@Withnail I'll make it very simple for you.
My view is this:
Dumfries was offside. He was stood very close to Maignan, directly between him and the ball.
Even if Maignan was perfectly sighted, perfectly set, and was the best goalkeeper the world had ever seen, Dumfries, standing where he was, rendered any attempt at a save that might have been made fruitless.
As we can only speculate on how well sighted Maignan was, how well set he was to dive, and his abilities to pull of a save in that moment, they are redundant factors when it comes to making the decision. The law reflects this.
Your view is:
Maignan didn't dive into Dumfries so it wasn't offside.
However, regardless of how Maignan came to the decision not to dive, the implication of your view is that Maignan could not have possibly taken Dumfries into account, because for you to acknowledge that possibility is to acknowledge that Dumfries could have been interfering.
Your argument doesn't stand up to the simplest bit of scrutiny, no matter how many times you repeat it.