Craft CMS - conditionally propagate entry and category titles

app.php

<?php
// File: config/app.php

/**
 * Yii Application Config
 *
 * Edit this file at your own risk!
 *
 * The array returned by this file will get merged with
 * vendor/craftcms/cms/src/config/app.php and app.[web|console].php, when
 * Craft's bootstrap script is defining the configuration for the entire
 * application.
 *
 * You can define custom modules and system components, and even override the
 * built-in system components.
 *
 * If you want to modify the application config for *only* web requests or
 * *only* console requests, create an app.web.php or app.console.php file in
 * your config/ folder, alongside this one.
 */

return [
    'modules' => [
        'my-module' => \modules\Module::class,
    ],
    'bootstrap' => ['my-module'],
];

Module.php

<?php
// File: modules\Module.php

namespace modules;

use Craft;
use craft\base\Element;
use craft\elements\Category;
use craft\elements\Entry;
use yii\base\Event;

/**
 * Custom module class.
 *
 * This class will be available throughout the system via:
 * `Craft::$app->getModule('my-module')`.
 *
 * You can change its module ID ("my-module") to something else from
 * config/app.php.
 *
 * If you want the module to get loaded on every request, uncomment this line
 * in config/app.php:
 *
 *     'bootstrap' => ['my-module']
 *
 * Learn more about Yii module development in Yii's documentation:
 * http://www.yiiframework.com/doc-2.0/guide-structure-modules.html
 */
class Module extends \yii\base\Module
{
    /**
     * Initializes the module.
     */
    public function init()
    {
        // Set a @modules alias pointed to the modules/ directory
        Craft::setAlias('@modules', __DIR__);

        // Set the controllerNamespace based on whether this is a console or web request
        if (Craft::$app->getRequest()->getIsConsoleRequest()) {
            $this->controllerNamespace = 'modules\\console\\controllers';
        } else {
            $this->controllerNamespace = 'modules\\controllers';
        }

        parent::init();

        if (Craft::$app->request->isCpRequest) {
            Event::on(Element::class, Element::EVENT_BEFORE_SAVE, function (Event $event) {
                // Static var to memoize titles
                static $titles = [];
                // Entry section handles we want titles to propagate for
                $sectionHandles = ['news', 'sitePages'];
                // For readability
                $element = $event->sender;

                // If the element is a Category or an Entry in a desired section ...
                if ($element instanceof Category ||
                    ($element instanceof Entry && in_array($element->section->handle, $sectionHandles))
                ) {
                    if (!$element->propagating) {
                        // This is the first save, so memoize the title
                        // and let it save
                        $titles[$element->id] = $element->title;
                    } else if (isset($titles[$element->id])) {
                        // This is a subsequent save while propagating
                        // so propagate the first saved title
                        $element->title = $titles[$element->id];
                    }
                }
            });
        }
    }
}