Mutating state does not re-render

NewQuizReacteasy~2 minimmutability, useState, Object.is

function List() {
  const [items, setItems] = useState(["a"]);

  function add() {
    items.push("b");
    setItems(items);
  }

  return (
    <>
      <button onClick={add}>Add</button>
      <p>{items.length}</p>
    </>
  );
}

What does the paragraph show after clicking Add once?

2, because the array now has two items

1, because React skips the update when the new state is the same reference

2, but React logs a mutation warning

It throws because state arrays are frozen