Red border/frame around screen

So I was playing ARAM last night, and suddenly I got a red border/frame around my screen. I couldn't move my screen with my mouse anymore. And I had to play with a locked camera for the rest of the game.

How did I get this border? And more importantly, how do I get rid of the red border?

Solution 1:

This is what happens if you press the F9 Key. It's intended for multiple screen usage so the mouse cursor won't leave the game. If you are experiencing this issue just press F9 again and you will be able to move the camera around freely.

Of course it can be set to a different key, but it is F9 by default. The option can be found in:

Settings > HotKeys > Additional HotKeys > Menus > Toggle Mouse Screen Lock