Source: modules/block/js/block.admin.es6.js

/**
 * @file
 * Block admin behaviors.
 */

(function ($, Drupal, debounce) {
  /**
   * Filters the block list by a text input search string.
   *
   * The text input will have the selector `input.block-filter-text`.
   *
   * The target element to do searching in will be in the selector
   * `input.block-filter-text[data-element]`
   *
   * The text source where the text should be found will have the selector
   * `.block-filter-text-source`
   *
   * @type {Drupal~behavior}
   *
   * @prop {Drupal~behaviorAttach} attach
   *   Attaches the behavior for the block filtering.
   */
  Drupal.behaviors.blockFilterByText = {
    attach(context, settings) {
      const $input = $('input.block-filter-text').once('block-filter-text');
      const $table = $($input.attr('data-element'));
      let $filterRows;

      /**
       * Filters the block list.
       *
       * @param {jQuery.Event} e
       *   The jQuery event for the keyup event that triggered the filter.
       */
      function filterBlockList(e) {
        const query = $(e.target).val().toLowerCase();

        /**
         * Shows or hides the block entry based on the query.
         *
         * @param {number} index
         *   The index in the loop, as provided by `jQuery.each`
         * @param {HTMLElement} label
         *   The label of the block.
         */
        function toggleBlockEntry(index, label) {
          const $label = $(label);
          const $row = $label.parent().parent();
          const textMatch = $label.text().toLowerCase().indexOf(query) !== -1;
          $row.toggle(textMatch);
        }

        // Filter if the length of the query is at least 2 characters.
        if (query.length >= 2) {
          $filterRows.each(toggleBlockEntry);
          Drupal.announce(
            Drupal.formatPlural(
              $table.find('tr:visible').length - 1,
              '1 block is available in the modified list.',
              '@count blocks are available in the modified list.',
            ),
          );
        } else {
          $filterRows.each(function (index) {
            $(this).parent().parent().show();
          });
        }
      }

      if ($table.length) {
        $filterRows = $table.find('div.block-filter-text-source');
        $input.on('keyup', debounce(filterBlockList, 200));
      }
    },
  };

  /**
   * Highlights the block that was just placed into the block listing.
   *
   * @type {Drupal~behavior}
   *
   * @prop {Drupal~behaviorAttach} attach
   *   Attaches the behavior for the block placement highlighting.
   */
  Drupal.behaviors.blockHighlightPlacement = {
    attach(context, settings) {
      // Ensure that the block we are attempting to scroll to actually exists.
      if (settings.blockPlacement && $('.js-block-placed').length) {
        $(context)
          .find('[data-drupal-selector="edit-blocks"]')
          .once('block-highlight')
          .each(function () {
            const $container = $(this);
            // Just scrolling the document.body will not work in Firefox. The html
            // element is needed as well.
            $('html, body').animate(
              {
                scrollTop:
                  $('.js-block-placed').offset().top -
                  $container.offset().top +
                  $container.scrollTop(),
              },
              500,
            );
          });
      }
    },
  };
})(jQuery, Drupal, Drupal.debounce);