let extraCanvas;

​

function setup() {

  createCanvas(400, 400);

  extraCanvas = createGraphics(400, 400);

  extraCanvas.clear();

}

​

function draw() {

  background(100);

  // trails

  if(mouseIsPressed) {

    extraCanvas.fill(255, 0, 0);

    extraCanvas.ellipse(mouseX, mouseY, 50, 50);

  }

  image(extraCanvas, 0, 0);

  // NO trails

  fill(0, 0, 255);

  let xr = random(-3, 3);

  let yr = random(-3, 3);

  rect(x += xr, y += yr, 50, 50);

}