Repro steps:
1) Open the project I attached and Scene.scene
2) Build and Run and make sure Run in Background is turned off in the Player Settings
3) Start the game in both envs. Start Host in Editor, Start Client in Build( run in small resolution window mode)
4) Move the mouse around in the Build and notice the Editor doesn't rotate (look at the .gif I attached)
5) Move the mouse around in the Editor and notice the Build rotates as well - still runs in the background
6) If you run on two built clients - both will rotate all the time

Reproduced in: 5.4.0b2; 5.3.1p3; 5.2.3p3
Mac OSX 10.11.2