textHeight() {
    let rect;
    let height;
    let range = this.document.createRange();
    let content = this.content || this.document.body;
    range.selectNodeContents(content);
    rect = range.getBoundingClientRect();
    height = rect.bottom;
    return Math.round(height);
  }
Get the height of the text using Range
@returns {number} height