[GTK] Запуск окна без рамки + нижний слой

нужно поставить аттрибут override_redirect у окна, в gtk помоему это называется тип окна - POPUP

void gtk_window_set_decorated (GtkWindow *window, gboolean setting);

By default, windows are decorated with a title bar, resize controls, etc. Some window managers allow GTK+ to disable these decorations, creating a borderless window. If you set the decorated property to FALSE using this function, GTK+ will do its best to convince the window manager not to decorate the window. Depending on the system, this function may not have any effect when called on a window that is already visible, so you should call it before calling gtk_window_show().

On Windows, this function always works, since there's no window manager policy involved.

Скорее всего, даже это функция не понадобится, а нужно создать окно определенного типа, например, с помощью

gtk_window_new (GTK_WINDOW_POPUP)

или установить тип с помощью

gdk_window_set_type_hint ()

void gdk_window_set_type_hint (GdkWindow *window, GdkWindowTypeHint hint);

The application can use this call to provide a hint to the window manager about the functionality of a window. The window manager can use this information when determining the decoration and behaviour of the window.

The hint must be set before the window is mapped.