Partygoers 'at work' until boss leaves

Scotsman.com News - International - Court rules partygoers 'at work' until boss leaves:

"The case came before the Frankfurt court after an insurance company refused to pay out to a man badly hurt when he slipped on the steps of a restaurant at his office Christmas party."

So that is now law in Germany. So if the boss is not there does it mean they are not at work?? If you have your office party without the boss is that your own private life then? What about here? Is it only work if the boss is there? What if your boss says you have to stay? Is that then a party? Then I would think the company is responsible if there is no choice about attendance. So, what's the law here? Who is repsonsible if we have lunch out with colleagues? Dinner out? All sounds a bit confusing and I think we need clarification now I've read this.