OrientationStackPanel

OrientationStackPanel.cs

using System;
using System.Collections.Generic;
using System.Diagnostics.Contracts;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;

namespace WPFLayoutTests
{
    public class OrientationStackPanel : StackPanel
    {
        private IEnumerable<FrameworkElement> ContainedControls => InternalChildren.Cast<FrameworkElement>();

        protected override Size ArrangeOverride(Size arrangeSize)
        {
            var leftPoint = new Point();
            var rightPoint = Orientation.Horizontal == Orientation ? new Point(arrangeSize.Width, 0) : new Point(0, arrangeSize.Height);

            ContainedControls.ForEach(element =>
            {
                Point location;

                if (Orientation.Horizontal == Orientation ? HorizontalAlignment.Right != element.HorizontalAlignment : VerticalAlignment.Bottom != element.VerticalAlignment)
                {
                    location = leftPoint;
                    leftPoint = Orientation.Horizontal == Orientation ? new Point(leftPoint.X + element.DesiredSize.Width, leftPoint.Y) : new Point(leftPoint.X, leftPoint.Y + element.DesiredSize.Height);
                }
                else
                {
                    rightPoint = Orientation.Horizontal == Orientation ? new Point(rightPoint.X - element.DesiredSize.Width, rightPoint.Y) : new Point(rightPoint.X, rightPoint.Y - element.DesiredSize.Height);
                    location = rightPoint;
                }

                element.Arrange(new Rect(location, new Size(element.DesiredSize.Width, element.DesiredSize.Height)));

            });

            return arrangeSize;
        }
    }
}

Example of use: