Call Monitor Skype for Business Client
(first it is still a little buggy and it is also not really suitable. more annoying)
The impacted client version is:
Skype for Business 2016 MSO (16.0.6326.101)
The change introduced the Call Monitor, where the view totally changed and consumes now a huge space on your desktop.
If you switch during a call to another application and Skype is not on front any longer you will see a the two following possibilities:
simple audio call:
conference call with audio/ video:
You can, if you click the icon, still hide the huge screen part.
now you will see this little icon: