YouTube has been testing different ways to help you keep watching videos on the web, while simultaneously letting you browse for more content or see details and comments. Just yesterday, the new Miniplayer PIP view was made official after months of tests, but there was also another miniplayer bar that would collapse the video at the top of the screen and let you scroll down to comments. Now, we're seeing yet another tests that's a little reminiscent of that, but it only affects the fullscreen view.
Some users are seeing a new "Scroll for details" option at the bottom of the YouTube window when in fullscreen. When they do scroll down, the video continues playing but they can read the description and check the comments. The video isn't collapsed anywhere, which is rather annoying, so you'll have to go back up to bring it into view. You can see how everything works in the GIF below.
This interaction reminds me a lot of Theater mode in YouTube — which I had never noticed until a few days ago. Perhaps YouTube is aware that most people either watch the video on the regular page or in fullscreen, and never activate Theater mode, so it's offering them most of the latter's same options but in fullscreen.
We first got tipped about this behavior a week ago, and both tipsters can only get the "Scroll for details" option to show up once every few attempts when browsing in Incognito. This still seems to be a limited server-side test.