First of all I would advise you to use the Close event of the Docking control to determine when a pane is closed instead of the PaneStateChange event.
When I pane is hidden it is not cleaned, but just hidden. If you need to clear the memory it uses you need to remove it from the Docking control and if you don't have other references the GC will do its job and clear it. The easies way to remove a pane from the Docking control is to use its RemoveFromParent method. For example you could handle the Close event and every time a pane is closed you could remove it to make sure it will clean the memory it occupies.
Of course there is a trade off - if you use the Save/load layout feature the content of the closed panes will not appear if you load a previously saved layout where these panes are not closed. This happens, because the content is removed from memory and the Docking control cannot load it.
Hope this information is helpful.
the Telerik team
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