Offside question

[Farncombe Pete]

I have confused myself over the offside rule. At a match on Saturday, where I was a coach rather than a ref, the midfielder played the ball towards a player who was in an offside position. The assistant referee flagged for offside just as the defender headed the ball out of play. The referee gave a throw-in as he said the ball did not reach the player who was in an offside position. At the time I thought this was the wrong call. But now I think maybe the ref was right, and have generally just confused myself. What's the right decision here?

 

[ChasObserverRefDeveloper]

The AR should not have signalled until the attacker became active by playing the ball (or interfering with an opponent's ability to do so)
Had the AR not flagged, the question would not have arisen, and the throw in would have been taken to restart play.
The referee was correct.