It's a bad thing? I don't think so, if i want to read something or access something on the sidebar I would want it to stay open, plus you can just press the X mark to close it
I know. when the collection panel is open, it takes a fixed portion of the screen, so what will be left is determined as all the screen instead, thus when you switch a video to full-screen, it is Correctly using all of the screen that is accessible to it.
Collections is designed to be present on the screen whether you toggle full screen or even watching a video. You have to close it if you're not using it.