code: function( currentStep, nextStep ) {
              if ( lastEntered === currentStep ) {
                  lib.util.triggerEvent( currentStep, "impress:stepleave", { next: nextStep } );
                  lastEntered = null;
              }
          }
docs: `onStepLeave` is called whenever the currentStep element is left but the event is triggered only if the currentStep is the same as lastEntered step.