GTK+3.0 window resize problem - only on ubuntu

[postanmb]

An app using libgtk-3 as GUI-lib shows strange behaviour since on ubuntu 18.04 an later.
The problem is, that a not resizeable window not sets its correct size, when it shows up a second and all further times - but the first time, the size is correct.
In that window a gtk element is inserted while showing up, so the size have to be recalculated.

This stange behaviour is only found on Ubuntu 18.04 and later (libgtk-3-0 >> 3.22.30-1ubuntu2 ONLY on ubuntu - not kubuntu).
On Debian systems (stable and testing) and Kubuntu systems of the same version all things are all right.
A short list, which versions have failures (os and libgtk-3-0 version) :

debian stretch with 3.22.11 is OK
debian buster with 3.24.5 is OK
ubuntu 17.10 with 3.22.25-0ubuntu0.1 is OK
ubuntu 18.04 (also LTS version) with 3.22.30-1ubuntu2 failure
ubuntu 18.10 with 3.24.4-0ubuntu1 failure

A screenshot of the correct first show of the window an the second on is attached.
A minimal example would also be attached in a replay to this report.

[postanmb (postanmb)]

A minimal example program to reproduce the behaviour.

In the example a click on "show subwindow" opens a second window, in which a component is inserted at showing up. On the first time all is ok like shown in the left part of the screenshot. But if you close the second (sub) window and open it again, then the is a wrong size calculation for the window size.
Further testing shows up, that this only happen, when the window is set to not resizeable. As soon as the window is set to resizeable, all is ok.

[Sebastien Bacher (seb128)]

Installing the Debian version of gtk on disco still shows the issue, it seems likely an upstream issue that should be reported on https://gitlab.gnome.org/GNOME/gtk/issues