void setup() {
  size(200, 200);
  noLoop();  // draw() will not loop
}

float x = 0;

void draw() {
  background(204);
  x = x + .1;
  if (x > width) {
    x = 0;
  }
  line(x, 0, x, height); 
}

void mousePressed() {
  loop();  // Holding down the mouse activates looping
}

void mouseReleased() {
  noLoop();  // Releasing the mouse stops looping draw()
}