util.getPosition - incorrect value for top with a 'sticky' picker

[StompyNZ]

When applying css to make the container div for the date picker sticky and you scroll down the page the picker opens further down the page than it should.

With the following css class applied the further you scroll down the further down the page the picker opens:
.datepicker-container {
position: -webkit-sticky;
position: sticky;
top: 0;
}

To fix in my current application I made the following change in utils.getPosition

y += elem.offsetParent.getBoundingClientRect().top before the while loop, instead of y += elem.offsetTop; inside the loop