Why doesn't Next.js router work when tested in Jest?

Question

I am rendering a component that uses Next.js router.

When rendering in jsdom using Jest/Testing Library via Testing Library's render method, the router always returns null.

For example, returning this component in Jest

const DashboardSidebar: FC<DashboardSidebarProps> = (props) => {
  const { onMobileClose, openMobile } = props;
  const router = useRouter();
  const { user } = useAuth();

  useEffect(() => {
    if (openMobile && onMobileClose) {
      onMobileClose();
    }
  }, [router.pathname]);

}

will give the error message

TypeError: Cannot read property 'pathname' of null

      145 |       onMobileClose();
      146 |     }
    > 147 |   }, [router.pathname]);

Same goes for router.isReady. router is null.

I am assuming that the rendering of the component in to jsdom will run the router, since the router is defined within the component.

I just have to make the router run so that components will render content, and I do not want to test its functionality; I am UI testing, so implementation details are unnecessary.

How does Nextjs router work under the hood, and why isn't it running, and how can I make the router run?