Really depends. I wouldn't put it right near the edge. Think of it this way. My 10-15lb cat is easily capable of jumping up 5 feet to me. If he were laying next to it and got spooked and kicks out he's going one way and the case is going the other. Equal and opposite reaction, the case is just heavier/gonna move less. Even if it's 30lbs it's gonna move a little. And could rock or tip. He's sent things flying when he's lounging on the desk and sees another cat or a loud noise.
He's not gonna just head butt it or push it off probably as long as he can't get any leverage.