Please consider the code below in the recently released Coolite 0.8 framework (works perfectly in Coolite 0.7).
When EnableEventValidation is set to 'false', clicking on the button will make the TextField disappear after the AsyncPostBack.

When its set to true, nothing strange occur.

<%@ Page Language="C#"  EnableEventValidation="false" ValidateRequest="false" %>

<%@ Register Assembly="Coolite.Ext.Web" Namespace="Coolite.Ext.Web" TagPrefix="ext" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title></title>
</head>
<body style="overflow: auto;">
    <form id="form1" runat="server">
    <asp:ScriptManager runat="server" ID="ScriptManager2">
    </asp:ScriptManager>
    <ext:ScriptManager ID="ScriptManager1" runat="server">
    </ext:ScriptManager>
    <ext:ViewPort ID="ViewPort1" runat="server">
        <Body>
            <ext:BorderLayout ID="BorderLayout1" runat="server">
                <North>
                    <ext:Panel runat="server">
                        <Body>
                            
                                <asp:UpdatePanel runat="server" ID="updMain">
                                    <ContentTemplate>
                                        <ext:TextField ID="txtSearchOld" runat="server" Width="270px" CtCls="txtSearch" EmptyText="empty text"
                                            FieldClass="txtSearch">
                                        </ext:TextField>
                                        <asp:Button ID="Button1" runat="server" />
                                    </ContentTemplate>
                                </asp:UpdatePanel>
                            

                        </Body>
                    </ext:Panel>
                </North>
                <Center>
                    <ext:Panel runat="server">
                    </ext:Panel>
                </Center>
            </ext:BorderLayout>
        </Body>
    </ext:ViewPort>
    </form>
</body>
</html>