CSS position absolute full width problem

You could set both left and right property to 0. This will make the div stretch to the document width, but requires that no parent element is positioned (which is not the case, seeing as #header is position: relative;)

#site_nav_global_primary {    
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
}

Demo at: http://jsfiddle.net/xWnq2/, where I removed position:relative; from #header

You need to add position:relative to #wrap element.

When you add this, all child elements will be positioned in this element, not browser window.

I have similar situation. In my case, it doesn't have a parent with position:relative. Just paste my solution here for those that might need.

position: fixed;
left: 0;
right: 0;