drupal/core/modules/layout_builder/js/layout-builder.es6.js

(($, { ajax, behaviors }) => {
  behaviors.layoutBuilder = {
    attach(context) {
      $(context).find('.layout-builder--layout__region').sortable({
        items: '> .draggable',
        connectWith: '.layout-builder--layout__region',
        placeholder: 'ui-state-drop',

        /**
         * Updates the layout with the new position of the block.
         *
         * @param {jQuery.Event} event
         *   The jQuery Event object.
         * @param {Object} ui
         *   An object containing information about the item being sorted.
         */
        update(event, ui) {
          // Check if the region from the event and region for the item match.
          const itemRegion = ui.item.closest('.layout-builder--layout__region');
          if (event.target === itemRegion[0]) {
            // Find the destination delta.
            const deltaTo = ui.item.closest('[data-layout-delta]').data('layout-delta');
            // If the block didn't leave the original delta use the destination.
            const deltaFrom = ui.sender ? ui.sender.closest('[data-layout-delta]').data('layout-delta') : deltaTo;
            ajax({
              url: [
                ui.item.closest('[data-layout-update-url]').data('layout-update-url'),
                deltaFrom,
                deltaTo,
                itemRegion.data('region'),
                ui.item.data('layout-block-uuid'),
                ui.item.prev('[data-layout-block-uuid]').data('layout-block-uuid'),
              ]
                .filter(element => element !== undefined)
                .join('/'),
            }).execute();
          }
        },
      });
    },
  };
})(jQuery, Drupal);