findmiddlediv.js

function findMiddleDiv(){

    var scrollTop = $(window).scrollTop();
    var winHeight = $(window).height();
    var middleHeight = winHeight / 2;

    var visibleElements = $('.platform-section');

    var $midElement = visibleElements.eq(0);
    var targetId,
        targetLink;
    var elementOffset,
        distanceFromTop = 0,
        currentDistance = 0;

    visibleElements.each(function(index, element) {
        elementOffset = $(this).offset().top;
        distanceFromTop = ( elementOffset - scrollTop );

        if( distanceFromTop <= middleHeight) {
            $midElement = $(element);
            platformPager.addClass("stick");
            currentDistance = distanceFromTop;
        }

    });

    if( currentDistance == 0 ){
        platformPager.removeClass("stick");
    }

    targetId = $midElement.attr('id');
    targetLink = $('a[data-target="#'+targetId+'"]');
    targetLink.addClass('active').parent('li').siblings().children('.pager-link').removeClass('active');

}