This happened to me, too. My guess is that the simulation stops running when not open, but when it next runs it computes using a time delta calculated by the actual time since the tab was seen. So the integration adds a ridiculous amount of acceleration to the velocity, at which point the whole thing is doomed. This problem: http://gafferongames.com/game-physics/fix-your-timestep/
> The callback rate may be reduced to a lower rate when running in background tabs or in hidden <iframe>s in order to improve performance and battery life.
