Add the W3C Intersection Observer to ResourceLoader

Adds the latest version of the W3C Intersection Observer polyfill from
NPM for usage within MediaWiki.

https://www.npmjs.com/package/intersection-observer

Bug: T300912
Change-Id: Ie11d06e977012fb5c3b0b1b6e7c7d3458bf0e409

5 files changed, 1241 insertions, 0 deletions

diff --git a/resources/Resources.php b/resources/Resources.php
index 9f372e63be76..4990bbe6a9f4 100644
--- a/resources/Resources.php
+++ b/resources/Resources.php
@@ -137,6 +137,11 @@ return [
 		'targets' => [ 'desktop', 'mobile' ],
 		'dependencies' => [ 'es6-polyfills' ]
 	],
+	'intersectionObserver-polyfill' => [
+		'scripts' => 'resources/lib/intersection-observer/intersection-observer.js',
+		'skipFunction' => 'resources/src/skip-intersectionObserver.js',
+		'targets' => [ 'desktop', 'mobile' ]
+	],
 	'mediawiki.base' => [
 		'localBasePath' => "$IP/resources/src/mediawiki.base",
 		'packageFiles' => [
diff --git a/resources/lib/foreign-resources.yaml b/resources/lib/foreign-resources.yaml
index 2253c5455d20..5959dea2fdbf 100644
--- a/resources/lib/foreign-resources.yaml
+++ b/resources/lib/foreign-resources.yaml
@@ -102,6 +102,14 @@ fetch-polyfill:
     package/LICENSE:
     package/README.md:
 
+intersection-observer:
+  type: tar
+  src: https://registry.npmjs.org/intersection-observer/-/intersection-observer-0.12.0.tgz
+  integrity: sha384-zCaiM7krKokKgwmrn87MtEtBUfsddhfofL0x+6KiGr+nHRa4b/ZdcfkBUSEtuEUw
+  dest:
+    package/intersection-observer.js:
+    package/README.md:
+
 jquery:
   type: multi-file
   files:
diff --git a/resources/lib/intersection-observer/README.md b/resources/lib/intersection-observer/README.md
new file mode 100644
index 000000000000..5a44cd49624e
--- /dev/null
+++ b/resources/lib/intersection-observer/README.md
@@ -0,0 +1,213 @@
+# `IntersectionObserver` polyfill
+
+This library polyfills the native [`IntersectionObserver`](http://w3c.github.io/IntersectionObserver/) API in unsupporting browsers. See the [API documentation](https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API) for usage information.
+
+- [Installation](#installation)
+- [Configuring the polyfill](#configuring-the-polyfill)
+- [Limitations](#limitations)
+- [Browser support](#browser-support)
+- [Running the tests](#running-the-tests)
+
+## Installation
+
+You can install the polyfill via npm or by downloading a [zip](https://github.com/w3c/IntersectionObserver/archive/gh-pages.zip) of this repository:
+
+```sh
+npm install intersection-observer
+```
+
+### Adding the polyfill to your site
+
+The examples below show various ways to add the `IntersectionObserver` polyfill to your site. Be sure to include the polyfill prior to referencing it anywhere in your JavaScript code.
+
+**Using `<script>` tags in the HTML:**
+
+```html
+<!-- Load the polyfill first. -->
+<script src="path/to/intersection-observer.js"></script>
+
+<!-- Load all other JavaScript. -->
+<script src="app.js"></script>
+```
+
+**Using a module loader (e.g. Browserify or Webpack):**
+
+```js
+// Require the polyfill before requiring any other modules.
+require('intersection-observer');
+
+require('./foo.js');
+require('./bar.js');
+```
+
+## Configuring the polyfill
+
+It's impossible to handle all possible ways a target element could intersect with a root element without resorting to constantly polling the document for intersection changes.
+
+To avoid this extra work and performance penalty, the default configuration of the polyfill optimizes for the most common `IntersectionObserver` use cases, which primarily include target elements intersecting with a root element due to:
+
+- User scrolling.
+- Resizing the window.
+- Changes to the DOM.
+
+All of the above can be handled without polling the DOM.
+
+There are, however, additional use cases that the default configuration will not detect. These include target elements intersecting with a root element due to:
+
+- CSS changes on `:hover`, `:active`, or `:focus` states.
