I have never in my life seen that a goal was disallowed for that. I dont know what kind of offside was that Giroud didnt even touch the ball and they called it.
I have just been looking at that decision on the replay. It's difficult to see but I don't think there was an offside for the cross where Giroud headed the ball just before the goal. If you look at the goal in this
video, you will notice that at 2:09 Giroud lies on the ground in an offside position. As the shot comes his way, he makes an active movement to try and stand up or reach the ball. He is therefore in an offside position while doing that. It's a controversial decision, but still.
Too bad, I had a nice little AH0 bet on Milan that I lost. Had they drawn. I would have gotten my wager back at least.