DockPanel.cs

public class DockPanel : Panel
{
    protected override Size ArrangeOverride(Size finalSize)
    {
        foreach (var control in Children)
        {
            if (control.GetValue(DockProperty) == Dock.Left)
            {
                ArrangeToLeft(finalSize, control);
            }

            if (control.GetValue(DockProperty) == Dock.Right)
            {
                ArrangeToRight(finalSize, control);
            }
        }

        return finalSize;
    }

    private static void ArrangeToRight(Size finalSize, IControl control)
    {
        var x = finalSize.Width - control.DesiredSize.Width;
        var width = control.DesiredSize.Width;

        control.Arrange(new Rect(x, 0, width, finalSize.Height));
    }

    private static void ArrangeToLeft(Size finalSize, IControl control)
    {
        control.Arrange(new Rect(0, 0, control.DesiredSize.Width, finalSize.Height));
    }

    public static readonly PerspexProperty<Dock> DockProperty = PerspexProperty.RegisterAttached<DockPanel, Control, Dock>("Dock");

    public static Dock GetDock(PerspexObject element)
    {
        return element.GetValue(DockProperty);
    }

    public static void SetDock(PerspexObject element, Dock dock)
    {
        element.SetValue(DockProperty, dock);
    }

    public enum Dock
    {
        Bottom,
        Left,
        Right,
        Top
    }
}