return el.className.baseVal === undefined ? el.className : el.className.baseVal
This only occurs in Firefox (found this on Windows 7 and Firefox 28.0).
At First thought this link as solution, but tries a very simple example (no jquery) yet the error occurs. Refer this jsfiddle.
This line 996 only hits on firefox and not other browser (checked on Safari 5.1.7, chrome 33 and ie 11 on windows) on this behaviour i.e. on dragging cursor beyond browser window.