Using document.addEventListener means it will work even if the DOM is updated without having to add new event listeners. If I'm not expecting the DOM to change I would be more inclined to do something like:

  document.querySelectorAll('.menu-wrapper')).forEach(menuWrapper => {
    const button = menuWrapper.querySelector('.menu-opener');
    const content = menuWrapper.querySelector('.menu-content');

    if (!content || !button) {
      return;
    }

    button.addEventListener(() => {
      button.setAttribute('aria-expanded', 'true');
      menu.showPopover();
    });

    content.addEventListener('toggle', e => {
      // reset back to aria-expanded=false on close
      if (e.newState == 'closed') {
        button.setAttribute('aria-expanded', 'false');
      }
    });
  });

The React example seems a little odd as well, if the "open" callback actually called "showPopover()" instead of only calling "setIsOpen" then the "useEffect" could be entirely redundant. The resulting code would be a lot clearer imo.