Make this retrocomputing scanline rock. Scan multiple lines in div is hard

Working fiddle be here: http://jsfiddle.net/WyXLB/1/

When a HTML element contains multiple lines of text, I want to scan over each one. At the moment, I am just scanning over the bounding rectangle of the entire element.

Some code is :

function run_scan(el,mask,callback) {
    // we need to go over each line of text in here
    // or somewhere
    var el_font_size = $(el).css('font');
    $('body').append(sizer);
    sizer.css({
        'position': 'absolute',
        'font' : el_font_size,
        'white-space' : 'pre' } );
    var real_box = window.getComputedStyle(sizer[0]);
    var real_width = parseFloat(real_box.width);
    var real_height = parseFloat(real_box.height);
    var lines = Math.round($(el).height()/real_height);
    var mask_offset = $(mask).offset();
    var origin_left = mask_offset.left;
    var duration = parseFloat($(mask).width())/2.0;
    $(mask).animate( {
        'left':origin_left+$(mask).width()
        },
        {
            duration:duration,
            complete:callback
        } );
}

How can I break the animation over the entire bounding rectangle, to scan over each line. I tried putting an animation->callback recursion in the above function, descending the mask by one real_height each call, but got unpredictable/chaotic behavior.

If you need only loop then it should look like this (inside run_scan)

    var i = 0;
    function loop() {
        if (++i < lines) {
            $(mask).animate({
                'left':origin_left+$(mask).width()
            },{
                duration:duration,
                complete:loop
            });
        } else {
            // last line
            callback();
        }
    }