Multilevel Menu but all in the same drop down?
Bootstrap has recently removed the sub-menu features from version 3.x, so it won’t support the multi-level menu anymore. But drupal 8 has multi-level menu structure. I am going to show you how to get back the sub menu features in bootstrap and how to render multi-level menu in the drupal 8. Just copy and paste the following css in your custom css, its has been taken from the bootstrap version 2.x with some custom changes, The following article would help to create multi-level menu in drupal 8