As I have mentioned earlier, the PanelBar would be able to work without a ScriptManager as long as you register the required scripts, e.g. MicrosoftAjax.js and MicrosoftAjaxWebForms.js.
I have tested the following simple markup that allowed the client-side object of the PanelBar to be properly initialized and the expand/collapse to work as expected. Keep in mind that ajax.js is a sample file where I pasted the content of MicrosoftAjax.js and MicrosoftAjaxWebForms.js
<%@ Page %>
<telerik:RadPanelBar RegisterWithScriptManager="false" runat="server">
<telerik:RadPanelItem Text="Root 1">
<telerik:RadPanelItem Text="Child 1"></telerik:RadPanelItem>
<telerik:RadPanelItem Text="Child 2"></telerik:RadPanelItem>
<telerik:RadPanelItem Text="Root 2"></telerik:RadPanelItem>
If you are using MVC, I would like to focus your attention to Telerik UI for ASP.NET MVC that is designed specifically for this framework
Another product you might be interested in is the Kendo UI for jQuery library that contains entirely client-side widgets which does not depend at all on the server framework:
Five days of Blazor, Angular, React, and Xamarin experts live-coding on twitch.tv/CodeItLive , special prizes and more, for FREE?! Register now for DevReach 2.0(20).