Scroll bars on the Mac are now just as wafer-thin as they are on the iPhone and iPad—and in some cases, they may disappear completely when you’re not actively scrolling.
Here’s how to get those scroll bars back visible at all times:
Click the Apple menu at the top-left of the screen, then select System Preferences.
Next, select the General preferences pane; it’s the very first one, up at the top.
Under the “Show scroll bars” heading, you’ll find three options: “Automatically based on input device,” “When scrolling,” and “Always.”
Go ahead and select that last “Always” option. You’re done!