When I click the screen share button, my screen shows an overlay with the option "Click to share screen. Esc to cancel" However, when I click Esc, the overlay stays over whichever screen my mouse moves to until I log out of the meeting.
I think the key does actually cancel the attempt to share because subsequent clicking on the screen does not initiate sharing. It's just that the overlay never gets removed.
Also, attempting to share again while the overlay is still up doesn't work either.