title: Intersection Observer doesn't observe new elements
body: <p>I need to observe elements in list and mark them as read when they appear in the user's viewport. But when I add new elements in the beginning of the array (unshift), observer doesn't work :(</p>
  <p>I use Vue, but I know that the problem does not correlate with it.</p>
  <p>Here is observer method, which don't fire for new elements:</p>
  <pre class="lang-js prettyprint-override"><code>onElementObserved(entries) {
        entries.forEach(({ target, isIntersecting}) =&gt; {
            if (!isIntersecting) {
              return;
            }
            
            this.observer.unobserve(target);
          
            setTimeout(() =&gt; {
              const i = target.getAttribute(&quot;data-index&quot;);
              this.todos[i].seen = true;
            }, 1000)
        });
      }
  </code></pre>
  <p><a href="https://codepen.io/andrej-rusinas/pen/gOwLewa" rel="nofollow noreferrer">codepen</a></p>