The issue is replicated in R3 2021 only.
<telerik:RadTabStrip runat="server" ID="RadTabStrip1" >
<Tabs>
<telerik:RadTab Text="Tab 1">
<Tabs>
<telerik:RadTab Text="fail"></telerik:RadTab>
</Tabs>
</telerik:RadTab>
<telerik:RadTab Text="Tab 2"></telerik:RadTab>
<telerik:RadTab Text="Tab 3"></telerik:RadTab>
</Tabs>
</telerik:RadTabStrip>
Solution from Admin:
Place the load/script below somewhere under the ScriptManager. It can be added in the MasterPage if needed:
<script>
Sys.Application.add_init(function fixTabStrip() {
if (Telerik && Telerik.Web && Telerik.Web.UI && Telerik.Web.UI.RadTabStrip) {
Telerik.Web.UI.RadTabStrip.prototype._childInserted = function (index, tab, owner) {
this._allTabs = null;
if (this._cachedSelectedTab) {
owner._setSelectedIndex(this._cachedSelectedTab.get_index());
this._cachedSelectedTab = null;
}
if (tab._cachedSelected) {
owner._setSelectedIndex(index);
tab._cachedSelected = false;
}
Telerik.Web.UI.RadTabStrip.callBaseMethod(this, "_childInserted", [index, tab, owner]);
if (tab.get_isBreak()) {
var breakElement = document.createElement("li");
breakElement.className = "rtsBreak";
owner.get_childListElement().insertBefore(breakElement, tab._element.nextSibling);
}
//if (tab.get_parent() && tab.get_parent() instanceof Telerik.Web.UI.RadTab && tab.get_parent().get_selected()) {
// $telerik.$(tab.get_parent().get_levelElement()).removeClass("rtsHidden");
//}
}
}
});
</script>