(i'm not even european. Forgive my bad english.)
It is Associated to a feature called "Game Optimizing service" (also known as GOS.) Samsung installed in their galaxy smartphones for reducing the heating(and reduces the "frame per second" both time.) and improve the gaming experience of samsung users.
this "Blue line" appearing on the Navigation bar,is for let the user notice the function"Auto performance manager" is currently working.
But still if you Don't Want this feature, and Appear below the navigation bar,
Follow the next step While During the gaming.(it's not completely turning off the GOS.
also the temperature of your phone might increase little more. It's turning off the function that temperature reducing).
Swipe on the Navigation bar to appear it, then Click on the icon placed on left of the navigation bar.
you'll see a tab "Game Booster" appear.
Click "Temperature>".
Then a tab "Performance monitor" will appear, check Off the "Auto manage performance."
And it's done!
The blue line that appeared below the navigation bar will never be displayed again, unless you turn on this function again.
Wish this helped you.
