How do I invalidate a control including it’s NonClient area?

You can do so as follows in your Control:

		private void InvalidateWindow()
		{
			NativeMethods.RedrawWindow(this.Handle, IntPtr.Zero, IntPtr.Zero, 
				0x0400/*RDW_FRAME*/ | 0x0100/*RDW_UPDATENOW*/
				| 0x0001/*RDW_INVALIDATE*/);
		}