When the bookmarks sidebar panel is open while I watch a YouTube video and I set the video to full screen, then it isn’t shown as full screen, because it shares the screen space with the bookmarks panel. The sidebar (with the bookmarks icon) disappears, though. This behavior is fairly recently, let’s say maybe two weeks or a bit less. Before that, it worked as expected - when switching a video to full screen, the bookmarks weren’t visible.
In case it is relevant (and indeed a bug and not intended behavior): I am on Linux with a Gnome desktop.
Why this is annoying: I have several YouTube channels that I am not subscribed to, because I only check them from time to time. I open my bookmarks and click through them one after the other and if I’m interested in a video, I watch in (in full screen). Because of this new behavior, I have to close the bookmarks, because I want to see the video in actual full screen. When I later open the bookmarks panel again, I have to open the category again and scroll to where I was before (there is at least one other topic on the latter issue already).