Extending Category Meta Box in WordPress admin

plugin.php

<?php

namespace Crumbls\Plugins\MetaBoxAlternate;

defined('ABSPATH') or exit(1);

/*
Plugin Name: Allow a filterd list for bloated category lists in the admin meta box
Plugin URI: http://crumbls.com
Description: Example of creating a third tab on the category selection box.  We used it to give our Editorial department the categories that we use for news stories, not similar category names.
Version: 0.0.1
Author: Chase C. Miller
*/

class AdminCategory {
    public function __construct() {
        add_action('add_meta_boxes', [$this, 'addMetaBoxes']);
    }

    // BizWest
    public function addMetaBoxes() {
        global $wp_meta_boxes;
        //$wp_meta_boxes

        // Find it first.
        $widget = false;
        foreach ($wp_meta_boxes['post'] as $p => &$priority) {
            if ($widget) {
                continue;
            }
            foreach ($priority as $context => &$widgets) {
                if ($widget) {
                    break;
                }
                if (array_key_exists('categorydiv', $widgets)) {
                    $widgets['categorydiv']['callback'] = [$this, 'metaCategory'];
                    return;
                }
            }
        }
    }

    public function metaCategory($post, $box) {
        $defaults = array('taxonomy' => 'category');
        if (!isset($box['args']) || !is_array($box['args'])) {
            $args = array();
        } else {
            $args = $box['args'];
        }
        $r = wp_parse_args($args, $defaults);
        $tax_name = esc_attr($r['taxonomy']);
        $taxonomy = get_taxonomy($r['taxonomy']);


        /** 
        * Below you will see the name Editorial.  
        * That is the new tab we are using.
        **/
        
        ?>
        <div id="taxonomy-<?php echo $tax_name; ?>" class="categorydiv">
            <ul id="<?php echo $tax_name; ?>-tabs" class="category-tabs">
                <li class="tabs"><a href="#<?php echo $tax_name; ?>-editorial"><?php _e('Editorial'); ?></a></li>
                <li class="hide-if-no-js"><a href="#<?php echo $tax_name; ?>-pop"><?php _e('Most Used'); ?></a></li>
                <li class="hide-if-no-js"><a href="#<?php echo $tax_name; ?>-all"><?php echo $taxonomy->labels->all_items; ?></a>
                </li>
            </ul>

            <?php
            $name = ($tax_name == 'category') ? 'post_category' : 'tax_input[' . $tax_name . ']';
            echo "<input type='hidden' name='{$name}[]' value='0' />"; // Allows for an empty term set to be sent. 0 is an invalid Term ID and will be ignored by empty() checks.
            ?>

            <div id="<?php echo $tax_name; ?>-editorial" class="tabs-panel">
                <?php

                // Category list here.
                $terms = get_terms( $tax_name, array(
                    'hide_empty' => false,
                ) );

        /**
        * $allow contains a list of parent categories that will show up in our list.
        **/
                $allow = [
                    34, // Industry News
                    95, // Newsletters
                ];

                if ($temp = array_filter($terms, function($e) use ($allow) {
                    return in_array($e->term_id, $allow) || in_array($e->parent, $allow);
                })) {
                    $terms = $temp;
                }

                // Reduce as needed.
                $args = [
                    'selected_cats' => array_map(function($e) {
                        return $e->term_id;
                    }, wp_get_object_terms(get_the_ID(), $tax_name))
                ];

                $walker = new \Walker_Category_Checklist;
                $output = call_user_func_array( array( $walker, 'walk' ), array( $terms, 0, $args ) );
                $output = str_replace('in-category-', 'in-editorial-category-', $output);

                // Reduce more.
                ?>
                <ul id="<?php echo $tax_name; ?>checklist" data-wp-lists="list:<?php echo $tax_name; ?>" class="categorychecklist form-no-clear">
                    <?php
                    echo $output;
                    ?>
                </ul>
            </div>

            <div id="<?php echo $tax_name; ?>-pop" class="tabs-panel" style="display: none;">
                <ul id="<?php echo $tax_name; ?>checklist-pop" class="categorychecklist form-no-clear">
                    <?php $popular_ids = wp_popular_terms_checklist($tax_name); ?>
                </ul>
            </div>


