filter() and is() match nothing for a positional pseudo-class left of a combinator (ul:first > li) #132

Description of the bug

filter() and is() match nothing when a positional pseudo-class (:first, :eq(), …) sits on a compound to the left of a combinator. find() handles the same selector:

$xml = '<?xml version="1.0"?><root><ul id="u1"><li id="a"/><li id="b"/></ul><ul id="u2"><li id="c"/></ul></root>';

qp($xml)->find('ul:first > li');          // a, b
qp($xml, 'li')->filter('ul:first > li');  // (none), expected a, b
qp($xml, 'li')->is('ul:first > li');      // false, expected true
qp($xml, 'li')->filter('ul:eq(1) > li');  // (none), expected c

Cause

In initialized mode (the candidates are tested as themselves), DOMTraverser resolves a non-subject compound that carries a positional pseudo-class by searching the candidates' descendants instead of the document. Found during #128 (DOMTraverser::filterNodes()), which preserves the existing behaviour.

Expected

The same as jQuery's .filter(): a candidate matches if it is one of the elements find() would return for the selector from the document.

QueryPath version

main at 9d04ccf.