I can't remove the red boxes. My last comment provides the screenshots that illustrate the problem.
This issue exists in my project and the JQuery UI Position Live demo; which is why I don't provide a jsfiddle because you can just go to the live demo to reproduce the error.
Steps to reproduce.
1) Go to the jQuery UI Position Live Demo site. http://jqueryui.com/position/
2) Set "my" to "left center"
3) Set "at" to "right center"
4) set "collision" to "flip flip"
5) drag the orange box to the far right of the iFrame so that the overflow appears.
6) You will see that the DOM objects being positioned by the position utility has an erroneous offset that makes it appear a bit off to the left.
Note that in my project I only set the position once, so it has nothing to do with how the live demo is setting the position as you drag it. The problem lies when you use the position utility on an object that is close to the css overflow.