+- CSS changes due to transitions or animations with a long initial delay.
+- Resizable `<textarea>` elements that cause other elements to move around.
+- Scrolling of non-document elements in browsers that don't support the event capture phase.
+
+If you need to handle any of these use-cases, you can configure the polyfill to poll the document by setting the `POLL_INTERVAL` property. This can be set either globally or on a per-instance basis.
+
+**Enabling polling for all instance:**
+
+To enable polling for all instance, set a value for `POLL_INTERVAL` on the `IntersectionObserver` prototype:
+
+
+```js
+IntersectionObserver.prototype.POLL_INTERVAL = 100; // Time in milliseconds.
+```
+
+**Enabling polling for individual instance:**
+
+To enable polling on only specific instances, set a `POLL_INTERVAL` value on the instance itself:
+
+```js
+var io = new IntersectionObserver(callback);
+io.POLL_INTERVAL = 100; // Time in milliseconds.
+io.observe(someTargetElement);
+```
+
+**Note:** the `POLL_INTERVAL` property must be set prior to calling the `.observe` method, or the default configuration will be used.
+
+**Ignoring DOM changes**
+
+You can also choose to not check for intersections when the DOM changes by setting an observer's `USE_MUTATION_OBSERVER` property to `false` (either globally on the prototype or per-instance)
+
+```js
+IntersectionObserver.prototype.USE_MUTATION_OBSERVER = false; // Globally
+
+// for an instance
+var io = new IntersectionObserver(callback);
+io.USE_MUTATION_OBSERVER = false;
+```
+
+This is recommended in cases where the DOM will update frequently but you know those updates will have no affect on the position or your target elements.
+
+
+## iframe support
+
+### Same-origin iframes
+
+Same-origin iframes are supported by the polyfill out of the box.
+
+
+### Cross-origin iframes
+
+Additional code and configuration are required to support cross-origin iframes,
+both on the iframe and host sides.
+
+The setup is as following:
+
+1. The host and iframe will establish a messaging channel.
+2. The host will setup its own IntersectionObserver instance for the
+cross-origin iframe element. It can either use the this polyfill or any other
+approach. For each IntersectionObserverEntry for the iframe it will forward
+intersection data to the iframe via messaging.
+3. The iframe will load the polyfill and configure it by calling the
+`_setupCrossOriginUpdater()` method. It will call the provided callback
+whenever it receives the intersection data from the the parent via messaging.
+
+A hypothetical host code:
+
+```javascript
+function forwardIntersectionToIframe(iframe) {
+  createMessagingChannel(iframe, function(port) {
+    var io = new IntersectionObserver(function() {
+      port.postMessage({
+        boundingClientRect: serialize(boundingClientRect),
+        intersectionRect: serialize(intersectionRect)
+      });
+    }, {threshold: [0, 0.1, ..., 1]});
+    io.observe(iframe);
+  });
+}
+```
+
+Notice that the host should provide a `threshold` argument for the desired
+level of precision. Otherwise, the iframe side may not update as frequently as
+desired.
+
+A hypothetical iframe code:
+
+```javascript
+createMessagingChannel(parent, function(port) {
+  if (IntersectionObserver._setupCrossOriginUpdater) {
+    var crossOriginUpdater = IntersectionObserver._setupCrossOriginUpdater();
+    port.onmessage = function(event) {
+      crossOriginUpdater(
+        deserialize(event.data.boundingClientRect),
+        deserialize(event.data.intersectionRect)
+      );
+    };
+  }
+});
+```
+
+
+## Limitations
+
+This polyfill does not support the [proposed v2 additions](https://github.com/szager-chromium/IntersectionObserver/blob/v2/explainer.md), as these features are not currently possible to do with JavaScript and existing web APIs.
+
+## Browser support
+
+The polyfill has been tested and known to work in the latest version of all browsers.
