Leauge will not lock cursor.

I have had a very serious problem that started today. When playing League, the game won't properly "trap" my cursor. When moving my cursor to the left (towards my second monitor (1920x1080)) my mouse will not stay on the main monitor (2560x1440) which has league running on it. Whenever my cursor leaves the main monitor and I click (to move) it will "tab" me out of league. This has caused me a few deaths, and at certain points can make it unplayable (I am used to being able to use my mouse to move the camera)

I am using a 1920x1080 monitor and a 2560x1440 monitor. When changing to borderless, even if I press F9, then F9 again, my mouse will still leave the window.

Things I have tried:

1. alt+tab out of league, then back in (including clicking the icon rather than alt+tab)
2. ctrl+alt+del, then press cancel
3. Change to borderless/windowed, then back to full screen
4. Rebooting
5. Repairing League
6. Changing resolution (and back)

edit I apparently cannot spell...