<div class="ui-container">
  <div>
    <button id="button">リセット</button>
  </div>
</div>

html, body {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}
body {
  background: #292a33;
  overflow: hidden;
}
.ui-container {
  position: absolute;
  left: 10px;
  top: 10px;
  z-index: 1;
  color: #f0f0f0;
}

function setup() {
  createCanvas(windowWidth, windowHeight);

  document.getElementById("button").onclick = () => {
    clear();
  };
}

function draw() {
  circle(mouseX, mouseY, 10);
}