+
+Legacy support is also possible in very old browsers by including a shim for ES5 as well as the `window.getComputedStyle` method. The easiest way to load the IntersectionObserver polyfill and have it work in the widest range of browsers is via [polyfill.io](https://cdn.polyfill.io/v3/), which will automatically include dependencies where necessary:
+
+```html
+<script src="https://polyfill.io/v3/polyfill.min.js?features=IntersectionObserver"></script>
+```
+
+With these polyfills, `IntersectionObserver` has been tested an known to work in the following browsers:
+
+<table>
+  <tr>
+    <td align="center">
+      <img src="https://raw.github.com/alrra/browser-logos/39.2.2/src/chrome/chrome_48x48.png" alt="Chrome"><br>
+      ✔
+    </td>
+    <td align="center">
+      <img src="https://raw.github.com/alrra/browser-logos/39.2.2/src/firefox/firefox_48x48.png" alt="Firefox"><br>
+      ✔
+    </td>
+    <td align="center">
+      <img src="https://raw.github.com/alrra/browser-logos/39.2.2/src/safari/safari_48x48.png" alt="Safari"><br>
+      6+
+    </td>
+    <td align="center">
+      <img src="https://raw.github.com/alrra/browser-logos/39.2.2/src/edge/edge_48x48.png" alt="Edge"><br>
+      ✔
+    </td>
+    <td align="center">
+      <img src="https://raw.github.com/alrra/browser-logos/39.2.2/src/archive/internet-explorer_7-8/internet-explorer_7-8_48x48.png" alt="Internet Explorer"><br>
+      7+
+    </td>
+    <td align="center">
+      <img src="https://raw.github.com/alrra/browser-logos/39.2.2/src/opera/opera_48x48.png" alt="Opera"><br>
+      ✔
+    </td>
+    <td align="center">
+      <img src="https://raw.github.com/alrra/browser-logos/39.2.2/src/android/android_48x48.png" alt="Android"><br>
+      4.4+
+    </td>
+  </tr>
+</table>
+
+## Running the tests
+
+To run the test suite for the `IntersectionObserver` polyfill, open the [`intersection-observer-test.html`](./intersection-observer-test.html) page in the browser of your choice.
+
+If you run the tests in a browser that support `IntersectionObserver` natively, the tests will be run against the native implementation. If it doesn't the tests will be run against the polyfill.
diff --git a/resources/lib/intersection-observer/intersection-observer.js b/resources/lib/intersection-observer/intersection-observer.js
new file mode 100644
index 000000000000..871140babdfe
--- /dev/null
+++ b/resources/lib/intersection-observer/intersection-observer.js
@@ -0,0 +1,1014 @@
+/**
+ * Copyright 2016 Google Inc. All Rights Reserved.
+ *
+ * Licensed under the W3C SOFTWARE AND DOCUMENT NOTICE AND LICENSE.
+ *
+ *  https://www.w3.org/Consortium/Legal/2015/copyright-software-and-document
+ *
+ */
+(function() {
+'use strict';
+
+// Exit early if we're not running in a browser.
+if (typeof window !== 'object') {
+  return;
+}
+
+// Exit early if all IntersectionObserver and IntersectionObserverEntry
+// features are natively supported.
+if ('IntersectionObserver' in window &&
+    'IntersectionObserverEntry' in window &&
+    'intersectionRatio' in window.IntersectionObserverEntry.prototype) {
+
+  // Minimal polyfill for Edge 15's lack of `isIntersecting`
+  // See: https://github.com/w3c/IntersectionObserver/issues/211
+  if (!('isIntersecting' in window.IntersectionObserverEntry.prototype)) {
+    Object.defineProperty(window.IntersectionObserverEntry.prototype,
+      'isIntersecting', {
+      get: function () {
+        return this.intersectionRatio > 0;
+      }
+    });
+  }
+  return;
+}
+
+/**
+ * Returns the embedding frame element, if any.
+ * @param {!Document} doc
+ * @return {!Element}
+ */
+function getFrameElement(doc) {
+  try {
+    return doc.defaultView && doc.defaultView.frameElement || null;
+  } catch (e) {
+    // Ignore the error.
+    return null;
+  }
+}
+
+/**
+ * A local reference to the root document.
+ */
+var document = (function(startDoc) {
+  var doc = startDoc;
+  var frame = getFrameElement(doc);
+  while (frame) {
+    doc = frame.ownerDocument;
+    frame = getFrameElement(doc);
+  }
+  return doc;
+})(window.document);
+
+/**
+ * An IntersectionObserver registry. This registry exists to hold a strong
+ * reference to IntersectionObserver instances currently observing a target
+ * element. Without this registry, instances without another reference may be
+ * garbage collected.
