Simple test:
1. Disable popup blocking.
2. Navigate a new tab to about:blank.
3. Open inspector.
4. x = 'moo';
5. window.open('', 'named');
6. In the new window, window.opener.x should be 'moo'.
7. Now go back to the original tab's inspector.
8. window.open('', 'named', 'noopener');
9. window.open() returns null, but the opened window did not disown its opener, and can still access window.opener.x.

The spec seems to suggest that the opener should still be disowned in this case.