I can't remember exactly, but notepad haven't multiple windows, it's using only one, only one handler as his MainWindowHandle..
On other side, SR_Client it's using multiple dialogs that can obtain the focus, then you have to focus the specific window to send the keys correctly (Or emulate it at least).
I was using AutoIt to avoid this issues and just send the keys to every window inside the SR_Client but SendMessage and PostMessage can be more usefull, multitask if used correctly.
You can take a look at