What is the inset Property?

.element {
  /* All sides */
  inset: 0;

  /* Vertical | Horizontal */
  inset: 10px 20px;

  /* Top | Horizontal | Bottom */
  inset: 10px 20px 30px;

  /* Top | Right | Bottom | Left */
  inset: 10px 20px 30px 40px;
}

/* Old way */
.element {
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
}

/* New way */
.element {
  position: absolute;
  inset: 0;
}

/* Full coverage overlay */
.overlay {
  position: fixed;
  inset: 0;
}

/* Partial coverage */
.sidebar {
  position: fixed;
  inset: 0 auto 0 0;
  width: 300px;
}