@5t3ph
> Important: This property only has an effect when the user’s system settings are not for “overlay” scrollbars, as in the default for Mac OS, where the scrollbar only appears as an overlay on content that is actively being scrolled.

As far as I know (I might be wrong), the default setting is "Automatically based on mouse or trackpad", not "When scrolling". With the "Automatic" setting, when you connect a mouse, which doesn't have touch scrolling, the scrollbars should appear. At least, that's how it works on my machine.