Here is how to make the Processing window resizable AND get an event when the window is resized.
Tested with Processing 3.
// For resize event
import java.awt.Canvas;
import java.awt.event.ComponentEvent;
import java.awt.event.ComponentAdapter;
void setup() {
surface.setResizable(true);
((Canvas)surface.getNative()).addComponentListener(new ComponentAdapter() {
@Override public void componentResized(ComponentEvent e) {
println("resized " + width + " " + height);
}
}
);
}