Fire event for when popup is toggled.

tp-popup.js

@@ -117,6 +117,13 @@ class TpPopup extends EventHelpers(Position(DomQuery(LitElement))) {
     this.listen(this, 'click', '_onClick');
   }
 
+  updated(changes) {
+    // Fire event only if isOpen wasn't undefined before. Otherwise an event is fired at the first render.
+    if (changes.has('isOpen') && changes.get('isOpen') !== undefined) {
+      this.dispatchEvent(new CustomEvent('is-open-changed', { detail: this.isOpen, bubbles: true, composed: true }));
+    }
+  }
+
   disconnectedCallback() {
     super.disconnectedCallback();
     this._cleanupEvents();