As a default behavior, Apple recently has hidden scroll bars in all windows until you hover your mouse over the window, or actually scroll in a window. Without scroll bars, in some cases, it makes it more difficult to ascertain that there might be more content "below the fold".
For example:
In the pictures of a browser window below, there are no visible scroll bars. So I might not realize that there is more content worth scrolling down for.
To make scroll bars always visible, do the following.
1. Go to the menus: Apple > System Preferences > General
2. In the "Show scroll bars" section, check the box "Always"
Go Back