Recently I ran into a strange issue where my usercontrols, when there are two or more of them managed by ajaxToolkit:ModalPopupExtender nested in another usercontrol, in RadAjaxPanel would lose input values between postbacks. There is no such issue with any other controls outside or the usercontrol parenting the problematic usercontrols.
As a revelation of my debugging attempts setting one of the problematic controls to visible false (and set it back to visible when they are supposed to appear in the modal popup) would solve this issue.
Here is my code:
Problematic controls:
01.<asp:Panel ID="FileDefinitionEdit" CssClass="sbpopup" style="display:none"  runat="server"    >02.                                03.              04.                                         <div class="popupholder" style="width:1000px">05.        <uc3:FileDefinitionEdit ID="FileDefinitionEditAscx" runat="server"  visible="false"  /> 06.                                             <div class="popbuttonarea">  <asp:Button ID="Button5"  OnClick="Button5_Click" CssClass="btn btn-primary btn-sm"  runat="server" Text="Insert"  />07.                        <asp:Button ID="Button6" CssClass="btn btn-primary btn-sm"   OnClick="Button6_Click"   runat="server" Text="Cancel" /></div> 08.                                </div>09. 10.                              11.    </asp:Panel> 12. 13. <asp:Panel ID="ConnectionStringEdit" CssClass="sbpopup"  style="display:none"  runat="server"   >14.                                15.                                     16.                                         <div class="popupholder" style="width:1000px">17.        <uc2:ConnectionStringEdit ID="ConnectionStringEditAscx" runat="server"   Visible="false"   /> 18.                                             <div class="popbuttonarea">  <asp:Button ID="Button4"  OnClick="Button4_Click" CssClass="btn btn-primary btn-sm"  runat="server" Text="Insert" />19.                        <asp:Button ID="Button3" CssClass="btn btn-primary btn-sm"   OnClick="Button6_Click"   runat="server" Text="Cancel" /></div> 20.                                </div>21. 22.         23.                                         24.    </asp:Panel> 25. 26. 27.    28.    <asp:Button ID="MPECTRL_Button"  CssClass="hiddenControl"   runat="server" Text="Button" /> 29. 30.    <ajaxToolkit:ModalPopupExtender  ID="PopupInsert_ModalPopupExtender" runat="server" TargetControlID="MPECTRL_Button"    PopupControlID="ConnectionStringEdit"      />  31.                           
Parent control of the problematic controls:
1.<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="ProcessesControl.ascx.cs"   Inherits="BizData.Perspective.DataGovernor.Controls.ProcessesControl" %>2.<%@ Register Src="ParametersControl.ascx" TagName="ParametersControl" TagPrefix="uc1" %>3.<%@ Register Src="ConnectionStringEdit.ascx" TagName="ConnectionStringEdit" TagPrefix="uc2" %>4.<%@ Register Src="FileDefinitionEdit.ascx" TagName="FileDefinitionEdit" TagPrefix="uc3" %>5. 6.<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="ajaxToolkit"  %>7. 8....
Master Page:
01.//Header02....03.<body style="margin: 0px; height: 100%; overflow: hidden;">04.    <form id="Form1" runat="server">05.        <telerik:RadScriptManager ID="RadScriptManager1" runat="server">06.        </telerik:RadScriptManager>07.        <telerik:RadStyleSheetManager ID="RadStyleSheetManager1" runat="server" />08.        <telerik:RadAjaxManager ID="RadAjaxManager1" runat="server" DefaultLoadingPanelID="RadAjaxLoadingPanel2">09.            <AjaxSettings>10.                <telerik:AjaxSetting AjaxControlID="RadAjaxPanel1">11.                    <UpdatedControls>12.                        <telerik:AjaxUpdatedControl ControlID="RadAjaxPanel1" LoadingPanelID="RadAjaxLoadingPanel2" />13.                    </UpdatedControls>14.                </telerik:AjaxSetting>15.            </AjaxSettings>16.        </telerik:RadAjaxManager>17.//Where all contents and controls reside:18.        <telerik:RadAjaxPanel ID="RadAjaxPanel1" runat="server" Width="100%" Height="100%">19. <telerik:RadSplitter ID="RadSplitter1" runat="server" Orientation="Horizontal" BorderStyle="None"20.                BorderSize="0" LiveResize="true" Width="100%" Height="100%" RegisterWithScriptManager="true"21.                RenderMode="Lightweight" >22. <telerik:RadPane ID="rpHeader" runat="server" BorderStyle="None" BorderWidth="0"23.                    Height="45px" CssClass="visibleOverflow">24....25.   </telerik:RadPane>26.            </telerik:RadSplitter>27. 28.        </telerik:RadAjaxPanel>29. 30.        <telerik:RadAjaxLoadingPanel ID="RadAjaxLoadingPanel2" runat="server" Transparency="10">31.            <div class="loading" style="text-align: left; padding-top: 45px; padding-left: 98%">32.                <asp:Image ID="Image2" runat="server" ImageUrl="images/loading1.gif" AlternateText="loading.."33.                    Width="16px" Height="16px" />34.            </div>35.        </telerik:RadAjaxLoadingPanel>36....37.//Datasources and footerMy wild guess would be too many usercontrols would exceed the viewstate capacity of some sort, resulting their input values to be truncated between postbacks.
Any ideas?