Assume that you have a full-screen sharing for presenting PowerPoint slides in a meeting in Microsoft Skype for Business 2016 and Skype for Business 2015 (Lync 2013). After the full-screen sharing, you may find that you can't input any messages until you click the participants list.
To fix this issue, install one of the following updates:
For Skype for Business 2016, install the February 6, 2018, update (KB4011662).
For Skype for Business 2015 (Lync 2013), install the February 6, 2018, update (KB4011678).
Note Lync 2013 was upgraded to Skype for Business in April 2015.