Screen divided into quarters during sharing

Weird thing happened this morning. A student of mine shared his screen with me, which we do roughly once per term. When he opened sharing for his desktop, I could see his web browser with a Zoom page showing - he was using the Zoom app, but had the browser open. His Apple Logic Pro application was already open under that window. Now, when he closed the web browser, only the top quarter of the Logic screen showed up, and behaved as expected - I could see his mouse pointer and the actions he was performing. But the rearming three quarters of my screen showed the web browser page he had closed. Like those three quadrants were frozen. We tried quitting an opening a new meeting (initiated at his end not mine) but the same thing happened. When his screen was already on Logic Pro before sharing, it looked right until he started doing things and then I could see that three quarters of my view were in fact frozen.

He is on an Intel Mac running OS X Mojave and I am on a Mac M1 running Sonoma, but I also tried to join the meeting on an old Mac laptop, same spec as the student’s and the same thing happened.

Anyone else experiences this mad behaviour?