This allows you the greatest flexibility in assigning which specific controls may invoke a page render, and is recommended, so that a developer will opt-in to respond to an event, rather than handling any events that may arise.
Note that when Update Panel controls are nested, when the Update Mode is set to Conditional, if the child Update Panel is triggered, but the parent is not, then only the child Update Panel will refresh.
Net project "Update Panel Examples" with a master page.
Add a Script Manager tag in the master page if one does not exist.
Create a separate class library "Update Panel Lib" in the project (or grab it from the attached source code). The control class "Foo Panel" extends a Place Holder control.