Custom scroll bar issue in Update panel

I have placed a div inside the Update panel for this div am adding jquery Custom scroll bar (size & color has been changing irrespective of the browsers). on page load it is working fine but when post back happens this scroll bar style is changing to default browser style.

After googleing i found a solution inside the update panel i placed Sys.Application.add_load(myFunctionName);

My problem is:

Inside the update panel i placed a button , on button click (onClick) event, a treeview control will be visible and OnSelectedNodeChange of treeview control Child nodes will be expanded, if the child's are more scroll bar is displayed, instead of showing 1 scroll bar it is showing multiple scroll bars (in this case 2 scroll bars because of i called two events (OnClick & OnSelectedNodeChange events))

MyCode :

<asp:UpdatePanel id="updatePanel1" runat="server">
<ContentTemplate>
     <script type="text/javascript">
                    Sys.Application.add_load(CustomScrollBar);
     </script>

<div class="scrollBar" height="300px">

<asp:Button ID="btnShowTreeView" runat="server" Text="Show Tree View"  onclick="btnShowTreeView_Click" />

<asp:TreeView ID="tvList" runat="server" OnSelectedNodeChanged="tvList_SelectedNodeChanged" Visible="false" ShowExpandCollapse="false">        
        <LeafNodeStyle  Font-Size="10px"/>                                                                                              
        </asp:TreeView>
</div>

</ContentTemplate>
    </asp:UpdatePanel>

In button click even am make tree view to visible="true" and here am binding the tree view from backend .cs file

Thanks in advance...