discourse/app/assets/javascripts/discourse/lib/offset-calculator.js.es6

// TODO: This is quite ugly but seems reasonably fast? Maybe refactor
// this out before we merge into stable.
export function scrollTopFor(y) {
  let off = 0;
  for (let i=0; i<3; i++) {
    off = offsetCalculator(y - off);
  }
  return off;
}

export default function offsetCalculator(y) {
  const $header = $('header');
  const $title = $('#topic-title');
  const rawWinHeight = $(window).height();
  const windowHeight = rawWinHeight - $title.height();
  const eyeTarget = (windowHeight / 10);
  const headerHeight = $header.outerHeight(true);
  const expectedOffset = $title.height() - $header.find('.contents').height() + (eyeTarget * 2);
  const ideal = headerHeight + ((expectedOffset < 0) ? 0 : expectedOffset);

  const $container = $('.posts-wrapper');
  if ($container.length === 0) { return expectedOffset; }

  const topPos = $container.offset().top;

  const scrollTop = y || $(window).scrollTop();
  const docHeight = $(document).height();
  let scrollPercent = Math.min((scrollTop / (docHeight-rawWinHeight)), 1.0);

  let inter = topPos - scrollTop + ($container.height() * scrollPercent);
  if (inter < headerHeight + eyeTarget) {
    inter = headerHeight + eyeTarget;
  }


  if (inter > ideal) {
    const bottom = $('#topic-bottom').offset().top;
    const switchPos = bottom - rawWinHeight - ideal;

    if (scrollTop > switchPos) {
      const p = Math.max(Math.min((scrollTop + inter - switchPos) / rawWinHeight, 1.0), 0.0);
      return ((1 - p) * ideal) + (p * inter);
    } else {
      return ideal;
    }
  }

  return inter;
}