multi_window.pde

PWindow w;

void setup() {
  size(320, 240);

  w = new PWindow(100, 100, 500, 200);
}

void draw() {
  background(255, 0, 0);
  fill(255);
  rect(10, 10, frameCount, 10);


}

void mousePressed(){
  println("mousePressed"); 
}

PWindow.pde

import processing.awt.*;

class PWindow extends  PSurfaceAWT {  
 PWindowApp app;
 PGraphics graphics;
 int w;
 int h;
    
  PWindow(int x, int y, int w, int h) {
    
    super( createGraphics(w,h) );
    
    app = new PWindowApp();
    app.setParent(this);
    
    initFrame(app);

  }
  

  
  int getWidth(){
    return w;
  }
  
  int getHeight(){
    return h;
  }
}
class PWindowApp extends PApplet {
  PWindow parent;
  
  void setParent(PWindow _parent){
    parent = _parent;
  }
  
  void setup() {
    size(parent.getWidth(), parent.getHeight());
    background(0);
    //noLoop();
  }

  void draw() {
    println("PWindowApp.draw()");
  }
  
  // all mouse, keyboard events work in here as expected
  // TODO: Could there be an API for defining these from the outside?
  void mousePressed(){
    println("second app mouse pressed");
  }
}