A GtkWindow can be one of these types. Most things you’d consider a “window” should have type #GTK_WINDOW_TOPLEVEL; windows with this type are managed by the window manager and have a frame by default (call gtk_window_set_decorated() to toggle the frame). Windows with type

GTK_WINDOW_POPUP are ignored by the window manager; window manager

keybindings won’t work on them, the window manager won’t 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_WINDOW_POPUP is used to implement widgets such as GtkMenu or tooltips that you normally don’t 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 gtk_window_set_decorated() for that.