Code

class DiagramViewEx : DiagramView
{
    public DiagramViewEx()
    {
        Diagram.BackBrush = new SolidBrush(Color.Transparent);
    }

    void PaintParentBackground(PaintEventArgs e)
    {
        if (Parent != null)
        {
            var rect = new Rectangle(Left, Top, Width, Height);

            e.Graphics.TranslateTransform(-rect.X, -rect.Y);

            try
            {
                using (var pea = new PaintEventArgs(e.Graphics, rect))
                {
                    pea.Graphics.SetClip(rect);
                    InvokePaintBackground(Parent, pea);
                    InvokePaint(Parent, pea);
                }
            }
            finally
            {
                e.Graphics.TranslateTransform(rect.X, rect.Y);
            }

            var behind = new Bitmap(Parent.Width, Parent.Height);
            foreach (Control c in Parent.Controls)
                if (c.Bounds.IntersectsWith(this.Bounds) && c != this)
                    c.DrawToBitmap(behind, c.Bounds);
            e.Graphics.DrawImage(behind, -Left, -Top);
            behind.Dispose();
        }
        else
        {
            e.Graphics.FillRectangle(
                SystemBrushes.Control, ClientRectangle);
        }
    }

    protected override void OnPaint(PaintEventArgs pe)
    {
        PaintParentBackground(pe);
        base.OnPaint(pe);
    }

    protected override void OnMouseMove(MouseEventArgs e)
    {
        base.OnMouseMove(e);

        if (Diagram.Interaction != null)
        {
            Diagram.Invalidate();
            Refresh();
        }
    }
}