Edit: 240321
on the "Zoom / Mirror" button displayed on YouTube Player at the bottom of the menu, there's a Mirror button. You just don't see it. If it's not FullScreen, you have to scroll down.

Now you don't have to scroll. Just need to update to v5.3.3. I let it all show.
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It's at the bottom of the menu. If you don't see it, scroll down.