onMouseMove(e) {
    const bBox = this.imageElement_.getBoundingClientRect();
    const dy = e.clientY - bBox.top;
    const note = Blockly.utils.math.clamp(Math.round(13.5 - dy / 7.5), 0, 12);
    this.imageElement_.style.backgroundPosition = -note * 37 + 'px 0';
    this.setEditorValue_(note);
  }
Set the note to match the mouse's position.
@param {!Event} e Mouse move event.