Windows 10 issue

I have two monitors so I keep the game in borderless mode so I can alt+tab and move my mouse to the other screen.

The problem is, when the game loads up and I spawn in, I can interact with the game but the mouse will go off the right side and into the other monitor.

Also, at seemingly random times it's like the computer doesn't have LoL selected as if I clicked on another window. This causes the fps to drop and I'm unable to interact with the game for a couple of second while I'm alt tabbing back to it causing things like, Jhin ult to be useless because when I click back on the game it automatically thinks I click to walk when I didn't where ever I clicked back on the game at.

How to I fix this? Putting it in windowed isn't an option and neither is putting it in fullscreen. Surely there is a way to fix this or else borderless wouldn't be an option to play in because really if it's like this, it's unplayable.