Feature: #109087 - Introduce BeforeBackendPageRenderEvent for BackendController 

Description 

A new PSR-14 event \TYPO3\CMS\Backend\Controller\Event\BeforeBackendPageRenderEvent has been introduced. It is dispatched in BackendController before the main backend page is rendered. It provides access to:

• $view ( ViewInterface ) – assign template variables to the backend top frame view
• $javaScriptRenderer ( JavaScriptRenderer ) – add custom JavaScript modules to the backend top frame
• $pageRenderer ( PageRenderer ) – add assets such as CSS files (marked @internal)

Example 

<?php

declare(strict_types=1);

namespace MyVendor\MyExtension\EventListener;

use TYPO3\CMS\Backend\Controller\Event\BeforeBackendPageRenderEvent;
use TYPO3\CMS\Core\Attribute\AsEventListener;
use TYPO3\CMS\Core\Page\JavaScriptModuleInstruction;

#[AsEventListener(identifier: 'my-extension/before-backend-page-render')]
final class BeforeBackendPageRenderEventListener
{
    public function __invoke(BeforeBackendPageRenderEvent $event): void
    {
        $event->javaScriptRenderer->addJavaScriptModuleInstruction(
            JavaScriptModuleInstruction::create(
                '@my-vendor/my-extension/backend-module.js'
            )
        );
    }
}

Impact 

It is now possible to add custom JavaScript modules and other assets to the TYPO3 backend top frame using the new PSR-14 event \TYPO3\CMS\Backend\Controller\Event\BeforeBackendPageRenderEvent .