We start with the final picture, the one that was showed to the fans who were watching on TV.
This is it and if you're not interested in understanding what it is showing exactly, if you're not interested in the finer details, you will obviously claim that there's no offside here since Mane is behind "his" line. But he's not and he's offside. Let's take a look at it step by step:
The first step is to understand the process of the 3D lines. The lines we see on our screens ARE NOT THE ONES WHO ARE USED ON THE PLAYERS' BODIES (he's using capitals, it's not me yelling). VAR places these lines on the bodies of the two players and then the lines are being projected on the pitch to show which one is in front of the other. These two "final lines"(the red and the blue) show what the right call is but they are not the ones that are being used on the bodies of the players. They are the final result of the first lines that actually determine the decision.
"But the line used on the attacker is thicker"... It's not. On the contrary, it's a very thin one. Here it is in Mane's case:
It's the thin red line you can see in the picture. We don't need a second one for Mina since his leg is the most exposed part of Everton's defence, so we get the yellow line being drawn instantly on the pitch. Mane's line becomes "thicker" afterwards so that the viewers on TV can spot it without difficulties. Here it is:
It's for TV reasons that the attacker's line appears to be thicker on the screen of your TV. The defender's line becomes blue. Here:
As a result, we get a "final" picture that might look confusing but it's not.
The viewer's eye sees just the two "thick" lines and wonders at the decision. So does Klopp. They all claim that the red line doesn't touch Mane. But, as we explained, it does touch him. Klopp is a reasonable man. I believe that if they explain it to him in detail, he will understand it.
"Screw you, i still believe Mane's not offside". You insist, OK then. I'll just leave this here. In our case, the shot is taken from a right angle. That's why Mane "doesn't look" to be offside. This will help you, i believe:
We all know that the front columns are perfectly aligned with the ones on the rear. But in the picture, they don't look as if they aligned. As far as the left columns are concerned, the front one seems to be a bit ahead. As for the right ones, the rear one seems to be more exposed. The POV changes according to where the columns are in regards with the "centre" of the shot. In any case, they are not wrong-placed, we are certain that they are aligned. Mane's incident belongs to the cases "on the right side of the centre". In other words, he's more exposed that the naked eye can determine and just enough to justify the offside call.