Hello Cliff Gibson,
You need to set the ExpandedPaneId property of the RadSlidingZone to an empty string in order to close an expanded sliding pane in code behind. However, as you mention that you are using AJAX, you need to make sure that the splitter control is updated during the AJAX request in order for the changes to take effect.
of the sliding zone and either the endRequest client-side event of the pagerequestmanager
or the ResponseScripts collection of RadAjaxManager/RadAjaxPanel
in case you are using RadAjax.
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