A Gtk.Window can be one of these types. Most things you would consider a "window" should have type Gtk.WindowType.TopLevel ; windows with this type are managed by the window manager and have a frame by default (call Window.Decorated to toggle the frame). Windows with type Gtk.Window.Popup are ignored by the window manager; window manager keybindings will not work on them, the window manager will not decorate the window with a frame, many GTK+ features that rely on the window manager will not work (e.g. resize grips and maximization/minimization).

Gtk.WindowType.Popup is used to implement widgets such as Gtk.Menu or tooltips that you normally do not think of as windows per se. Nearly all windows should be Gtk.Window.Toplevel. In particular, do not use Gtk.Window.Popup just to turn off the window borders; use Window.Decorated for instead.