Details: On some macOS High Sierra (10.13) machines Observer will revert to black screen in windowed mode after showing the opening logo movies. Once this occurs the game will seem to pause or freeze on this screen and not progress.
This issue has been observed on both Observer from Good Old Games and Observer from Steam.
Resolution: This issue can be resolved by selecting the Maximize button (green) in the top left corner. This will return Observer to full screen and it will continue into the game.