SpNotebookPage>>#beCloseable seems to have no effect

[ClotildeToullec]

More specifically, it does not seem to make the page closeable (i.e. having the small x button).

To reproduce:

notebook := SpNotebookPresenter new.

notebookPage := SpNotebookPage 
            title: 'Uncloseable' 
            provider: [ notebook newButton
            		label: 'Make closeable';
            		action: [ notebookPage
					title: 'Closeable';
					beCloseable ];
            		yourself ].
	
notebook addPage: notebookPage.
notebook open

Behavior observed in P11 and P12.