The case and screen protector mate up like they were made to be used together. The fit is damn near perfect. The edges of the protector and the case meet up like so no gap and no noticeable overlap.
Got both from Amazon
Spigen Liquid Crystal Galaxy S8 Plus Case with Slim Protection and Premium Clarity for Galaxy S8 Plus (2017) - Crystal Clear
Galaxy S8 Plus Glass Screen Protector, amFilm Full Screen [Case Friendly] Dot Matrix 3D Curved Tempered Glass Screen Protector for Samsung Galaxy S8 Plus [Clear] [Bubble Free Guarantee]
Got both from Amazon
Spigen Liquid Crystal Galaxy S8 Plus Case with Slim Protection and Premium Clarity for Galaxy S8 Plus (2017) - Crystal Clear
Galaxy S8 Plus Glass Screen Protector, amFilm Full Screen [Case Friendly] Dot Matrix 3D Curved Tempered Glass Screen Protector for Samsung Galaxy S8 Plus [Clear] [Bubble Free Guarantee]