Flickering with Socket and Plug

[René Hansen <rene hansen-audio de>]

Hey GTK list,

I've uploaded a demo app on https://github.com/rehans/GtkSocketPlugDemo

It is very simple and just creates a GtkWindow with a GtkSocket child. A 
GtkPlug with an GtkDrawingArea attached is connected to the socket. The 
GtkDrawingArea redraws itself every 16ms and simply paints an image. 
(Some things in the demo might not be very meaningful but it is just to 
demo the problem)

While this demo works on Ubuntu 16.04 (tested on 3 PCs) with
libgtk-3-0:amd64  3.18.9-1ubuntu3.3  amd64  GTK+ graphical user 
interface library

it starts flickering constantly on Ubuntu 17.04 with
libgtk-3-0:amd64  3.22.11-0ubuntu3  amd64  GTK+ graphical user interface 
library

The Raspberry PI 3 with (Debian Stretch installed) shows the same 
behavior like Ubuntu 17.04 with
libgtk-3-0:armhf  3.22.11-1+rpi3  armhf  GTK+ graphical user interface 
library

(I've checked the GTK Version with "dpkg -l libgtk-3-0" from Terminal)

Does anyone know what is causing this behavior and can help me out here? 
Anything I can improve? Is this a GTK bug which was introduced after 3.18.9?

René