By design, only a single pane can be Expanded / Docked , If the pane contains a Docked / Expanded property this should allow multiple sliding panes to have it set. Having this property to the sliding zone avoid such misunderstandings.
In addition, expanding and docking are self excluding operations when applied to a single sliding pane, i.e. docked pane is not expanded and vise versa - in case of multiple sliding panes, you can have one pane docked and another pane expanded. Thus if you want the pane to be docked you need to set a pane to be Docked when the page is loaded you need to set DockedPaneId property.
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