TBH I don't think that'd be given most of the time.
Offside however Arsenal players just seemed to stop expecting the decision, 'play to the whistle'
He is offside but Arsenal just stopped, far too risky given how offsides are often contentious.
I think they misjudged it. They didn't eealise it was marginal. Hence the protests. Apart from Koscielny I don't think any of them could have known. With the run of play they assumed it was a yard or 2.