public void set_opacity (double opacity)
Set this to render as partially transparent, with opacity 0 being fully transparent and 1 fully opaque.
(Values of the opacity parameter are clamped to the [0,1] range.)
For toplevel windows this depends on support from the windowing system that may not always be there. For instance, On X11, this works only on X
screens with a compositing manager running. On Wayland, there is no per-window opacity value that the compositor would apply. Instead, use
`gdk_window_set_opaque_region (window, NULL)` to tell the compositor that the entire window is (potentially) non-opaque, and draw your content
with alpha, or use
gtk_widget_set_opacity to set an overall opacity for your widgets.
For child windows this function only works for non-native windows.
Support for non-toplevel windows was added in 3.8.
a top-level or non-native Window