tree.js

(function ($, Drupal, drupalSettings, once) {
  'use strict';

  /**
   * Attaches the JS.
   */
  Drupal.behaviors.TaxonomyManagerMergeTree = {
    attach: function (context, settings) {
      document.addEventListener('taxonomy_manager-tree-select', function (event) {
        let data = event.detail || {};

        if (data.tree.getSelectedNodes().length < 1) {
          // Disable the button if less the two terms are selected.
          document.getElementById('edit-merge').disabled = true;
        } else {
          let $mergeButton = document.getElementById('edit-merge');
          $mergeButton.disabled = false;
          if ($mergeButton.classList.contains('is-disabled')) {
            $mergeButton.classList.remove('is-disabled');
          }
        }
      });
    }
  };

})(jQuery, Drupal, drupalSettings, once);