responses to mouse wheel scrolling when the DropDown
is opened and the observed by you behavior is not expected - please, check if something in your code prevents that behavior from working. More detailed information concerning the mouse support of RadComboBox
you could find on the following article from our help documentation:
We hope this will help you.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.