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
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.