Modify scrollbar check, stop static nav shift

* Modify check for scrollbar to account for IE10/11
* Stop static nav shift on modal open
* Fixes #9855

Tested working:
* Debian 7/Iceweasel 17.0.10
* Debian 7/Chrome 32.0.1700.107
* Windows 7/IE 10
* Windows 7/Chrome 33
* OSX 10.9/Firefox 27.0.1