Combo 's problem when scroll

i am using combo Version: 4.0.3,it is in tabbar content.I works fine in chrom,but in IE(8,11),when scroll ,the list of combo can not attach the combo

to resolve this problem,I do something like this
change var j = + a - f; in _aOfsRect: function(d)


var j =  - f;

and it can work correctly in chrom and IE