即使OnClientClick返回true

Question

单击“确认”后，“ btnSubmit_Click”不会在后面的代码中被调用。在OnClientClick上调用ClosePopup并返回true，该值应回发并在服务器端调用Submit。有什么想法为什么不起作用？

<ajaxToolkit:ModalPopupExtender ID="mpe" runat="server" TargetControlId="btnCopyShip" 
    PopupControlID="ModalPanel" BackgroundCssClass="ui-widget-overlay"
    CancelControlID="btnCancel" behaviorid="modalwithinput"
    OnCancelScript="ClearUI();"/>

<asp:Panel ID="ModalPanel" runat="server" style="background-color:White; width:320px; padding:0px 10px 10px 10px; border:1px Black; display: none;height:100px" cssClass="shadow">
<asp:UpdatePanel ID="uPnlNewShip" runat="server">
<ContentTemplate>
    <fieldset style="width:300px;">
        <legend style="font-weight:bold;font-size: medium">Copy Ship</legend>
        <label for="lblShip" style="padding-left: 0px" >Ship: </label>

        <asp:DropDownList runat="server" ID="ddlNewShips" style="width:150px">
        </asp:DropDownList><%-- ValidationGroup="popup"--%>

        <asp:RequiredFieldValidator ID="popupRequiredFieldValidator" runat="server"
            ControlToValidate="ddlNewShips" Display="None"
            ErrorMessage="Required Field" SetFocusOnError="True"
            InitialValue="0">
        </asp:RequiredFieldValidator> <%--ValidationGroup="popup"--%>

        <%--<ajaxToolkit:ValidatorCalloutExtender runat="server"
                targetcontrolid="popupRequiredFieldValidator"
                id="popupValidatorCalloutExtender"
                behaviorid="ddlValidator" enabled="True">
            </ajaxToolkit:ValidatorCalloutExtender>--%>
    </fieldset>

    <br /><br />

    <span style="float: right"> 
        <asp:Button ID="btnCancel" runat="server" Text="Cancel" OnClick="btnCancel_Click" CausesValidation="False"/>
        <asp:Button ID="btnSubmit" runat="server" Text="Confirm" OnClick="btnSubmit_Click"  OnClientClick="return ClosePopup()"/>  
    </span>
</ContentTemplate>
</asp:UpdatePanel>
</asp:Panel>

function ClosePopup()
{
    if (IsValid())
    {
        ClearUI();
        $find('modalwithinput').hide();
        return true;
    }
    else
        return false;
}