[css-cascade-6] Scoping the subject of a `:has()` selector

This issue was raised on social media by @mayank99. The goal is to select a scope that has a given element in scope. However, since @scope only requires the 'subject' to be in-scope, and the contents of has are not selecting the subject, there is not currently any way to achieve this.

Here's a code-pen for experimenting.

It seems like this might be solvable (though not elegant) with a :scope-end selector (already proposed in another thread) that matches lower boundary elements:

@scope (my-scope) to (my-scope) {
  :scope:has(a:not(:scope-end a)) { … }
}

(You can get close to that solution by replacing :scope-end with my-scope - but it becomes fragile with further nesting.)

Since this issue seems specific to the :has() selector, the other approach might be changes on that end:

• Add a :scope-has() selector that does a scoped version of the same thing. It's clear and to the point, but maybe a bit of a special case?
• Resolve :has() differently when it's used on the :scope() selector. This is even more of a heuristic special case, but it really gets to the point of the issue. We want to know if the scope has something.

Other ideas?

The relevant spec text is:

Their selectors can only match elements that are in scope. (This only applies to the subject; the rest of the selector can match unrestricted.)

Start with the scoped style rules section of CSS Cascade 6 and the linked issue 8617 about :scope-end. Use the linked CodePen to reproduce the scoping cases and compare the proposed :scope-has(), :scope-end, and :has() approaches. Done means the Working Group has chosen and specified a resolution for selecting scoped subjects.