Position a Div "Fixed" Vertically and "Absolute" Horizontally within a "Position:Relative" Container Div

With JQuery, use the scrollLeft() property of the document! This would work

$(window).scroll(function(event) {
   $("#blue-box").css("margin-left", 400-$(document).scrollLeft());
});

See also

http://jsfiddle.net/zhQkq/9/

Good luck!

Edit: If you want it to use your preset margin-left instead of a hard-coded "400", use

$(window).scroll(function(event) {
   $("#blue-box").css("margin-left", $("#blue-box").css("margin-left")-$(document).scrollLeft());
});

Using vanilla javascript would be something like this:

var bb = document.getElementById('blue-box');
window.addEventListener('scroll',function(event){
    bb.style.marginLeft = window.scrollX + 'px';
});

In modern browsers, as of 2020, you should try to use the CSS position:fixed; instead of JavaScript positioning because it is widely supported now.