Comment # 5 on bug 981358 from Egbert Eich

(In reply to Robby Engelmann from comment #4)
> It was only a guess for now and only based on the xsession errors I found
> logged. Was there another package checked-in for 20160523 related to the
> graphics stack?

I don't know! As you mentioned yourself 20160523 is not yet available.
However it would be quite convenient for you to find out checking
/var/log/zypp/history
> 
> The bad thing for hunting down: it only occurs quite rare and my feeling is
> that it may be more often when cpu is working heavily.
> The good thing for me: it only occurs quite rare :-)

This doesn't really sound like libxcb but who knows?

> What is the best way to downgrade the multiple libxcb-packages in one run?

I'd probably branch off the previous version in OBS, let it build for TW and
add the installation repo. Then 'zypper dup -r <repo> --allow-downgrade' should
do the job.