On the Mac, scroll bars are essential for both orienting yourself and navigating within a Web page or document window. But they may not appear unless you hover the pointer over the right spot or start scrolling with a gesture on a trackpad or a turn of a mouse scroll wheel. This has been the default behavior since about macOS 10.7 Lion. If it bothers you as much as it bothers me, go to the Apple menu > System Preferences > General, and under Show Scroll Bars, select Always. That way, scroll bars will always be visible without you having to guess where they are or perform some incantation to reveal them.