Scrollbar: none ?

... a simulation of a spectre on desktop ...

(not responsive, to view on desktop only)

Background

Triggering for this improvisation was the scrollbar-width property in the Editors Draft of the CSS Scrollbars Module Level 1 (ED 14 November 2018, work in progress), and the Github-opinions about it.
 

Box 1

This is the "worst case scenario". Imagine this. The company of the site has bought a template of a skilled designer. In the template, accompanied by a manual, are all the tools to get the site easily usable for mobile and desktop devices.
Company happy: the old website didn't pay too much attention to mobile (a growing amount of their customers).
The young PR assistant is the webmaster of the company, and is mostly concerned about the content of the site ("the rest is done by
the CMS"). In his/her enthusiasm (s)he had overlooked the paragraph in the manual in which was explained that with a hidden scrollbar (animated by touch) something extra has to be done for desktops.
Focused on cell phones, (s)he asked his/her friends: "Does it work on your GSM?". Now the friends are of the generation that never touches a desktop, so the answers were: "Yes! Fully responsive! Funny scrollbars! It is working like gangbusters!"

In the meantime a desktop visitor of the site doesn't see that there is more in Box 1. - A mouse exploration doesn't do anything (unless (s)he touches by accident the scroll wheel on the mouse when the mouse pointer is in the box).

Box 2

In this scenario the scrollbar is animating for a moment, if the desktop visitor has the luck to mouse slowly over the right border of Box 2.
Then (s)he can go back. But if found, the scrollbar is "thin" and difficult to grab with the mouse pointer.

Box 3

This scenario has a "custom scrollbar", facilitated by a scrollbar-width:none rule. - An animation points to the Down and Up buttons.
In this clean minimal design the Down and Up buttons are the only remnants of the old scrollbar. These small buttons are friendly for
touch devices, but again hard to use for desktop visitors; and too many scrollbar variations can be confusing.

Partial solutions?

Maybe a mandatory indication that the element is scrollable, which will conjure up the scrollbar; more or less like this:

What could help for 'thin' scrollbars on devices with a pointer (desktops/laptops), is to extend the Visible thin scrollbar with
an invisible (transparent) but Active surface left and right. Then the grabbing area for the pointer can be as width as the default
scrollbar, and is easy to catch.

Other conditions?

Realizing that improper use by "an unknown part of CSS-users worldwide" is inevitable, I think it would be great if there could
be certain mandatory conditions: applications other than the platform scrollbar(s) are only permitted/working if the conditions
are fulfilled. Maybe some rules could be made platform/feature-dependent in some way, in the spirit of:

• (default)
  • scrollbar-width: "auto" (default platform-size)
  • scrollbar-width: "thin" (thin platform-size, with defined min-width of ..px) (desktop usability)
  • scrollbar-width: <length> only for custom scrollbar; min-width = thin platform-size; max-width = default platform-size
  • scrollbar-width: ("none" = not allowed, neutralized by error handling)
   
• @media (hover: none) {
  • scrollbar-width: "auto" (default platform-size)
  • scrollbar-width: "thin" (thin platform-size, with defined min-width of ..px) (can be smaller than the default thin)
  • scrollbar-width: <length> only for custom scrollbar; no min-width; max-width = default platform-size
  • scrollbar-width: "none"
  }