You don't even need to do that. I found a demo for the page visibility api[1] (presumably what they use to detect whether the tab is focused), and found that having the window not minimized is sufficient to make the tab think it's focused. That is, all you have to do is move the tab to a new window, and click your main window. Now your training video is in the background and not taking up any screen space (you can mute the tab if you want), but the page doesn't realized it's not focused/visible.
[1] https://jsfiddle.net/0GiS0/cAG5N/