I have an image pop-up ability on my website (see this screenshot), in order to show users the full resolution picture when they click on a smaller version on the page.

This is the current CSS that positions it:

div#enlargedImgWrapper {
    position: absolute;
    top: 30px;
    left: 55px;
    z-index: 999;
}

The problem now is that if I click on an image further down the page, the window still appears in the top left corner of the page, where I can't see it until I scroll back up. I need it to appear relative to the window, whatever its current position relative to the document is.

Note: I don't want to use position: fixed; as some images might be taller than the screen, so I want users to be able to scroll along the image as well.

My idea was to use JS to change the top value:

var scrollValue = ???;
document.getElementById('enlargedImgWrapper').style.top = scrollValue+30 + 'px';

How can I detect by how much the user has scrolled down the page (var scrollValue)?
Or is there a 'better' way to do this?

Thanks!