Those of you experiencing client window selection issues under Windows 10, should note that in setting compatibility mode to a previous windows version on a game or app, can cause certain software not not find the window or task properly. I believe that Win10 is isolating the process a further layer, maybe a container when doing so. Setting the client.exe to normal operation under Win10 was the only way that I have found to correct these selection issues.
Others trying to access multiple windows at once should also remember that as an operating system, Windows (all versions) are only designed to take UI input from a single window at a time. In other words, the keys and mouse clicks you send can only access 1 window at a time. If you need to send commands to another window, you will need to select it, thus making it the foreground window, prior to sending anything. The only way to access different windows in a truley simultaneous manner, would be with additional machines. Be they virtual (VMWare) or actual machines and sending commands over the LAN to the other computers.