IE11 performance issues

[UnrealKazu]

In IE11, using this directive results in extremely laggy scrolling. Using the profiler I've traced the problem back to the getCSS method added in this commit: ensure 'left' value is the original css value.

IE11's engine is apparently not optimized for the series of operations being done in getCSS. Reverting back to the way the left property is calculated before the commit (using $elem.css('left')) results again in a smooth scrolling experience.

What was the underlying idea of this method? I understand that there is some sort of use case, so I'm curious what this use case is.

I've not tested this in other IE versions, but it would not surprise me if the problem exists in IE<11 as well.