If a theme chooses to use the 'settings' block instead of flat navigation (Such as the popular Adaptable theme) then "Turn editing on" has been removed in the class 'settings_navigation' in 'lib/navigationlib.php' by MDL-67924 which was a change for the 'Boost' theme, however, that theme does not use the settings block so the impact could not have been anticipated to be beyond the scope of the intent of the improvement.  Thus the code in the navigationlib.php needs putting back and an alternative solution found.

Thus missing:

and this is the code: