(and it is present in WPF Extented Toolbox Avalon)
To reproduce just dock AvalonDock.MVVMTestApp "Properties" panel along with "Noname "document as tabbed document and click "X" button.
Comments: ** Comment from web user: cfneese **
This is a result of the control template for the LayoutDocumentTabItem control binding the "X" button to CloseCommand, whereas all of the other "X" buttons are bound to HideCommand. The Hide command is only available on LayoutAnchorableItem, so it is not available (without some logic) to LayoutDocumentTabItem, which has a LayoutItem.
There are workarounds:
1. Bind the LayoutAnchorableItem's CloseCommand to a command on the VM.
2. Subscribe to the Closing event of LayoutAnchorable, then hide the LayoutAnchorable and cancel the close. This can be done using an attached property on the DockingManager.
3. Rework the control template for the LayoutDocumentTabItem.
I think that the best way to fix this would be to introduce a new command on LayoutItem called HideOrCloseCommand that would test the type of its LayoutElement and close it if it is a LayoutDocument, but hide it if it is a LayoutAnchorable. Then the LayoutDocumentTabItem's default template should bind the "X" button to the HideOrCloseCommand.