void gtk_widget_set_opacity ( GtkWidget* widget, double opacity )
widget to be rendered partially transparent,
with opacity 0 being fully transparent and 1 fully opaque. (Opacity values
are clamped to the [0,1] range.).
This works on both toplevel widget, and child widgets, although there
are some limitations:
For toplevel widgets this depends on the capabilities of the windowing system. On X11 this has any effect only on X screens with a compositing manager running. See gtk_widget_is_composited(). On Windows it should work always, although setting a window’s opacity after the window has been shown causes it to flicker once on Windows.
For child widgets it doesn’t work if any affected widget has a native window, or disables double buffering.