AjaxContentPanels or something to that effect. Those things were a nightmare. At the time, asp.net pretended to be stateful by bundling up the entire state of the page into "ViewState" and passing it back and forth client to server. Getting that to work with those panels was more work than just ajax-ing the content and injecting it with jquery.
In the Microsoft-verse, this might also draw some comparisons to the more modern server-side blazor.
In the Microsoft-verse, this might also draw some comparisons to the more modern server-side blazor.