title: Apply CSS transition to element in Angular
body: <p>I have the following popup element on and Angular 12 application:</p>
  <pre><code>&lt;div class=&quot;window&quot; [hidden]=&quot;!active&quot; (clickOutside)=&quot;close()&quot;&gt;
    &lt;ng-content select=&quot;[window]&quot;&gt;&lt;/ng-content&gt;
  &lt;/div&gt; 
  </code></pre>
  <p>Is it possible to apply a CSS transition the DIV shows / hides?</p>
  <p>I was considering changing its opacity.</p>