discourse/app/assets/stylesheets/mobile/components/sidebar/edit-navigation-modal-form/modal.scss

.sidebar__edit-navigation-modal-form {
  .modal-inner-container {
    width: 35em;
  }

  .sidebar__edit-navigation-modal-form__filter {
    flex-direction: column;

    .sidebar__edit-navigation-modal-form__filter-dropdown {
      margin-left: 0;
      width: 100%;
    }
  }
}