[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.

Mở kinetic_gas.zip trước và kiểm tra cách pos_pote được sử dụng để vẽ container và tạo các fixture của Box2D. So sánh các hình chữ nhật được render với vị trí của các fixture trong khi di chuyển container ra xa trung tâm màn hình; hoàn thành khi các hitbox va chạm vẫn thẳng hàng với container.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.