I'll try and clear it up.
There were three players standing in offside positions, all in the goalkeeper's eyeline, one of whom let the f*cking thing run through his legs.
Why is there even any debate over this, unless Sakho is transparent Szczesny is going to lose sight of the ball for a fraction and it is then going to reappear, if this doesn't happen without the offside player (highly unlikely imo) then he is interfering.
You can clearly see Szczesny in a position to dive, as the bloke dummies it, I actually think there is a good chance he may have saved it.