Updated June 10th, 2019.
For some reason, starting in Mac OS 10.7 the scroll bars don’t show up. At least not until you mouse over them. That’s just silly. Here’s how you get them back.
Apple Menu/System Preferences/General/”Show scroll bars: Always”
See below (MacOS 10.13)
Here’s the General preference pane for Mojave (macOS 10.14).