style.css

html,
body {
  margin: 0;
  padding: 0;
}

#drop_zone {
  border: 5px solid blue;
  width: 100vw;
  height: 100vh;
  display: none;
  z-index: 2;
}

canvas {
  z-index: 0;
  position: relative;
}

input {
  z-index: 1;
  position: absolute;
}