(T);
    int numVisuals = VisualTreeHelper.GetChildrenCount(parent);
    for (int i = 0; i < numVisuals; i++)
    {
        Visual v = (Visual)VisualTreeHelper.GetChild(parent, i);
        child = v as T;
        if (child == null)
        {
            child = GetVisualChild<T>(v);
        }
        if (child != null)
        {
            break;
        }
    }
    return child;
}

This may help you..

private void ListViewScrollViewer_PreviewMouseWheel(object sender, System.Windows.Input.MouseWheelEventArgs e)
{
   ScrollViewer scv = (ScrollViewer)sender;
   scv.ScrollToVerticalOffset(scv.VerticalOffset - e.Delta);
   e.Handled = true;
 }