Here is my solution. The trick is to use the OnClientActiveTabChanged
event. Quite obvious, isn't it?
<asp:Content ID="Foobar" ContentPlaceHolderID="MainContentPlaceHolder" runat="Server">
<script type="text/javascript">
function CanSave(sender,e)) {
var tabContainer = window.$get('<%=tcMain.ClientID%>');
if (tabContainer != undefined && tabControl != null) {
tabContainer = tabControl.control;
document.getElementById('<%=ButtonSave.ClientID %>').disabled = (tabContainer.get_activeTabIndex() == 0);
}
}
</script>
<asp:Button ID="ButtonSave" runat="server" Enabled="false" ... />
<ajaxToolkit:TabContainer
runat="server"
ID="tcMain"
OnClientActiveTabChanged="CanSave">
...