Stop :autofill matching every element

[jdalton]

:autofill and :-webkit-autofill are handled by the pseudo_nop group, which breaks out of the compile loop without emitting a test. A resolver with no test accepts whatever it is given, so :autofill matches every element in the document and input:autofill matches every input.

Detail, and how it was checked

The group is claimed and then abandoned:

else if ((match = selector.match(Patterns.pseudo_nop))) {
  break;
}

Nothing is added to the compiled source, so the resolver answers true for whatever reaches it. On <input id=a><p id=b>x</p>:

selector	master 2e9498f	with this patch	jsdom's engine
:autofill	html, head, body, a, b	nothing	nothing
input:autofill	a	nothing	nothing
p	b	b	b

Emitting a test that never passes keeps the selector valid to write, which is what the group is for, and answers it the way a host with no autofill state should.

The case 'autofill' further down the same switch is unreachable today, because this group claims the selector first. Routing there would be the browser-accurate answer, and it belongs with #178: the call it makes is a raw e.matches(), which is the call that recurses under a host whose matcher routes back into this engine.

References: the spec, the browser source, and what each part was reasoned from

This patch applies to master on its own, and applies cleanly alongside the others in this series in any order.

• Spec: selector autofill — :autofill matches a control the user agent has autofilled.
• Chromium: selector_checker.cc#L2778 — the state is read from the control, not inferred.
• MDN: :autofill.

Found while reviewing #178, where the unguarded e.matches() in the unreachable handler turned up.