Draggable within container fails on Safari

When adding containment to a draggable element in Safari the element becomes only vertically draggable.

Adding the following to the example at http://docs.jquery.com/UI/Draggables/draggable shows the problem:

$(".block").draggable({containment: 'parent'});

One interesting observation is that if "parent" is body one can drag a little bit across the screen, whereas is "parent" is a div it is totally locked horizontally.