index.html

<style>
  .is-hidden { display: none; }
  .is-visible { display: block; }
</style>

<div class="app-root">
  <div class="some-component">
    <div class="some-component-child-a">
      <button jsaction="foo">Click here</button>
    </div>
  </div>

  <div class="another-component is-hidden">
    <div class="another-component-child-b">
      Show this on [jsaction=foo] click
    </div>
  </div>
</div>