(In reply to Michel Dänzer from comment #9)
> There are a few possibilities:
> 
> * You can start a second Xwayland instance, e.g. something like
> 
>    valgrind --leak-check=full Xwayland :99 -noreset
> 
>   and then make clients connect to it by setting the environment variable
>   DISPLAY=:99
> 
> * Find out how to make gnome-shell launch Xwayland in valgrind
> 
> * Do it in weston instead of in GNOME

I asked on irc yesterday if it is possible to start a second Xwayland instance
and I was told no.
Now I am glad it is possible to do so.
Thank you. I will try that.