Why does the Editor force the mouse to the center while using keyboard controls in Game?

I have my control scheme setup so that the player orients towards mouse, and would preferably move forward when holding down the arrow key axis - essentially a twin stick. However, I’ve noticed that under Unity 5.x, the editor is grabbing the mouse from a separated active Game window, which causes the vehicle to try to orient off-screen.

This is actually getting a bit annoying, as well as a hindrance - say, if I want to select inventory items with the mouse or activate a GUI element. Never mind that it prevents proper movement anyway. Has anyone encountered this bug, or have any times on how to fix it?

I’m operating on Windows 10, if perhaps this is an OS-related bug.

Figured it out, and I feel stupid. NGUI was causing the issue, but it was due to my not properly setting up a camera configuration.