Hello John Reynolds,
in SitePanelbar user control is binding declaratively to a site map using a SiteMapDataSource
control. When the page is loaded for the first time, we explicitly bind the Panelbar
to set some styles for the currently selected item. So removing (!Page.IsPostBack) will not break the proper functionality of SitePanelbar. But I tried to reproduce your problem by setting SitePanelbar and a modal popup on the master page with no success. Panelbar
was rendered properly in case of modal popup postback. If the information does not help solve the issue, please send us your master page to investigate it locally.
All the best,
the Telerik team