how to find whether ajaxautocomplete extender scroll is click or not

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ASP.NET

Hi,
I have textbox with a ajaxautocomplete extender ,as

 <asp:TextBox ID="TextBox1" onblur="callfunction()" runat="server"></asp:TextBox>
<ajaxToolkit:AutoCompleteExtender
     runat="server"
    ID="autoComplete1"
    TargetControlID="TextBox1"
    ServiceMethod="GetCompletionList"
    ServicePath="AutoComplete.asmx"
    MinimumPrefixLength="2"
    CompletionInterval="1000"
    EnableCaching="true"
    CompletionSetCount="20"
    CompletionListCssClass="autocomplete_completionListElement"
    CompletionListItemCssClass="autocomplete_listItem"
    CompletionListHighlightedItemCssClass="autocomplete_highlightedListItem"
    DelimiterCharacters=";, :"
    ShowOnlyCurrentWordInCompletionListItem="true">      
</ajaxToolkit:AutoCompleteExtender>

function callfunction(){
alert()
}

Whenever the list is autopopulated,a sroll like a dropdownlist is displayed ,
I want the scroll to be displayed ,
If i click on the item which is populated, callfunction() function is getting not getting executed,
but the problem is if i click on the scroll, callfunction() is getting executed,

I dont want when user click on the scroll, the callfunction() should be executed.
How to find whether scroll is click or not ??.so that i can stop callfunction to be executed that time.