A custom nav walker for WordPress that uses the Bulma navbar styles (from bikubi/wp-bulma-navbar-walker)

bulma-nav-walker.php

<?php
/**
 * Custom nav menu walker with Bulma support
 *
 * @link https://developer.wordpress.org/reference/classes/walker_nav_menu/
 *
 * @package Bureau
 */

class Bulma_Nav_Menu extends Walker_Nav_Menu {

	public function start_lvl( &$output, $depth = 0, $args = array() ) {
		if ( isset( $args->item_spacing ) && 'discard' === $args->item_spacing ) {
			$t = '';
			$n = '';
		} else {
			$t = "\t";
			$n = "\n";
		}
		$indent = str_repeat( $t, $depth );

		$classes = array( 'navbar-dropdown', 'sub-menu' );

		$class_names = join( ' ', apply_filters( 'nav_menu_submenu_css_class', $classes, $args, $depth ) );
		$class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';

		$output .= "{$n}{$indent}<div$class_names>{$n}";
	}

	public function end_lvl( &$output, $depth = 0, $args = array() ) {
		if ( isset( $args->item_spacing ) && 'discard' === $args->item_spacing ) {
			$t = '';
			$n = '';
		} else {
			$t = "\t";
			$n = "\n";
		}
		$indent = str_repeat( $t, $depth );
		$output .= "$indent</div>{$n}";
	}

	public function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
		if ( isset( $args->item_spacing ) && 'discard' === $args->item_spacing ) {
			$t = '';
			$n = '';
		} else {
			$t = "\t";
			$n = "\n";
		}
		$indent = ( $depth ) ? str_repeat( $t, $depth ) : '';

		$classes = empty( $item->classes ) ? array() : (array) $item->classes;
		$classes[] = 'navbar-item';
		$classes[] = 'menu-item-' . $item->ID;

		if ($args->walker->has_children) {
			$classes[] = 'has-dropdown';
			$classes[] = 'is-hoverable';
		}

		$args = apply_filters( 'nav_menu_item_args', $args, $item, $depth );

		$class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args, $depth ) );
		$class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';

		$id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args, $depth );
		$id = $id ? ' id="' . esc_attr( $id ) . '"' : '';

		if ( $args->walker->has_children ) {
			$output .= $indent . '<div' . $id . $class_names .'>';
		}
		else {
			$output .= $indent;
		}

		$atts = array();
		$atts['title']  = ! empty( $item->attr_title ) ? $item->attr_title : '';
		$atts['target'] = ! empty( $item->target )     ? $item->target     : '';
		$atts['rel']    = ! empty( $item->xfn )        ? $item->xfn        : '';
		$atts['href']   = ! empty( $item->url )        ? $item->url        : '';

		if ($args->walker->has_children) {
			$atts['class'] = 'navbar-link';
		}

		$atts = apply_filters( 'nav_menu_link_attributes', $atts, $item, $args, $depth );

		$attributes = '';
		foreach ( $atts as $attr => $value ) {
			if ( ! empty( $value ) ) {
				$value = ( 'href' === $attr ) ? esc_url( $value ) : esc_attr( $value );
				$attributes .= ' ' . $attr . '="' . $value . '"';
			}
		}

		$title = apply_filters( 'the_title', $item->title, $item->ID );
		$title = apply_filters( 'nav_menu_item_title', $title, $item, $args, $depth );

		$item_output = $args->before;
		if ($args->walker->has_children) {
			$item_output .= '<a'. $attributes .'>';
		}
		else {
			$item_output .= '<a'. $id . $class_names . $attributes .'>';
		}
		$item_output .= $args->link_before . $title . $args->link_after;
		$item_output .= '</a>';
		$item_output .= $args->after;

		$output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
	}

	public function end_el( &$output, $item, $depth = 0, $args = array() ) {
		if ( isset( $args->item_spacing ) && 'discard' === $args->item_spacing ) {
			$t = '';
			$n = '';
		} else {
			$t = "\t";
			$n = "\n";
		}
		if ( $args->walker->has_children ) {
			$output .= "</div>{$n}";
		}
		else {
			$output .= "{$n}";
		}
	}

}