Screen sharing not working properly

[elmoG]

I was using Google Slides in Presenter view on my new Macbook. I highlighted the window containing the actual slide (as opposed to the window displaying my notes), but when I moved it on my screen, to make it smaller or just so I could read my notes in the window behind, the person I was speaking with would see the shared screen move, and partly disappear (e.g if I moved it to the bottom of my screen). I never had this problem on my old Mac. I've attached a screengrab so you can see what I mean. What's the solution  @Anonymous ? Thanks.