Disable minimize and maximize buttons for the preview window
Ubuntu 18.04.1, GTK 3.22.20, Gnome 3.28.2.
When the preview window is displayed a title bar contains minimize and maximize buttons.
The maximize button do nothing at the moment. I think it is better to remove it, as making it work does not make much sense - the preview window is something temporary/ephemeral.
The minimize button works... somewhat. It minimizes the preview window, but you cannot really 'un-minimize' it. It is not visible in Nautilus windows list. Also if you minimize the preview window and then click other file in Nautilus you need to hit Space key 3 times to show the preview for this file.