First of all on-screen visible area is known as Viewport.

(image is taken from OP. Cleared and edited in Photoshop)

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So all you need is to detect all elements in your Viewport.

This can be achieved using many plugins for jQuery, but I’ll explain you on one example, which is called as jQuery withinviewport

Link to source and documentation on: [ withInViewport – Github ]

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Step 1:

Download plugins and include jQuery framework and withinviewport plugin in your script:

<script src="http://code.jquery.com/jquery-1.7.min.js"></script>
<script src="withinViewport.js"></script>
<script src="jquery.withinviewport.js"></script>

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Step 2:

Initialise function on scroll event:

$(window).bind("scroll", function() {
    //your code placeholder
});

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Step 3:

Use withinviewport selector to get all elements in you Viewport and by each element add class to corresponding list-item in your #timeline container:

$("#elements > div").withinviewport().each(function() {
   $('#timeline > li[view-id="'+$(this)[0].id+'"]').addClass('active');
});

Step 4:

Put all together:

$(window).bind("scroll", function() {

    //clear all active class
    $('#timeline > li').removeClass('active');

    //add active class to timeline
    $("#elements > div").withinviewport().each(function() {
         $('#timeline > li[view-id="'+$(this)[0].id+'"]').addClass('active');
    });
});

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Also this plugin gives you opportunity to set top, bottom, left and right offset for view-port.

See demo here: http://patik.com/code/within-viewport/