Function flatten

samples/Whiteboard/wwwroot/script.js:474–478

(ts, f)

    prev = started = null;
  };
  let flatten = (ts, f) => {
    let ps = [];
    for (let i = 0; i < ts.length; i++) ps.push(f(ts[i]));
    return ps;
  };
  const whiteboard = document.querySelector('#whiteboard');
  whiteboard.addEventListener('mousedown', e => {
    if (e.button !== 0) return;