In a meeting, if you share you local screen we cover the resulting image that is streamed to the meeting - only on the side of the person sharing!
This has a reason: If we wouldn't cover up the meeting itself, the person sharing their screen would see something like this:
To prevent this, we cover the screen with the Desktop sharing is active! box.
You can see if the desktop is successfully shared at the borders of the screen, where we intentionally left out a little bit to show you that it's working