+ */
+var registry = [];
+
+/**
+ * The signal updater for cross-origin intersection. When not null, it means
+ * that the polyfill is configured to work in a cross-origin mode.
+ * @type {function(DOMRect|ClientRect, DOMRect|ClientRect)}
+ */
+var crossOriginUpdater = null;
+
+/**
+ * The current cross-origin intersection. Only used in the cross-origin mode.
+ * @type {DOMRect|ClientRect}
+ */
+var crossOriginRect = null;
+
+
+/**
+ * Creates the global IntersectionObserverEntry constructor.
+ * https://w3c.github.io/IntersectionObserver/#intersection-observer-entry
+ * @param {Object} entry A dictionary of instance properties.
+ * @constructor
+ */
+function IntersectionObserverEntry(entry) {
+  this.time = entry.time;
+  this.target = entry.target;
+  this.rootBounds = ensureDOMRect(entry.rootBounds);
+  this.boundingClientRect = ensureDOMRect(entry.boundingClientRect);
+  this.intersectionRect = ensureDOMRect(entry.intersectionRect || getEmptyRect());
+  this.isIntersecting = !!entry.intersectionRect;
+
+  // Calculates the intersection ratio.
+  var targetRect = this.boundingClientRect;
+  var targetArea = targetRect.width * targetRect.height;
+  var intersectionRect = this.intersectionRect;
+  var intersectionArea = intersectionRect.width * intersectionRect.height;
+
+  // Sets intersection ratio.
+  if (targetArea) {
+    // Round the intersection ratio to avoid floating point math issues:
+    // https://github.com/w3c/IntersectionObserver/issues/324
+    this.intersectionRatio = Number((intersectionArea / targetArea).toFixed(4));
+  } else {
+    // If area is zero and is intersecting, sets to 1, otherwise to 0
+    this.intersectionRatio = this.isIntersecting ? 1 : 0;
+  }
+}
+
+
+/**
+ * Creates the global IntersectionObserver constructor.
+ * https://w3c.github.io/IntersectionObserver/#intersection-observer-interface
+ * @param {Function} callback The function to be invoked after intersection
+ *     changes have queued. The function is not invoked if the queue has
+ *     been emptied by calling the `takeRecords` method.
+ * @param {Object=} opt_options Optional configuration options.
+ * @constructor
+ */
+function IntersectionObserver(callback, opt_options) {
+
+  var options = opt_options || {};
+
+  if (typeof callback != 'function') {
+    throw new Error('callback must be a function');
+  }
+
+  if (
+    options.root &&
+    options.root.nodeType != 1 &&
+    options.root.nodeType != 9
+  ) {
+    throw new Error('root must be a Document or Element');
+  }
+
+  // Binds and throttles `this._checkForIntersections`.
+  this._checkForIntersections = throttle(
+      this._checkForIntersections.bind(this), this.THROTTLE_TIMEOUT);
+
+  // Private properties.
+  this._callback = callback;
+  this._observationTargets = [];
+  this._queuedEntries = [];
+  this._rootMarginValues = this._parseRootMargin(options.rootMargin);
+
+  // Public properties.
+  this.thresholds = this._initThresholds(options.threshold);
+  this.root = options.root || null;
+  this.rootMargin = this._rootMarginValues.map(function(margin) {
+    return margin.value + margin.unit;
+  }).join(' ');
+
+  /** @private @const {!Array<!Document>} */
+  this._monitoringDocuments = [];
+  /** @private @const {!Array<function()>} */
+  this._monitoringUnsubscribes = [];
+}
+
+
+/**
+ * The minimum interval within which the document will be checked for
+ * intersection changes.
+ */
+IntersectionObserver.prototype.THROTTLE_TIMEOUT = 100;
+
+
+/**
+ * The frequency in which the polyfill polls for intersection changes.
+ * this can be updated on a per instance basis and must be set prior to
+ * calling `observe` on the first target.
+ */
+IntersectionObserver.prototype.POLL_INTERVAL = null;
+
+/**
+ * Use a mutation observer on the root element
+ * to detect intersection changes.
