You are not referring to a nautilus window. You are referring to a GTK3 File - Open dialog, part of the standard GTK toolkit, which can be used by programs build on this toolkit.
That the "Maximize" option is grayed out indeed is unexpected, because, despite what the dialog suggests, it can be maximized. It may be worthwhile to file this issue with the developers if it has not yet been filed. Even in the most recent GTK versions, the "Maximize" option is grayed out.
You can quickly maximize the dialog using your Window manager's shortcut key to maximize a Window. In many desktop environments, the traditional shortcut key Alt+F10 will allow you to toggle between maximized and restored. On the standard Ubuntu desktop, you can also use Super+Up. Use Super+Down to restore.
You are not referring to a nautilus window. You are referring to a GTK3 File - Open dialog, part of the standard GTK toolkit, which can be used by programs build on this toolkit.
That the "Maximize" option is grayed out indeed is unexpected, because, despite what the dialog suggests, it can be maximized. It may be worthwhile to file this issue with the developers if it has not yet been filed. Even in the most recent GTK versions, the "Maximize" option is grayed out.
You can quickly maximize the dialog using your Window manager's shortcut key to maximize a Window. In many desktop environments, the traditional shortcut key Alt+F10 will allow you to toggle between maximized and restored. On the standard Ubuntu desktop, you can also use Super+Up. Use Super+Down to restore.