Neos\Neos\Controller\Backend\SettingsController::editPreviewAction PHP Method

    public function editPreviewAction()
    {
        $this->response->setHeader('Content-Type', 'application/json');
        $configuration = new PositionalArraySorter(Arrays::getValueByPath($this->settings, 'userInterface.editPreviewModes'));
        return json_encode($configuration->toArray());
    }