LibWeb: Add HTMLOptionsCollection selected index property

Author: https://github.com/bplaat
Commit: 5decf4b33c
Pull-request: https://github.com/SerenityOS/serenity/pull/23895
Reviewed-by: https://github.com/shannonbooth

Userland/Libraries/LibWeb/HTML/HTMLOptionsCollection.cpp

@@ -111,4 +111,17 @@ WebIDL::ExceptionOr<void> HTMLOptionsCollection::add(HTMLOptionOrOptGroupElement
     return {};
 }
 
+// https://html.spec.whatwg.org/#dom-htmloptionscollection-selectedindex
+WebIDL::Long HTMLOptionsCollection::selected_index() const
+{
+    // The selectedIndex IDL attribute must act like the identically named attribute
+    // on the select element on which the HTMLOptionsCollection is rooted.
+    return verify_cast<HTMLSelectElement>(*root()).selected_index();
+}
+
+void HTMLOptionsCollection::set_selected_index(WebIDL::Long index)
+{
+    verify_cast<HTMLSelectElement>(*root()).set_selected_index(index);
+}
+
 }