I think that it is better to have the position relative to the window otherwise when the browser is resized your position will not be accurate.
I'm designing for a tablet app, so the window won't be resized. However, does this mean that window.pageXOffset can apply to a scrolling div as well as the whole window? Can I do something along the lines of ("scrollingbox").window.pageXOffset or is that totally off the mark?
Here is how I would find the left pos of a static element. You probably can add this to a function for a scroller element.
Rectangle on the stage with a click event and alert to see the position:
var btn = sym.$("Rectangle");
var leftPos = parseFloat(btn.css('left'));
// see position
alert("Left position is: " + leftPos);