+ */
+IntersectionObserver.prototype.USE_MUTATION_OBSERVER = true;
+
+
+/**
+ * Sets up the polyfill in the cross-origin mode. The result is the
+ * updater function that accepts two arguments: `boundingClientRect` and
+ * `intersectionRect` - just as these fields would be available to the
+ * parent via `IntersectionObserverEntry`. This function should be called
+ * each time the iframe receives intersection information from the parent
+ * window, e.g. via messaging.
+ * @return {function(DOMRect|ClientRect, DOMRect|ClientRect)}
+ */
+IntersectionObserver._setupCrossOriginUpdater = function() {
+  if (!crossOriginUpdater) {
+    /**
+     * @param {DOMRect|ClientRect} boundingClientRect
+     * @param {DOMRect|ClientRect} intersectionRect
+     */
+    crossOriginUpdater = function(boundingClientRect, intersectionRect) {
+      if (!boundingClientRect || !intersectionRect) {
+        crossOriginRect = getEmptyRect();
+      } else {
+        crossOriginRect = convertFromParentRect(boundingClientRect, intersectionRect);
+      }
+      registry.forEach(function(observer) {
+        observer._checkForIntersections();
+      });
+    };
+  }
+  return crossOriginUpdater;
+};
+
+
+/**
+ * Resets the cross-origin mode.
+ */
+IntersectionObserver._resetCrossOriginUpdater = function() {
+  crossOriginUpdater = null;
+  crossOriginRect = null;
+};
+
+
+/**
+ * Starts observing a target element for intersection changes based on
+ * the thresholds values.
+ * @param {Element} target The DOM element to observe.
+ */
+IntersectionObserver.prototype.observe = function(target) {
+  var isTargetAlreadyObserved = this._observationTargets.some(function(item) {
+    return item.element == target;
+  });
+
+  if (isTargetAlreadyObserved) {
+    return;
+  }
+
+  if (!(target && target.nodeType == 1)) {
+    throw new Error('target must be an Element');
+  }
+
+  this._registerInstance();
+  this._observationTargets.push({element: target, entry: null});
+  this._monitorIntersections(target.ownerDocument);
+  this._checkForIntersections();
+};
+
+
+/**
+ * Stops observing a target element for intersection changes.
+ * @param {Element} target The DOM element to observe.
+ */
+IntersectionObserver.prototype.unobserve = function(target) {
+  this._observationTargets =
+      this._observationTargets.filter(function(item) {
+        return item.element != target;
+      });
+  this._unmonitorIntersections(target.ownerDocument);
+  if (this._observationTargets.length == 0) {
+    this._unregisterInstance();
+  }
+};
+
+
+/**
+ * Stops observing all target elements for intersection changes.
+ */
+IntersectionObserver.prototype.disconnect = function() {
+  this._observationTargets = [];
+  this._unmonitorAllIntersections();
+  this._unregisterInstance();
+};
+
+
+/**
+ * Returns any queue entries that have not yet been reported to the
+ * callback and clears the queue. This can be used in conjunction with the
+ * callback to obtain the absolute most up-to-date intersection information.
+ * @return {Array} The currently queued entries.
+ */
+IntersectionObserver.prototype.takeRecords = function() {
+  var records = this._queuedEntries.slice();
+  this._queuedEntries = [];
+  return records;
+};
+
+
+/**
+ * Accepts the threshold value from the user configuration object and
+ * returns a sorted array of unique threshold values. If a value is not
+ * between 0 and 1 and error is thrown.
+ * @private
+ * @param {Array|number=} opt_threshold An optional threshold value or
+ *     a list of threshold values, defaulting to [0].
+ * @return {Array} A sorted list of unique and valid threshold values.
+ */
+IntersectionObserver.prototype._initThresholds = function(opt_threshold) {
+  var threshold = opt_threshold || [0];
+  if (!Array.isArray(threshold)) threshold = [threshold];
+
+  return threshold.sort().filter(function(t, i, a) {
+    if (typeof t != 'number' || isNaN(t) || t < 0 || t > 1) {
+      throw new Error('threshold must be a number between 0 and 1 inclusively');
+    }
+    return t !== a[i - 1];
+  });
+};
+
+
+/**
+ * Accepts the rootMargin value from the user configuration object
+ * and returns an array of the four margin values as an object containing
+ * the value and unit properties. If any of the values are not properly
+ * formatted or use a unit other than px or %, and error is thrown.
