web: Ensure custom element is only registered once

If it's registered twice, there might be a problem in the user's setup.

However, instead of letting the error be thrown, we'll just silently
ignore it and hope that the registered element is compatible with how
they use it.

Fixes #1043.

web/src/giscus.ts

@@ -2,10 +2,17 @@ import { html, css, LitElement, PropertyDeclaration } from 'lit';
 import { customElement, property } from 'lit/decorators.js';
 import { createRef, ref, Ref } from 'lit/directives/ref.js';
 
+function safeCustomElement(tagName: string) {
+  // Prevents re-registering an element.
+  return customElements.get(tagName)
+    ? (v: ReturnType<ReturnType<typeof customElement>>) => v
+    : customElement(tagName);
+}
+
 /**
  * Widget element for giscus.
  */
-@customElement('giscus-widget')
+@safeCustomElement('giscus-widget')
 export class GiscusWidget extends LitElement {
   private GISCUS_SESSION_KEY = 'giscus-session';
   private GISCUS_DEFAULT_HOST = 'https://giscus.app';