How to overlay a div (or any element) over a table row (tr)?

You need to make the overlay div have an absolute position. Also use the position() jQuery method for top and left positions of the row – here are the missing pieces:

var rowPos = $divBottom.position();
bottomTop = rowPos.top;
bottomLeft = rowPos.left;

//
$divOverlay.css({
    position: 'absolute',
    top: bottomTop,
    left: bottomLeft,
    width: bottomWidth,
    height: bottomHeight
});