MooTools position calculation does not work correct if the element is position outside the screen if the page is centered within an document with an odd size.

There is no difference if the element is position with left or margin-left

The problem is that the bounds are floating point values in some browsers.

Original source of the getOffsets method:

return {
    x: bound.left.toInt() + elemScrolls.x + ((isFixed) ? 0 : htmlScroll.x) - html.clientLeft,
    y: bound.top.toInt()  + elemScrolls.y + ((isFixed) ? 0 : htmlScroll.y) - html.clientTop
};

Modified source which solves the problem

return {
    x: Math.round(bound.left) + elemScrolls.x + ((isFixed) ? 0 : htmlScroll.x) - html.clientLeft,
    y: Math.round(bound.top)  + elemScrolls.y + ((isFixed) ? 0 : htmlScroll.y) - html.clientTop
};

Following browser show the problem: Google Chrome, Firefox 16
Following browser DO NOT show the problem: IE9, Opera