Screen sharing pauses or shows a black box when I go to another window

Windows laptop, zoom and windows updates all done and rebooted. 

Lately if I'm sharing a video with someone - specifically sharing that window - if I flick to another window (to take notes), the screen sharing pauses and/or shows the other person a black box. 

It never used to do this, I used to be able to keep sharing the video while taking my notes. It's really stuffing me around!

I don't want to share the notes window - I just want the window with the video to continue being shared with the other person.