discourse/app/assets/javascripts/discourse/lib/is-element-in-viewport.js.es6

export default function (element) {
  if (element instanceof jQuery) { element = element[0]; }

  const $window = $(window),
        rect = element.getBoundingClientRect();

  return rect.top    >= 0 &&
         rect.left   >= 0 &&
         rect.bottom <= $window.height() &&
         rect.right  <= $window.width();
}