[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.