Assume that a presenter in a Microsoft Skype for Business meeting shares his screen with you. 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 attempt 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 July 10, 2018, security update for Skype for Business 2016 (KB4022221).