Showing the preview section of fluidpages.

ext_localconf.php

<?php
$GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['cms/layout/db_layout.php']['drawHeaderHook'][] = \My\Extension\Hooks\PagePreviewRenderer::class . '->render';

PagePreviewRenderer.php

<?php
namespace My\Extension\Hooks;

use FluidTYPO3\Fluidpages\Provider\PageProvider;
use TYPO3\CMS\Backend\Controller\PageLayoutController;
use TYPO3\CMS\Core\Utility\GeneralUtility;
use TYPO3\CMS\Extbase\Object\ObjectManager;
use FluidTYPO3\Flux\Service\WorkspacesAwareRecordService;
use FluidTYPO3\Flux\View\PreviewView;

class PagePreviewRenderer {

    /**
     * @param array $params
     * @param PageLayoutController $pageLayoutController
     * @return string
     */
    public function render(array $params, PageLayoutController $pageLayoutController) {
        /** @var ObjectManager $objectManager */
        $objectManager = GeneralUtility::makeInstance(ObjectManager::class);

        /** @var WorkspacesAwareRecordService $recordService */
        $recordService = $objectManager->get(WorkspacesAwareRecordService::class);

        /** @var PageProvider $pageProvider */
        $pageProvider = $objectManager->get(PageProvider::class);

        $row = $recordService->getSingle('pages', '*', $pageLayoutController->id);
        $form = $pageProvider->getForm($row);

        #prevent rendering of the grid twice
        $form->setOption(PreviewView::OPTION_PREVIEW, array(
            PreviewView::OPTION_MODE => PreviewView::MODE_NONE
        ));

        list($previewHeader, $previewContent, $continueDrawing) = $pageProvider->getPreview($row);

        return $previewContent;
    }

}