Created attachment 309374 [details]
Polari use case for having focusable widget as GtkListBox header.

At the moment GtkListBox assumes all focusable widgets are of type GtkListBoxRow. If I make GtkListBox headers of type GtkButton which is focusable, then it does not seem possible to focus it using tab. If the button is clicked it does receives focus, but pressing tab will then crash the application.

If focusable widget in the header of GtkListBox are unsupported/expected to break, that should at least be documented.

If it helps, the attached screenshot shows the use case in Polari - The header is a button which becomes sensitive in event of error in which it can be clicked and detailed error information can be shown.