ASP面板控件中的下拉列表问题

Question

我正在使用asp Panel 控件，其中我有一个下拉列表列表，可以选择不同的时区。

然而，当我选择任何时区时，当我下次打开弹出窗口时，它会消失。

你能建议或提供一些指示吗？

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 <asp:PlaceHolder ID="DisplayMode" runat="server">

                <div id="ListBranch" style="text-align: left">
                    <asp:Button ID="AddBranch" runat="server" Text="Add Branch" OnClick="AddBranch_Click" />

                    <act:ModalPopupExtender ID="ModalPopupExtenderBranch" runat="server" TargetControlID="AddBranch" PopupControlID="pnlAddBranch"
                        CancelControlID="btnCancel" BackgroundCssClass="modalBackground">
                    </act:ModalPopupExtender>

                    <act:ToolkitScriptManager ID="smBranch" runat="server">
                    </act:ToolkitScriptManager>
                    <div>
                        <br />
                        <asp:GridView runat="server" ID="gvBranchList" DataKeyNames="BranchID" AutoGenerateColumns="false" Font-Size="Small">
                            <RowStyle BackColor="#EFF3FB" />
                            <FooterStyle BackColor="#507CD1" Font-Bold="True" ForeColor="White" />
                            <PagerStyle BackColor="#2461BF" ForeColor="White" HorizontalAlign="Center" />
                            <HeaderStyle BackColor="#336699" Font-Bold="True" ForeColor="White" />
                            <AlternatingRowStyle BackColor="White" />
                            <Columns>

                                <asp:BoundField DataField="Branch Name" HeaderText="Branch Name" />
                                <asp:BoundField DataField="Timezone" HeaderText="Timezone" />

                                <asp:TemplateField HeaderText="Edit">
                                    <ItemTemplate>
                                        <asp:Button ID="Edit" runat="server" Text="Edit" OnClick="Edit_Click" />
                                    </ItemTemplate>
                                </asp:TemplateField>

                            </Columns>
                        </asp:GridView>
                        <br />
                        <asp:Label ID="lblresult" runat="server" />
                        <asp:Button ID="btnShowPopup" runat="server" Style="display: none" />
                        <asp:Button ID="UpdateBranch" CommandName="Update" runat="server" Text="Ok" OnClick="Ok_Click" Style="display: none" />


                        <asp:Panel ID="pnlAddBranch" runat="server" BackColor="White" Height="80%" Width="100%" Font-Size="Small" Style="top: 40px !important;">
                            
                            <table style="border: solid 2px #336699; width: 100%; height: 100%; font-size: small;" cellpadding="0" cellspacing="0">
                                <tr style="background-color: #336699">
                                    <td colspan="2" style="height: 5%; color: White; font-weight: bold; font-size: larger" align="center">Branch Details</td>
                                </tr>

                                <tr>
                                    <td align="left">Branch Name:  </td>
                                    <td>
                                        <asp:TextBox ID="BranchName" runat="server"></asp:TextBox>
                                    </td>
                                </tr>
                                <tr>
                                    <td align="left">Timezone: </td>
                                    <td>
                                        <asp:DropDownList ID="ddlTimezones" AutoPostBack="false" OnSelectedIndexChanged="ddlTimezones_SelectedIndexChanged" runat="server" />
                                    </td>
                                </tr>
                                <tr>
                                    <td align="left">Assigned Holidays: </td>
                                    <td>
                                        <asp:CheckBoxList ID="chkAssignedHolidays" runat="server" RepeatColumns="3" Font-Size="Small" />
                                    </td>
                                </tr>

                                <tr>
                                    <td></td>
                                    <td>
                                        <asp:Button ID="Ok" CommandName="Add" runat="server" Text="Ok" OnClick="Ok_Click" />
                                        <asp:Button ID="Cancel"  Commandname="Cancel" runat="server" Text="Cancel" OnClick="Cancel_Click" />
                                    </td>
                                </tr>
                                <tr>
                                    <td colspan="2">
                                        <asp:Label ID="lblBranchID" runat="server" Style="display: none"></asp:Label>
                                    </td>
                                </tr>
                            </table>
                        </asp:Panel>
                    </div>

                </div>

            </asp:PlaceHolder>

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