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.
(Featured image by Taylor Wilcox on Unsplash)