title: React-Spring transition 'leave' animation not working
body: <p>When spring items leave the DOM nothing (no leave animation) happens.</p>
  
  <p>Full CodeSandbox: <a href="https://codesandbox.io/s/jzz6xv1y4w" rel="nofollow noreferrer">https://codesandbox.io/s/jzz6xv1y4w</a></p>
  
  <pre><code>const Todo = ({ todo, onDeleteClick }) =&gt; {
    const transition = useTransition(todo, null, {
      from: { opacity: 0, transform: "translateY(-10px)" },
      enter: { opacity: 1, transform: "translateY(0)" },
      leave: { opacity: 0, transform: "translateY(10px)" }
    });
    return transition.map(
      ({ item, props, key }) =&gt;
        item &amp;&amp; (
          &lt;TodoContainer style={props} key={key}&gt;
            &lt;span&gt;{todo}&lt;/span&gt;
            &lt;button onClick={onDeleteClick}&gt;X&lt;/button&gt;
          &lt;/TodoContainer&gt;
        )
    );
  };
  </code></pre>