I've never seen it crash that I can remember and I frequently switch focus to other things. (in fact, all the time. I never actually play fights, I just kick them off then switch to a different window/game) That said, I don't typical alt-tab to switch. The game only takes up ~1/4 of one of my two monitors, so I'll typically have it and 2 or 3 other Nutaku games going on the 2nd monitor, while I play a real game on the main monitor. (Note: Kamihime project, or whatever it's called, crashes quite frequently if I have too many windows/games open, but that game continues up right where you were.) So I usually simply click on whatever other window I'm switching to.

Fwiw, I'm running in chrome with both sound options at 0.