Screen.getRgbaVisual

Gets a visual to use for creating windows with an alpha channel. The windowing system on which GTK+ is running may not support this capability, in which case null will be returned. Even if a non-null value is returned, its possible that the window’s alpha channel won’t be honored when displaying the window on the screen: in particular, for X an appropriate windowing manager and compositing manager must be running to provide appropriate display.

This functionality is not implemented in the Windows backend.

For setting an overall opacity for a top-level window, see [gdk.window.Window.setOpacity].