Took me a while to figure this out, but, and if I understand what's going on and you didn't realize it you're going to smack yourself, but...
Try clicking on that tiny window with everyone's video feed on it. It gets bigger again into a full (and resizable) window with everyone's video feed, while the window you are sharing is still being shared (and outlined in red).
Apologies if I misunderstand or this doens't apply to you (I'm on MacOS), but it literally took me months of being frustrated with that situation before I realized clicking on the tiny window would restore it to a full window, so I figured that might be you too. I forget, maybe it requires a double-click.