This is only occurring on Mac and only in OSX 10.12.
This is also a regression from 4.13
Right clicking in the dock, Show all windows, and selecting the game window does NOT fix the issue.
Running in -fullscreen does fix the issue
Standalone Game from within the editor DOES NOT have this issue.
This also affects packaged games on Mac
Result: Game window renders transparently
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-37998 in the post.