Show field on node edit page

Came across this and GaëlG's worked great when there was a link set. If one was not set, it would cause errors. So, I modified one part of it:

function MY_MODULE_save_menu_link_fields(array $form, FormStateInterface $form_state) {
  if ($link = _ys_core_get_link($form_state)) {
    // Only save the menu item extras if there is a menu link with a route.
    if ($link->getUrlObject()->getRouteName()) {
      $form_display = EntityFormDisplay::load('menu_link_content.' . $link->getMenuName() . '.default');
      assert($form_display instanceof EntityFormDisplay);
      $form_display->extractFormValues($link, $form['menu']['link']['extra'], $form_state);
      $link->save();
    }
  }
}

The change is the if ($link->getUrlObject()->getRouteName()) { which checks to see if there is an actual route in the link. If so, go ahead and save the menu item extras fields. If not, ignore it.

Thank you all. Thanks GaëlG and codechefmarc for #6 and #8. I got far with these!

I needed to patch Plugin/Field/FieldWidget/MenuItemExtrasViewModeSelectorSelect.php to work with link view modes on node add and edit forms. The widget was picking up the node storage view modes rather than the menu_link_content view modes given to us by extras. This change seems to work in node forms and menu forms, hopefully others can confirm.