You can get the OS scrollbar gutter width as an integer in a --CSS-var on :root
https://codepen.io/propjockey/pen/ogzovYg?editors=1100
(to have a gutter at all you'll have to, on mac, turn on scrollbars to "always", etc, and refresh to ever see it non-zero)
But it's bugged in Safari:
https://bugs.webkit.org/show_bug.cgi?id=310597







