BackendLayoutView.php

<?php
namespace Smichaelsen\MyExt\Backend\View;

use FluidTYPO3\Flux\Service\ContentService;

class BackendLayoutView extends \TYPO3\CMS\Backend\View\BackendLayoutView {

	/**
	 * Flux doesn't work well with TYPO3 backend layouts. The colPos item that is added in ext_tables.php is discarded by TYPO3's
	 * BackendLayoutView. That's why we re-add it here.
	 *
	 * @param array $parameters
	 */
	public function colPosListItemProcFunc(array $parameters) {
		parent::colPosListItemProcFunc($parameters);
		$parameters['items'][] = [
			$this->getLanguageService()->sL('LLL:EXT:flux/Resources/Private/Language/locallang.xlf:tt_content.tx_flux_container'),
			ContentService::COLPOS_FLUXCONTENT
		];
	}

}