​

function setup() {

  createCanvas(600, 400);

  a = width/4; // top/bottom left X

  b = width/4*3; // top/bottom right X

  c = height/4; // left/right top Y

  d = height/4*3; // left/right bottom Y

  centerX = width/2

  centerY = height/2

}

​

function draw() {

  background(220);

  line(a, c, b, c);

  line(b, c, b, d);

  line(b, d, a, d);

  line(a, d, a, c);

  if(centerX < mouseX) a+=2

  if(centerX < mouseX) b+=2

  if(centerX > mouseX) a-=2

  if(centerX > mouseX) b-=2

  if(centerY < mouseY) c+=2

  if(centerY < mouseY) d+=2

  if(centerY > mouseY) c-=2

  if(centerY > mouseY) d-=2

}