RadPropertyGrid should not invalidate the properties when the help bar is resized.

Workaround: 

  class MyRadPropertyGrid : RadPropertyGrid
    {
        protected override void WndProc(ref Message m)
        {
            switch (m.Msg)
            {
                case 0x7b:
                    Point point;
                    int x = Telerik.WinControls.NativeMethods.Util.SignedLOWORD(m.LParam);
                    int y = Telerik.WinControls.NativeMethods.Util.SignedHIWORD(m.LParam);
                    if (((int)((long)m.LParam)) == -1)
                    {
                        point = new Point(this.Width / 2, this.Height / 2);
                    }
                    else
                    {
                        point = this.PointToClient(new Point(x, y));
                    }

                    this.PropertyGridElement.PropertyTableElement.ProcessContextMenu(point);
                    return;
            }

            base.WndProc(ref m);
        }
    }

Here is the code that should be used for custom sub-properties

private void OnCreateItem(object sender, CreatePropertyGridItemEventArgs e)
{
    e.Item = new MyPropertyGridItem((PropertyGridTableElement)sender, e.Parent);
}

As visual studio would be happy of you only provide a constructor with one argument when you inherit from PropertyGridItem you have no way of knowing that you need a second one where the parent should be the second argument.

The proper way to handle this would be to set the parent internally.