[Vala] Getting semi-transparent window

[Mark Messer <messerbras gmail com>]

Hi All,

My question is maybe a trivial one, but two days ago I knew nothing about
Vala. I found it pretty useful though and easy to understand (I wrote for
Gtk in C and Python, I have some experience with Cpp and C#). All in all -
I actually like it.
But to the point. I started from getting GtkBuilder as it provides an
automated (semi) build chain on meson. It imposes a certain structure of
projects, with main application loop separated and the rest in a namespace.
So far so good. As I last wrote something for Gtk some time ago I had to
switch from colormap to visual when it comes to transparency. But with
namespace and a little odd main loop devoid of any control common with
standard approach I have no idea where should I (and whether should I) put
Cairo on repaint.
Are there any resources or examples aimed at semi-transparent main windows?