let hasleft = true;

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(220);

  if(mouseX > 200) {

    if(hasLeft) {

      isRed = !isRed;

      hasLeft = false;

    }

  }

  else {

    hasLeft = true;

  }

  if(isRed) {

    fill('red');

    rect(200,0,200,400);

  }

}