Setting a RadGridView element instance as content for a RadPanelBarItem, causes the RadGridView to not show all of its columns.

As a workaround to this, you could set the IsScrollViewerInsideItemsEnabled property of the RadPanelBar element, to True, and you could control the expanded length of the PanelBarItem which holds the RadGridView instance. This could be done by utilizing the ExpandedLength property.