In CSSSelectorParser::consumeRelativeScopeSelector we sometimes insert a PseudoClassType::RelativeScope pseudo-class at the front of the selector argument of a :has().  This is what ensures that the matching of the selector argument does not break out of the :has scope.

But we only do this if (a) we don't already mention :scope in the selector argument, or (b) the :has() scope combinator is not DescendantSpace.  But we do need this synthesized RelativeScope pseudo-class, otherwise this will alert true when it shouldn't:

<div id=x><span></span></div>
<script>
alert(x.matches(":has(:scope span)"));
</div>