<com:TContent ID="Content"> <com:TActivePanel ID="Main"> <com:TPanel ID="A" Visible="true"> <label>T1</label> <com:TTextBox ID="T1" /> <com:TActiveButton Text="T1" ValidationGroup="G1" OnClick="clicked" /> <com:TRequiredFieldValidator ControlToValidate="T1" Text="T1-error" ValidationGroup="G1" /> </com:TPanel> <com:TPanel ID="B" Visible="false"> This is B </com:TPanel> <com:TActiveButton Text="showB" OnCallback="showBcallback" /> <com:TActiveButton Text="showA" OnCallback="showAcallback" /> </com:TActivePanel> </com:TContent>