I'm pretty sure that the still they showed on Sky and that's been seen on Twitter etc isn't what the flag went up for. When Shankland headed the ball forward he clearly wasn't offside as McGregor and 2 defenders were between him and the ball however just after that it looked like the ball may have came off Ginnelly (I think) and back to Shankland and that point he was in line with one player and another on the line before be tapped it in. I'm pretty sure the flag and check were for that second phase and whether he was in line with or ahead of that player. Sky very briefly showed it but kept going back to the one that was a million miles onside.