+ * @private
+ * @param {string=} opt_rootMargin An optional rootMargin value,
+ *     defaulting to '0px'.
+ * @return {Array<Object>} An array of margin objects with the keys
+ *     value and unit.
+ */
+IntersectionObserver.prototype._parseRootMargin = function(opt_rootMargin) {
+  var marginString = opt_rootMargin || '0px';
+  var margins = marginString.split(/\s+/).map(function(margin) {
+    var parts = /^(-?\d*\.?\d+)(px|%)$/.exec(margin);
+    if (!parts) {
+      throw new Error('rootMargin must be specified in pixels or percent');
+    }
+    return {value: parseFloat(parts[1]), unit: parts[2]};
+  });
+
+  // Handles shorthand.
+  margins[1] = margins[1] || margins[0];
+  margins[2] = margins[2] || margins[0];
+  margins[3] = margins[3] || margins[1];
+
+  return margins;
+};
+
+
+/**
+ * Starts polling for intersection changes if the polling is not already
+ * happening, and if the page's visibility state is visible.
+ * @param {!Document} doc
+ * @private
+ */
+IntersectionObserver.prototype._monitorIntersections = function(doc) {
+  var win = doc.defaultView;
+  if (!win) {
+    // Already destroyed.
+    return;
+  }
+  if (this._monitoringDocuments.indexOf(doc) != -1) {
+    // Already monitoring.
+    return;
+  }
+
+  // Private state for monitoring.
+  var callback = this._checkForIntersections;
+  var monitoringInterval = null;
+  var domObserver = null;
+
+  // If a poll interval is set, use polling instead of listening to
+  // resize and scroll events or DOM mutations.
+  if (this.POLL_INTERVAL) {
+    monitoringInterval = win.setInterval(callback, this.POLL_INTERVAL);
+  } else {
+    addEvent(win, 'resize', callback, true);
+    addEvent(doc, 'scroll', callback, true);
+    if (this.USE_MUTATION_OBSERVER && 'MutationObserver' in win) {
+      domObserver = new win.MutationObserver(callback);
+      domObserver.observe(doc, {
+        attributes: true,
+        childList: true,
+        characterData: true,
+        subtree: true
+      });
+    }
+  }
+
+  this._monitoringDocuments.push(doc);
+  this._monitoringUnsubscribes.push(function() {
+    // Get the window object again. When a friendly iframe is destroyed, it
+    // will be null.
+    var win = doc.defaultView;
+
+    if (win) {
+      if (monitoringInterval) {
+        win.clearInterval(monitoringInterval);
+      }
+      removeEvent(win, 'resize', callback, true);
+    }
+
+    removeEvent(doc, 'scroll', callback, true);
+    if (domObserver) {
+      domObserver.disconnect();
+    }
+  });
+
+  // Also monitor the parent.
+  var rootDoc =
+    (this.root && (this.root.ownerDocument || this.root)) || document;
+  if (doc != rootDoc) {
+    var frame = getFrameElement(doc);
+    if (frame) {
+      this._monitorIntersections(frame.ownerDocument);
+    }
+  }
+};
+
+
+/**
+ * Stops polling for intersection changes.
+ * @param {!Document} doc
+ * @private
+ */
+IntersectionObserver.prototype._unmonitorIntersections = function(doc) {
+  var index = this._monitoringDocuments.indexOf(doc);
+  if (index == -1) {
+    return;
+  }
+
+  var rootDoc =
+    (this.root && (this.root.ownerDocument || this.root)) || document;
+
+  // Check if any dependent targets are still remaining.
+  var hasDependentTargets =
+      this._observationTargets.some(function(item) {
+        var itemDoc = item.element.ownerDocument;
+        // Target is in this context.
+        if (itemDoc == doc) {
+          return true;
+        }
+        // Target is nested in this context.
+        while (itemDoc && itemDoc != rootDoc) {
+          var frame = getFrameElement(itemDoc);
+          itemDoc = frame && frame.ownerDocument;
+          if (itemDoc == doc) {
+            return true;
+          }
+        }
+        return false;
+      });
+  if (hasDependentTargets) {
+    return;
+  }
+
+  // Unsubscribe.
