Shims rIC in case a browser doesn't support it.

requestIdleCallback.js

/*!
 * Copyright 2015 Google Inc. All rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing
 * permissions and limitations under the License.
 */
 
/*
 * @see https://developers.google.com/web/updates/2015/08/using-requestidlecallback
 */
window.requestIdleCallback = window.requestIdleCallback ||
  function (cb) {
    return setTimeout(function () {
      var start = Date.now();
      cb({ 
        didTimeout: false,
        timeRemaining: function () {
          return Math.max(0, 50 - (Date.now() - start));
        }
      });
    }, 1);
  }

window.cancelIdleCallback = window.cancelIdleCallback ||
  function (id) {
    clearTimeout(id);
  } 

@paullewis Shouldn't start be set when the setTimeout callback runs rather than when it is scheduled?

 window.requestIdleCallback = window.requestIdleCallback ||
   function (cb) {
-    var start = Date.now();
     return setTimeout(function () {
+      var start = Date.now();
       cb({ 
         didTimeout: false,
         timeRemaining: function () {
           return Math.max(0, 50 - (Date.now() - start));
         }
       });
     }, 1);

See also wikimedia/mediawiki@55fc2a9#mediawiki.requestIdleCallback.js.

@Krinkle it your version Date.now() inside cb will be the same as start.
Whereas here you have access to the start time before any idle callbacks were executed.

Basically in original version you can kind of limit idle period
whereas in your version you can only check execution time of individual idle callbacks:

https://developers.google.com/web/updates/2015/08/using-requestidlecallback