您会考虑取消 UpdatePanel 吗?我能够使用您的原始代码重现故障。删除 UpdatePanel 后,故障不再可重现:
<fieldset>
<legend>Role</legend>
<table width="500">
<tr>
<td>A</td>
<td valign="top">
<asp:DropDownList ID="Roles" runat="server" AutoPostBack="True" OnSelectedIndexChanged="roles_SelectedIndexChanged">
</asp:DropDownList>
</td>
<td valign="top">
<%-- <asp:UpdatePanel ID="UpdatePanel" runat="server">
<ContentTemplate>--%>
<asp:TextBox ID="txtAuthority" Text="Authorities" runat="server" CssClass="txtboxaschkbox" Visible="false"></asp:TextBox>
<asp:Panel runat="server" ID="PnlAuth" Visible="false" CssClass="PnlDesign">
<cc1:EnhancedCheckboxList ID="Authorities" runat="server">
</cc1:EnhancedCheckboxList>
</asp:Panel>
<cc2:PopupControlExtender runat="server" ID="PceSelectAuthority" TargetControlID="txtAuthority"
PopupControlID="PnlAuth" Position="Bottom"/>
<%-- </ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="Roles" />
</Triggers>
</asp:UpdatePanel>--%>
</td>
</tr>
<tr>
<td>B</td>
<td valign="top">
<asp:DropDownList ID="Roles2" runat="server" AutoPostBack="True" OnSelectedIndexChanged="roles2_SelectedIndexChanged">
</asp:DropDownList>
</td>
<td valign="top">
<%-- <asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>--%>
<asp:TextBox ID="txtAuthority2" Text="Authorities" runat="server" CssClass="txtboxaschkbox" Visible="false"></asp:TextBox>
<asp:Panel runat="server" ID="PnlAuth2" Visible="false" CssClass="PnlDesign">
<cc1:EnhancedCheckboxList ID="Authorities2" runat="server">
</cc1:EnhancedCheckboxList>
</asp:Panel>
<cc2:PopupControlExtender runat="server" ID="PceSelectAuthority2" TargetControlID="txtAuthority2"
PopupControlID="PnlAuth2" Position="Bottom"/>
<%-- </ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="Roles2" />
</Triggers>
</asp:UpdatePanel>--%>
</td>
</tr>
</table>
</fieldset>