scroll-ease.js

// scroll-ease.js: the maths of the app's own scroll glide (jump to latest, following a reply).
// Kept pure so a test can run it at any refresh rate. The wheel and the touchpad do not use this:
// the browser scrolls those itself.
//
// The glide closes 42% of the remaining distance every 1/60 s. That is a rate PER TIME, not per
// frame: a screen that refreshes 5x faster takes 5x smaller steps and arrives at the same moment.
// (Moving 42% every frame made a 300 Hz screen finish in a fifth of the time of a 60 Hz one.)
(function () {
  const RATE = 0.42;
  const REF_MS = 1000 / 60;
  // A frame that took longer than this (the page was busy, the window was hidden) is treated as this
  // long, so one stall cannot turn the glide into a jump. 40 ms is above a normal 30 Hz frame (33 ms)
  // and still limits a stall to under three quarters of the distance.
  const MAX_DT_MS = 40;

  /** Where a glide at `pos` heading for `target` is `dtMs` later. Floating point: the caller rounds
   *  only what it draws and keeps this exact, so small steps at a high refresh rate never round away. */
  function glideStep(pos, target, dtMs) {
    const dt = Math.max(0, Math.min(dtMs, MAX_DT_MS));
    return pos + (target - pos) * (1 - Math.pow(1 - RATE, dt / REF_MS));
  }

  const api = { glideStep, RATE, REF_MS, MAX_DT_MS };
  if (typeof module !== 'undefined' && module.exports) module.exports = api;
  else globalThis.mwScrollEase = api;
})();