code:
Interop.Call('user32', 'RedrawWindow', ChatWnd.Handle, 0, 0, 0x001 /* RDW_INVALIDATE */);
This would be better to use. Just send the window the WM_PAINT message to have it paint itself.
code:
Interop.Call('user32', 'SendMessageW', ChatWnd.Handle, 0x000F /* WM_PAINT */, 0, 0);