A custom element is defined AFTER its declarative shadow root was already parsed into the DOM. During the upgrade, what happens with the existing shadow root and `attachShadow`?

<my-el> <template shadowrootmode="open">...</template> </my-el> <script> class MyEl extends HTMLElement { constructor() { super(); this.attachShadow({ mode: 'open' }); // what happens? } } customElements.define('my-el', MyEl); </script>