by you behavior of the unpinned RadPane
is an expected one in the current implementation of RadDocking.
When a pane is unpinned, it is placed in a Popup control. In WPF all Popups are shown at the top pf the application above all windows. Because of that the pane is placed and visualized on top of the rectangle. Unfortunately, currently we could not suggest you any proper solution to reorder the positions - this is a Framework behavior.
We hope the provided information will help you. Please, do not hesitate to contact us if you have any additional questions or concerns regarding Telerik controls.
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