This issue is expected since Q3 2012 release some breaking changes were made to the RadComboBox control appearance as listed here. The width of the RadComboBox sprite is decreased from 2000px to
1000px. So in this case if your RadComboBox control is wider than 1000px on rendering in the browser,
there will be a gap between the arrow
and the input field. The only solution in such cases would be creating a
new sprite image for RadComboBox with the desired dimensions.