The new slider for video description was introduced some time ago, but now it’s even better! If you opened the video description and decided to switch to landscape mode, it will stay open on the right side of the screen. This way you can keep playing it while the video is playing.
Apart from that, if the video has chapters, you will be able to browse through them from the right section as well. If you like reading comments while watching videos, you can do that there as well. Unfortunately, you may need to exit full screen mode to switch between the different sections.
h / t Derek