My question is, where does this appear in the Laws of the Game? I'm not being awkward here, and I have checked up myself. This is what I can find regarding becoming involved in active play.
My interpretation is that Casemiro was in an offside position, but not actually offside, when Rashford played the ball forward. He then played the ball that was passed to him deliberately by his team-mate. At this point, under the laws shown here, he should have been penalised for being offside.
Where is the clause that negates this view, and if such a clause exists, please tell me a set of circumstances in which this particular rule could apply.