https://drafts.csswg.org/css-overflow-4/#scrollbar-gutter-property

The overflow-* values set on the root or body elements are propagated to the viewport following the rules specified in section 3.5. Overflow Viewport Propagation of the CSS Overflow spec. The element from which the value was propagated will subsequently get a used value of visible.

The purpose of this issue is to ask what happens when scrollbar-gutter is used on an element from which overflow may be propagated to the viewport.

The expected outcome IMHO would be that the element gets a gutter that matches the scrollbar used by the viewport.

In any case, I think that the spec should cover this situation