title: How can I force a re-render of a lit-element grandchild?
body: <p>I have the following structure</p>
  
  <pre><code>&lt;my-app&gt;
     &lt;my-modal&gt; &lt;!-- 
       &lt;my-form&gt;&lt;/my-form&gt;
     &lt;/my-modal&gt;
  &lt;/my-app&gt;
  </code></pre>
  
  <p><code>my-modal</code> is designed as a generic wrapper, which I use to wrap many different dialogues within my app. In it I listen for a location change, and <code>render html '&lt;slot&gt;&lt;/slot&gt;'</code></p>
  
  <p>My problem is that, although <code>my-modal</code> is behaving correctly by showing and hiding the form, the form itself is never re-rendered, and so is always displayed showing stale content. I am currently kludging something using <code>IntersectionObserver</code> to fire when visible, but this feels really hacky and is causing other issues.</p>
  
  <p>Is there a way that my-modal can force its children to re-render, even though the only child it knows about is <code>&lt;slot&gt;&lt;/slot&gt;</code>. I don't want <code>my-app</code> to know anything about <code>my-modal</code>'s behaviours.</p>