The new slider for video description was introduced a while ago but now it got even better! If you have the video description opened and decided to go to the landscape mode, it will remain being opened on the right side of the screen. In this way, you can continue reading it while the video is playing.
Apart from this, if the video has chapters, you will be able to browse them from the right section as well. If you like to read comments while watching videos, you can do it there as well. Unfortunately, you may need to exit from the full-screen mode in order to switch between different sections.