Function install_document_root 

pub fn install_document_root(world: &mut World, root: Entity)

Record root as the DocumentRoot.

Every assembly records it, because everything that writes the root - set_root_class, the desktop’s class-change watcher - asks here rather than guessing from the hierarchy. It used to be recorded by the desktop alone, which is why set_root_class reached nothing in a page or a server render.