You code is going to give you the bottom left corner of the element. By starting with:
var offsetTop = $(sender).outerHeight();
All positioning works to the top left point of the element. If you loop through offsetLeft and offsetTop like you have in your example. You will get the distance from the top left corner of the element to the top left corner of the page. But where you start with the element height you'll also have this factored in to give you the bottom left.
This result will always be the same if elements are absolutely positioned and anchored to the top left.
Only if you have centre, right or bottom anchors or relative (%) sizes will a change of browser size effect the position of the element.
The following is a snippit from the script I use with my applications.
var uiHelper = {
ElementPos: function(el) {
var result = { x: 0, y: 0 };
while(el != null) {
if(el.offsetLeft) {
result.x += el.offsetLeft;
result.y += el.offsetTop;
}
el = el.parentNode;
}
}
}