Seems like there are still some menus (and in the case of Physics basically all the properties) that stayed behind the 2.9 UI polishing.
Particularly the Border Collisions’ menu in the Fluid>Domain property seems to do everything to not be visible 
. Other section that I didn’t show in the video but still has some floating menu is the particle System.
@julianeisel is this part of your “area”? Maybe can be tackled for the 3.0 release in order to have a more coherent UI?