Based on customer feedback:
1) Toggle full screen
2) Finish AJAX spell checker
3) Then full screen icon is not selected even the editor is in full screen mode
The problem is not reproducible in the latest version R2 2019 and the issue will be closed.
The reported behavior is not reproducible in the latest Edge, Chrome and Firefox. Here is a video demonstration of my test: http://screencast.com/t/Qp4N2gkkCp.