            <div id="<?php echo $tax_name; ?>-all" class="tabs-panel"  style="display: none;">
                <ul id="<?php echo $tax_name; ?>checklist" data-wp-lists="list:<?php echo $tax_name; ?>"
                    class="categorychecklist form-no-clear">
                    <?php wp_terms_checklist($post->ID, array('taxonomy' => $tax_name, 'popular_cats' => $popular_ids)); ?>
                </ul>
            </div>

            <?php if (current_user_can($taxonomy->cap->edit_terms)) : ?>
                <div id="<?php echo $tax_name; ?>-adder" class="wp-hidden-children">
                    <a id="<?php echo $tax_name; ?>-add-toggle" href="#<?php echo $tax_name; ?>-add"
                       class="hide-if-no-js taxonomy-add-new">
                        <?php
                        /* translators: %s: add new taxonomy label */
                        printf(__('+ %s'), $taxonomy->labels->add_new_item);
                        ?>
                    </a>
                    <p id="<?php echo $tax_name; ?>-add" class="category-add wp-hidden-child">
                        <label class="screen-reader-text"
                               for="new<?php echo $tax_name; ?>"><?php echo $taxonomy->labels->add_new_item; ?></label>
                        <input type="text" name="new<?php echo $tax_name; ?>" id="new<?php echo $tax_name; ?>"
                               class="form-required form-input-tip"
                               value="<?php echo esc_attr($taxonomy->labels->new_item_name); ?>" aria-required="true"/>
                        <label class="screen-reader-text" for="new<?php echo $tax_name; ?>_parent">
                            <?php echo $taxonomy->labels->parent_item_colon; ?>
                        </label>
                        <?php
                        $parent_dropdown_args = array(
                            'taxonomy' => $tax_name,
                            'hide_empty' => 0,
                            'name' => 'new' . $tax_name . '_parent',
                            'orderby' => 'name',
                            'hierarchical' => 1,
                            'show_option_none' => '&mdash; ' . $taxonomy->labels->parent_item . ' &mdash;',
                        );

                        /**
                         * Filters the arguments for the taxonomy parent dropdown on the Post Edit page.
                         *
                         * @since 4.4.0
                         *
                         * @param array $parent_dropdown_args {
                         *     Optional. Array of arguments to generate parent dropdown.
                         *
                         * @type string $taxonomy Name of the taxonomy to retrieve.
                         * @type bool $hide_if_empty True to skip generating markup if no
                         *                                      categories are found. Default 0.
                         * @type string $name Value for the 'name' attribute
                         *                                      of the select element.
                         *                                      Default "new{$tax_name}_parent".
                         * @type string $orderby Which column to use for ordering
                         *                                      terms. Default 'name'.
                         * @type bool|int $hierarchical Whether to traverse the taxonomy
                         *                                      hierarchy. Default 1.
                         * @type string $show_option_none Text to display for the "none" option.
                         *                                      Default "&mdash; {$parent} &mdash;",
                         *                                      where `$parent` is 'parent_item'
                         *                                      taxonomy label.
                         * }
                         */
                        $parent_dropdown_args = apply_filters('post_edit_category_parent_dropdown_args', $parent_dropdown_args);

                        wp_dropdown_categories($parent_dropdown_args);
                        ?>
                        <input type="button" id="<?php echo $tax_name; ?>-add-submit"
                               data-wp-lists="add:<?php echo $tax_name; ?>checklist:<?php echo $tax_name; ?>-add"
                               class="button category-add-submit"
                               value="<?php echo esc_attr($taxonomy->labels->add_new_item); ?>"/>
                        <?php wp_nonce_field('add-' . $tax_name, '_ajax_nonce-add-' . $tax_name, false); ?>
                        <span id="<?php echo $tax_name; ?>-ajax-response"></span>
                    </p>
                </div>
            <?php endif; ?>
        </div>
        <script>
            jQuery(document).ready(function($) {
                setTimeout(function() {
                    jQuery('#category-tabs li:first-child a').click();
                }, 300);
                jQuery('#categorydiv input[type="checkbox"]').change(function(e) {
                    var value = jQuery(this).val();

                    jQuery('#categorydiv input[value='+value+']').not(jQuery(this)).prop("checked", this.checked);
                })
            });
        </script>
        <?php
    }
}