The CollapsiblePanel is an ASP.NET AJAX extender control in the AJAX Control Toolkit. This particular extender control extends the default behavior of the standard ASP.NET Panel to collapse and expand when a particular element on the page is clicked (it will scroll open or closed when this trigger element is clicked). It's very easy and elegant to use, but one common question seems to be - when a full post back is triggered - what is the current (or most recent) state of the panel - collapsed or expanded?

The way to find out is by inspecting the ClientState property on the CollapsiblePanel's instance:

If ClientState is true, then the panel is collapsed; if the ClientState is false, then the panel is expanded.