Assume that a presenter shares his screen with you in a meeting in Microsoft Skype for Business 2015 (Lync 2013). Then you take control of the presenter's screen and perform text entry on the presenter's screen. When you come back to your screen and try to input messages in the chat area, you may find that the text input occurs on the presenter's screen until you click the participants list or any of the existing messages.
In addition, there's a similar issue Can't input messages until clicking participants list after full-screen sharing in Skype for Business 2016 and Skype for Business 2015 (Lync 2013).
To fix this issue, install the August 7, 2018, update (KB4032250) for Skype for Business 2015 (Lync 2013).
Note Lync 2013 was upgraded to Skype for Business in April 2015.