+  var unsubscribe = this._monitoringUnsubscribes[index];
+  this._monitoringDocuments.splice(index, 1);
+  this._monitoringUnsubscribes.splice(index, 1);
+  unsubscribe();
+
+  // Also unmonitor the parent.
+  if (doc != rootDoc) {
+    var frame = getFrameElement(doc);
+    if (frame) {
+      this._unmonitorIntersections(frame.ownerDocument);
+    }
+  }
+};
+
+
+/**
+ * Stops polling for intersection changes.
+ * @param {!Document} doc
+ * @private
+ */
+IntersectionObserver.prototype._unmonitorAllIntersections = function() {
+  var unsubscribes = this._monitoringUnsubscribes.slice(0);
+  this._monitoringDocuments.length = 0;
+  this._monitoringUnsubscribes.length = 0;
+  for (var i = 0; i < unsubscribes.length; i++) {
+    unsubscribes[i]();
+  }
+};
+
+
+/**
+ * Scans each observation target for intersection changes and adds them
+ * to the internal entries queue. If new entries are found, it
+ * schedules the callback to be invoked.
+ * @private
+ */
+IntersectionObserver.prototype._checkForIntersections = function() {
+  if (!this.root && crossOriginUpdater && !crossOriginRect) {
+    // Cross origin monitoring, but no initial data available yet.
+    return;
+  }
+
+  var rootIsInDom = this._rootIsInDom();
+  var rootRect = rootIsInDom ? this._getRootRect() : getEmptyRect();
+
+  this._observationTargets.forEach(function(item) {
+    var target = item.element;
+    var targetRect = getBoundingClientRect(target);
+    var rootContainsTarget = this._rootContainsTarget(target);
+    var oldEntry = item.entry;
+    var intersectionRect = rootIsInDom && rootContainsTarget &&
+        this._computeTargetAndRootIntersection(target, targetRect, rootRect);
+
+    var rootBounds = null;
+    if (!this._rootContainsTarget(target)) {
+      rootBounds = getEmptyRect();
+    } else if (!crossOriginUpdater || this.root) {
+      rootBounds = rootRect;
+    }
+
+    var newEntry = item.entry = new IntersectionObserverEntry({
+      time: now(),
+      target: target,
+      boundingClientRect: targetRect,
+      rootBounds: rootBounds,
+      intersectionRect: intersectionRect
+    });
+
+    if (!oldEntry) {
+      this._queuedEntries.push(newEntry);
+    } else if (rootIsInDom && rootContainsTarget) {
+      // If the new entry intersection ratio has crossed any of the
+      // thresholds, add a new entry.
+      if (this._hasCrossedThreshold(oldEntry, newEntry)) {
+        this._queuedEntries.push(newEntry);
+      }
+    } else {
+      // If the root is not in the DOM or target is not contained within
+      // root but the previous entry for this target had an intersection,
+      // add a new record indicating removal.
+      if (oldEntry && oldEntry.isIntersecting) {
+        this._queuedEntries.push(newEntry);
+      }
+    }
+  }, this);
+
+  if (this._queuedEntries.length) {
+    this._callback(this.takeRecords(), this);
+  }
+};
+
+
+/**
+ * Accepts a target and root rect computes the intersection between then
+ * following the algorithm in the spec.
+ * TODO(philipwalton): at this time clip-path is not considered.
+ * https://w3c.github.io/IntersectionObserver/#calculate-intersection-rect-algo
+ * @param {Element} target The target DOM element
+ * @param {Object} targetRect The bounding rect of the target.
+ * @param {Object} rootRect The bounding rect of the root after being
+ *     expanded by the rootMargin value.
+ * @return {?Object} The final intersection rect object or undefined if no
+ *     intersection is found.
+ * @private
+ */
+IntersectionObserver.prototype._computeTargetAndRootIntersection =
+    function(target, targetRect, rootRect) {
+  // If the element isn't displayed, an intersection can't happen.
+  if (window.getComputedStyle(target).display == 'none') return;
+
+  var intersectionRect = targetRect;
+  var parent = getParentNode(target);
+  var atRoot = false;
+
+  while (!atRoot && parent) {
+    var parentRect = null;
+    var parentComputedStyle = parent.nodeType == 1 ?
