Position feature fails with scrolled elements

In large pages, when the elements are scrolled, the position feature is not working right.

Example :

• Two html components : a div container (extra large) and a little square. Actually is the same example that appears as position demo (http://api.jqueryui.com/position/) but with a big container.
• Move the square in order to its top-left corner match the top-left corner of the container.

$( "#square" ).position( {my: "left top", at: "left top", of: "#container"} );

• When the scroll is at the bottom. The square is moved to the bottom-left side of the container.

JSFiddle here : https://jsfiddle.net/Ljpcpkpt/1/