One of my friend asked me that he is getting difficulty to put scroll bar for the AutoComplete(Atlas) Control. I tried to put scroll bar for him. same thing I'm explaining here.. May interest you guys.
By default Autocomplete control does not provide the scroll bar, in-order to have scroll bar you need to define panel or Div to attach as dropdown area and apply style on that to get scroll bar.
Define Panel for the dropdown area for the autocomplete control
<asp:Panel ID="dropdown" Height="100px" ScrollBars=Vertical runat=server></asp:Panel>
Set the "DropDownPanelID" same as panel id.