gistfile1.js

    Plugin.prototype.init = function(){
        var self = this,
        offset = $(this.element).offset(); //need to inject test data here
        $(window).scroll(function(event){
            //event.target is usually the object that jQuery was called on
            //I could have used this or $(window), but neither is testable
            if(elementIsOutsideViewport(event.target, offset)){
                self.elementScrolledOff(); 
            } 
            else{
                self.elementScrolledOn();
            }
        });
    }

    Plugin.prototype.elementScrolledOn = function(){
    }

    Plugin.prototype.elementScrolledOff = function(){    
    }


    function elementIsOutsideViewport(viewport,offset){
        var $viewport = $(viewport); //need to inject test data here
        return $viewport.scrollTop() > offset.top || $viewport.scrollLeft() > offset.left;
    }