[Splitscreen] Window focus isn't correctly given to left or right

[isXander]

After the second window attaches, there is no way to regain proper window focus on the first one, and if the second is unfocused, neither window ever gets focus. This prevents keyboard input, and may be the underlying cause of the necessity to pass window focus from the parent, since they can never get it themselves.

[isXander]

Potentially fix by manually giving the client windows focus on cursor enter/leave (this works), when the parent window is focused.