Hover over the 2nd box in the testcase. You will notice that the positioned pseudo-element does not move, even though it should, according to the first and third divs (which have the exact same styling as the 2nd one when hovered).
Testcase works fine in other browsers.
The bug is also present in Webkit nightlies, and I tested in both OSX and Windows.
I'm seeing the same issue when border-box-sizing is used: child element position does not update when the width of the parent element's border changes. (No pseudo-elements involved.)