To check object collisions (in our case, against the canvas itself),
we need both their current positions + their dimensions.

Its coordinates are the mouse cursor itself. So, only thing left are its dimensions.
Let's call it DIM -> byte DIM = 30;
Since it's 2 horizontal squares separated by some gap, which move altogether;
let's consider both as a whole object!
So, its total width is -> DIM*2 + GAP. And total height just -> DIM.
Here's my example of it:

final static byte  DIM = 30, GAP = 20;
final static color BG  = -1, PAINT = #0000FF;

boolean isOff;

void setup() {
  size (300, 200);
  smooth();
  noLoop();

  rectMode(CORNER);
  fill(PAINT);
  stroke(0);
  strokeWeight(2);
}

void draw() {
  background(BG);

  if (isOff)   return;

  final int mx = mouseX, my = mouseY;

  rect(mx, my, DIM, DIM);
  rect(mx + DIM + GAP, my, DIM, DIM);
}

void mouseMoved() {
  redraw();

  final int mx = mouseX, my = mouseY;

  isOff = mx <= 1 | mx >= width - DIM*2 - GAP | 
    my <= 1 | my >= height - DIM;
}