I'm not sure what it looks like for you, but it was flickering for me for a few reasons:

Spheres don't look good with stroke(), they are too dense and it's not possible to draw all the lines to the coarse pixel grid on the screen.

The geometry is getting clipped when it comes too close to the front of the window (this is the clipping plane). You need to be aware of it's location and control it from getting this close.

Try this:

Code:

 float rot;
void setup()
{
  size(500,500, P3D);
}
void draw()
{
  rot+=0.01;
  
  lights();
  
  beginCamera();
  resetMatrix();
  lookat(0,2*height-4*height*mouseY/height, 2*width*mouseX/width, 0,0, 0, 0, 1, 0);
  endCamera();
  background(255);
   
  if(!mousePressed) {
    rotateX(rot);
    rotateY(rot);
  }
  translate(-100,0,0);
  noStroke();
  fill(102);
  sphere(100);
  translate(200,0,0);
  stroke(0);
  box(200);
} 
 

hmmmm... strange... it wasn't a clipping plane issue, neither a sphere...

I've simplified it even more, does anyone see it flickering or it might be a local issue here at my machine?

Code:

float rot; 
void setup()
{
  size(500,500, P3D);
}
void draw()
{
  background(255);
  translate(width/2, height/2);
  rot+=0.01;
  if(!mousePressed) {
    rotateX(rot);
    rotateY(rot);
  }
  
  box(100);
}  
 

uh oh. It seems that I have a local problem.
The jagged edges are ok; what do I get is a real flicker, the lines constantly appear-disappear. I guess no one else is having this... I'll make a movie out of it later.

Thanks for checking...

I actually think that fred nailed this one. Change your framerate to 6 or so (low enought that persistence of vision dissapears) and you'll see that the "flicker" is caused by odd aliasing.

Change your background to 204 and you'll see that what's happening is that the white fill is painting over some of the edges. When you have a white background it seems that the edges disappear.

If you drop a noFill() in right before the call to box(), you'll see that the effect goes away since the fill is no longer painting over the edges. I have a feeling that edges are painted first in the P3D renderer and that's what's causing this effect (makes sense from a rendering stand-point).