Why does the video screen appear on my phone app without a way to make it go away

Since a new release, when I share my screen on the phone, a video popup appears blocking 1/3 or so of the screen.  This is very disruptive to my screensharing where I"m supposed to be demoing my app for people who are going to buy it.  Can you please let me know how I can remove this popup so I can just demo full screen?