Issues with mouse not completely switching screens

[Jeannie 0]

I have 2 windows machines (server is 10, client is 8.1) when I switch to the client machine, and hold the mousebutton down to drag or select etc, the mouse is clicking on the main screen as well. I'm playing some games in picture in picture mode and if I swap to the client machine the main on also clicks every time I click or spins my characters around when i hold the mouse button down and move the mouse.

 

My main question is: how do I make the mouse fully separate between screens? Seems a bit weird - I've tried just about every setting I could test and there's been no change. I'm running 1.13.1-stable-063519a8 on both server and client. The keyboard works great, no echoing on the main server screen, but the mouse does funny stuff when clicking or holding the mouse buttons down in the client.

This is not an intermittent issue whatsoever, it happens every time as is 100% reproducible. Is there a configuration item that I'm missing perhaps?