How to prevent ajax toolkit DropDownExtender from closing on click?

hi every body

i have a treeView in DropDownExtender, The problem is that when i want expand each treenode, dropdownlist closes and i need to reopen it to select a node.

How do i make it so the dropdownlist dosn't close until i select a node?

<asp:Panel ID="pnlCategory" runat="server"><div id="Div2" class="TreeContiner"><asp:TreeView ID="CategoryTreeView2" runat="server" ShowLines="true" LineImagesFolder="images/treeRTLlines"
                                                CssClass="AspNet-TreeView" OnSelectedNodeChanged="CategoryTreeView2_SelectedNodeChanged"><NodeStyle CssClass="AspNet-TreeView-Nodes" /><SelectedNodeStyle CssClass="AspNet-TreeView-Selected-Nodes" /></asp:TreeView></div></asp:Panel><asp:DropDownExtender ID="DropDownExtender1" runat="server" TargetControlID="txtDropDownCategory"
                                        DropDownControlID="pnlCategory"></asp:DropDownExtender>

i have