Does something block this commit to be merged?

Are you using sub pixels when setting the left and top?

Update the PR to use String.toFloat instead.

The problem mostly occurs on slideshows if you use an element centered with auto margins within an element of odd width or using percents in width/padding.

The pull request has been updated.

This is going to be on hold, until we get a spec to check for this. You're welcome to add one yourself, or wait about a week to get a hold of the pending PR that reduces the complexity to test MooTools.

I have rebased the previous commit and written a test spec.

PhantomJS version 1.9.8 used by the karma runner is based on a webkit version released around 2011 before subpixel alignment was added to webkit.
EDIT: I broke the builds again, tested locally against phantomjs v2 …

Is a test spec needed for this change? If the browser itself does not support subpixel alignment it is just fine, but I can't think of a way to test this usefully without breaking the PhantomJS build …

If something is missing please let me know.

LGTM but travis failed

Travis fails on PhantomJS tests. We could add a bypass to phantom.js in the specs for this.

The test case has been modified to compare the result of getPosition with the output of getBoundingClientRect.
All tests pass now without testing for PhantomJS