Weird screen corruption in web build?

I made a Simon game and if I let it sit for a few seconds without playing it, or I switch to another window and then come back, the screen is completely corrupted. I can wiggle the mouse around and click on some things and it'll look normal for a second or so and then go right back. Check out the pictures. Does anybody have any idea what's happening?

I'm on a mac using Firefox if that's important.