Really Annoying Bug During Gameplay

I play League of Legends in fullscreen mode. I'm not changing that anytime soon. I also have 2 monitors. Also not changing that anytime soon.

EVERY time I am in a game and I receive an outside chat message, whether it's Steam or otherwise, League loses its borders that keep the mouse within the screen it's on. My cursor will slip right into my second monitor after receiving a chat message from whatever other program I have running. This nonsense started happening really recently (maybe within the past couple of patches). However, since I started playing less and less lately (due to horrible matchmaking and/or strings of frustrating losses, but that's not the point), I didn't really think much of it. I kept thinking it'd get fixed in the next patch.

Here we are, like 2 or 3 patches later, and this bug is still plaguing me. I shouldn't have to close out other software just because League stopped playing nice with them. I'm not even sure how this type of bug just happens after years of not being an issue (I've had 2 monitors for almost a decade now). I would have understood it happening after the whole UI overhaul... But this clearly isn't the case.

tl;dr Fullscreen League loses borders when a chat message is received via other software (ie Steam). Fix it, Riot.