core-bundle/src/Resources/contao/dca/tl_layout.php
public function checkFramework($value)
	{
		if (empty($value))
		{
			return '';
		}

		$array = Contao\StringUtil::deserialize($value);

		if (empty($array) || !\is_array($array))
		{
			return $value;
		}

		if (($i = array_search('responsive.css', $array)) !== false && !\in_array('layout.css', $array))
		{
			array_insert($array, $i, 'layout.css');
		}

		return $array;
	}
public function checkFramework($value)
	{
		if (empty($value))
		{
			return '';
		}

		$array = Contao\StringUtil::deserialize($value);

		if (empty($array) || !\is_array($array))
		{
			return $value;
		}

		if (($i = array_search('responsive.css', $array)) !== false && !\in_array('layout.css', $array))
		{
			array_insert($array, $i, 'layout.css');
		}

		return $array;
	}
Auto-select layout.css if responsive.css is selected (see #8222)

@param mixed $value

@return string