Window.setOverrideRedirect

An override redirect window is not under the control of the window manager. This means it won’t have a titlebar, won’t be minimizable, etc. - it will be entirely under the control of the application. The window manager can’t see the override redirect window at all.

Override redirect should only be used for short-lived temporary windows, such as popup menus. #GtkMenu uses an override redirect window in its implementation, for example.