'Opens menu by clicking on the first child of the source element.'

def open(self):

    if (self.is_open() is False):
        dropdown = self._get_element(*self._dropdown_locator)
        def predicate(d):
            classes = dropdown.get_attribute('class').split()
            return (self._active_cls in classes)
        self._wait_until(predicate)
        dropdown.click()
        self._wait_till_element_visible(self._menu_container_locator)
