Forever I've used the macOS Command-Shift-4 screenshot utility to measure things. Pressing it gets you a little crosshairs cursor which you can click-and-drag to take a screenshot but, crucially, has little numbers that tell you the width/height of the selection in pixels. It's crude, but ever so useful.
In macOS, users have the ability to set when they want to see scrollbars in windows. This affects all windows in the operating system, including in web browsers. They have three choices:
- Automatically based on mouse or trackpad
- When scrolling
Which means you are either in a state where you see them all the time, or a state where you only see them once scrolling has been started through some other means.