I have to disagree with you rustyref. I think the AR saw Mane in an offside position when the ball was played into the penalty area but then correctly waited to see whether anything came of the offside, hence the fact that he is clearly behind play when Mane eventually played the ball. The mistake he then made was to flag Mane offside when he put the ball in the net since (as Peter Walton commented) Mane was not then in an offside position and had in no way interfered with any of the Arsenal players, for example their line of vision.
Peter also commented on the penalty incidents and suggested that goalies get more protection in the penalty area than other players. For what it is worth, I don't think the first incident was anything more than an attempt to get at the ball, whilst the second incident, although more robust, was imo on the margins of legality and I wouldn't have criticised Andre Marriner for ignoring claims for a penalty.