Draggable div not constrained within containing div when window scrollbars moved

I've looked at other open bugs; #3568 looked similar, but I have checked, and this problem still seems to persist with the SVN version.

I have a container div. I have a square div within that, which I set as draggable, and add the "parent" constraint. This all works perfectly, but when the browser window is small enough to induce scrollbars and the scrollbar is not at its original position, the square can be dragged outside of the container - seemingly by the amount by which I've scrolled from the default scrollbar position.

There's a little video of the problem here:

http://www.bixbite.co.uk/jquery/drag_problem/drag_problem.html

A live example of the code is here (I've attached the HTML to the ticket as well):

http://www.bixbite.co.uk/jquery/drag_problem/test-svn.html