IIRC, the default setting for "Show scrollbars" is "Automatic based on mouse or trackpad": if you have a mouse connected, they're always visible, while if you have a trackpad, they only show up when you're scrolling. (There is no "never visible" setting.)

Your half right : if you have a mouse connected they only show up when you're scrolling

They coincided with using trackpads by default on the desktop.

The scrollbar appears if you use a scroll gesture.

It’s obvious with web pages, and with most content when you can scroll, and removal of visual noise and increasing the content area is good.

I personally use a mouse, and so I have them turned on, but generally I like Apple removing unnecessary clutter.

It’s really no big deal at all.

Scrollbars aren't visual noise, they are an indicator that tells you a) that there's more stuff than fits on the screen, b) where you are spatially, in relation to all that stuff, and c) how much more stuff there is, relative to what's shown on your screen. Hiding them leads to users often not figuring out that there's more stuff, and scrolling will get them there.

FWIW, pages are supposed to flash their scrollbars on load to indicate these.

Is that on load, or when the user finally switches to the tab they opened few minutes before by middle-clicking the link?

When they switch to the tab.

It’s when the control displaying the content first appears on the actual display.

Honestly, I've never seen that behavior in the wild. Is it Apple-specific?

It exists on Apple platforms.

If nobody else is doing it, then perhaps it is Apple specific.

> Scrollbars aren't visual noise, they are an indicator that tells you a) that there's more stuff than fits on the screen, b) where you are spatially, in relation to all that stuff, and c) how much more stuff there is, relative to what's shown on your screen.

They are visual noise if that information isn’t relevant to you.

If you’re proofreading a long book, they are likely to be more useful than just browsing the web.

> Hiding them leads to users often not figuring out that there's more stuff

Does it?

> and scrolling will get them there.

Really? With most content it’s fairly clear from the content itself.