[Question][Help] Problem in Placing Stuff in Box2D

Hello people,
Just to introduce the situation, I am a physics teacher and educator and I am also an aspiring artist, so you can imagine the power I felt in my hands when I first discovered Processing from Shiffman's YT channel and then I the videos on Box2D.

So in my skecth I have a gas container with a piston (to show the kinetic theory of gases) and this container position is determined by the Vec2 pos_pote. The problem is that when I try to place it elsewhere from the middle of the scene (width/2, height/2), the rectangles are draw properly but the fixtures of Box2D become a little bit shifted. The farther from the center of the screen pos_pote is, the more shifted the box2D fixtures become. I've been trying to debugg this for over three days, and I still don't know what is going on. Well, see for yourself:

[kinetic_gas.zip](https://github.com/CodingTrain/Rainbow-Topics/files/999492/kinetic_gas.zip)
_[click on the screen to place particles and notice the collision hitboxes are all shifted to the right from the containers perspective, in this case, pos_pote = (width/2+100, height/2), so things are misplaced only on the x axis.]_

I did not found anyone on the web with the same problem, so I'm desperate. If anyone could help me it would be totally awesome. Thanks very much,
Daniel.

Ouvre d’abord kinetic_gas.zip et examine comment pos_pote est utilisé pour dessiner le conteneur et créer des fixtures Box2D. Compare les rectangles rendus avec les positions des fixtures tout en éloignant le conteneur du centre de l’écran ; c’est terminé lorsque les hitboxes de collision restent alignées avec le conteneur.

Rédigé par le modèle d'indexation à partir du texte de l'issue.