title: Meaning of v-slot:activator="{ on }"
body: <p>Looking at the <a href="https://vuetifyjs.com/en/components/toolbars#toolbar" rel="noreferrer">Vuetify example code for <code>v-toolbar</code></a>, what is the purpose of <code>v-slot:activator="{ on }"</code>? For example:</p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;template v-slot:activator="{ on }"&gt;
    &lt;v-toolbar-title v-on="on"&gt;
      &lt;span&gt;All&lt;/span&gt;
      &lt;v-icon dark&gt;arrow_drop_down&lt;/v-icon&gt;
    &lt;/v-toolbar-title&gt;
  &lt;/template&gt;
  
  &lt;script&gt;
    export default {
      data: () =&gt; ({
        items: [
          'All', 'Family', 'Friends', 'Coworkers'
        ]
      })
    }
  &lt;/script&gt;
  </code></pre>
  
  <p>As far as I can see, <code>on</code> is not a defined variable anywhere, so I don't see how this is working. When I try it in my project, Internet Explorer throws an error on the <code>&lt;template v-slot:activator="{ on }"&gt;</code>, but if I remove it, the page renders.</p>