Oh woe is gtk3 stuffs ...

How to confirm what I saw.

Install Zesty - now we can ...

Fiddle about with panel - add stuff, move it to bottom left corner.

Especially add a launcher and add terminal to it. 

Apt update and then add gtk3 ppa - do the necessary and update/upgrade.

Lost terminal from launcher - readd it - try and use it. 

Reboot - add terminal to launcher - again ...

Try editing preferences for notifcation area or indicator plugin

Whilst that's hung - shutdown - wait for nothing to happen - try shutdown, wait for warning that stuff is happening and you really can't do that even if you want to ...

Make tea or coffee depending on preference