Pressing browser back while in lightbox sends user away from my site

Im using a lightbox button to play a video. I have some custom css to make it fullscreen, and Im using the &autoplay=1 trick to get it to play on load.

The issue is that if someone presses back on their browser (which feels like the natural thing to do on mobile when you have finished watching the video) it sends the user back to the page they were looking at before they loaded my page. (my whole site is only one page)

Is there a way to make the lightbox add a querystring or something to the URL so back button just closes the lightbox? Any other work arounds?


Here is my site Read-Only:
(how to share your site Read-Only link)