Extends bulma.io CSS framework's Menu component to make sub-menu items expand and collapse. Requires jQuery and Font Awesome.

bulma_collapsible_menu.js

$("ul").addClass('menu-list').not(':first').addClass('is-closed');
$(".is-closed").siblings('a').addClass('menu-has-children has-closed');

$("a.menu-has-children").on('click', function(){
  var firstUL = $(this).siblings('ul');
  if(firstUL.hasClass('is-closed'))
  {
    $(this).removeClass('has-closed');
    firstUL.removeClass('is-closed');	
  }
  else
  {
    $(this).addClass('has-closed');
    firstUL.addClass('is-closed');
  }
});

bulma_collapsible_menu.scss

ul.menu-list {
    a.menu-has-children {
        &:after {
            font-family: Font Awesome\ 5 Free;
            font-weight: 900;
            margin-left: 8px;
            content: "\f107";
            float: right;
        }
        
        &.has-closed:after {
            content: "\f105";
        }
    }

    &.is-closed {
        display: none;
    }
}