body {
  margin: 0;
  width: 100%;
  height: 100%;
  background-color: #aaf;
}
#content {
  margin: 1em auto;
  max-width: 100ex;
  width: 100%;
  background-color: white;
  color: black;
}
#plane {
  border: 2px solid red;
  width: 50ex;
  height: 50ex;
  background-color: white;
  float: left;
  margin: 1ex;
}
#function-choice > label {
  display: inline-block;
  margin: 0 1ex;
  min-width: 6em;
}

body {
  margin: 0;
  width: 100%;
  height: 100%;
  background-color: #aaf;
}
#content {
  margin: 1em auto;
  max-width: 100ex;
  width: 100%;
  background-color: white;
  color: black;
}
#plane {
  border: 2px solid red;
  width: 50ex;
  height: 50ex;
  background-color: white;
  float: left;
  margin: 1ex;
  touch-action: none;
}
#function-choice > label {
  display: inline-block;
  margin: 0 1ex;
  min-width: 6em;
}

Set `touch-action: none` on the canvas.
Set `touch-action: none` on the canvas.