+        window.getComputedStyle(parent) : {};
+
+    // If the parent isn't displayed, an intersection can't happen.
+    if (parentComputedStyle.display == 'none') return null;
+
+    if (parent == this.root || parent.nodeType == /* DOCUMENT */ 9) {
+      atRoot = true;
+      if (parent == this.root || parent == document) {
+        if (crossOriginUpdater && !this.root) {
+          if (!crossOriginRect ||
+              crossOriginRect.width == 0 && crossOriginRect.height == 0) {
+            // A 0-size cross-origin intersection means no-intersection.
+            parent = null;
+            parentRect = null;
+            intersectionRect = null;
+          } else {
+            parentRect = crossOriginRect;
+          }
+        } else {
+          parentRect = rootRect;
+        }
+      } else {
+        // Check if there's a frame that can be navigated to.
+        var frame = getParentNode(parent);
+        var frameRect = frame && getBoundingClientRect(frame);
+        var frameIntersect =
+            frame &&
+            this._computeTargetAndRootIntersection(frame, frameRect, rootRect);
+        if (frameRect && frameIntersect) {
+          parent = frame;
+          parentRect = convertFromParentRect(frameRect, frameIntersect);
+        } else {
+          parent = null;
+          intersectionRect = null;
+        }
+      }
+    } else {
+      // If the element has a non-visible overflow, and it's not the <body>
+      // or <html> element, update the intersection rect.
+      // Note: <body> and <html> cannot be clipped to a rect that's not also
+      // the document rect, so no need to compute a new intersection.
+      var doc = parent.ownerDocument;
+      if (parent != doc.body &&
+          parent != doc.documentElement &&
+          parentComputedStyle.overflow != 'visible') {
+        parentRect = getBoundingClientRect(parent);
+      }
+    }
+
+    // If either of the above conditionals set a new parentRect,
+    // calculate new intersection data.
+    if (parentRect) {
+      intersectionRect = computeRectIntersection(parentRect, intersectionRect);
+    }
+    if (!intersectionRect) break;
+    parent = parent && getParentNode(parent);
+  }
+  return intersectionRect;
+};
+
+
+/**
+ * Returns the root rect after being expanded by the rootMargin value.
+ * @return {ClientRect} The expanded root rect.
+ * @private
+ */
+IntersectionObserver.prototype._getRootRect = function() {
+  var rootRect;
+  if (this.root && !isDoc(this.root)) {
+    rootRect = getBoundingClientRect(this.root);
+  } else {
+    // Use <html>/<body> instead of window since scroll bars affect size.
+    var doc = isDoc(this.root) ? this.root : document;
+    var html = doc.documentElement;
+    var body = doc.body;
+    rootRect = {
+      top: 0,
+      left: 0,
+      right: html.clientWidth || body.clientWidth,
+      width: html.clientWidth || body.clientWidth,
+      bottom: html.clientHeight || body.clientHeight,
+      height: html.clientHeight || body.clientHeight
+    };
+  }
+  return this._expandRectByRootMargin(rootRect);
+};
+
+
+/**
+ * Accepts a rect and expands it by the rootMargin value.
+ * @param {DOMRect|ClientRect} rect The rect object to expand.
+ * @return {ClientRect} The expanded rect.
+ * @private
+ */
+IntersectionObserver.prototype._expandRectByRootMargin = function(rect) {
+  var margins = this._rootMarginValues.map(function(margin, i) {
+    return margin.unit == 'px' ? margin.value :
+        margin.value * (i % 2 ? rect.width : rect.height) / 100;
+  });
+  var newRect = {
+    top: rect.top - margins[0],
+    right: rect.right + margins[1],
+    bottom: rect.bottom + margins[2],
+    left: rect.left - margins[3]
+  };
+  newRect.width = newRect.right - newRect.left;
+  newRect.height = newRect.bottom - newRect.top;
+
+  return newRect;
+};
+
+
+/**
+ * Accepts an old and new entry and returns true if at least one of the
+ * threshold values has been crossed.
+ * @param {?IntersectionObserverEntry} oldEntry The previous entry for a
+ *    particular target element or null if no previous entry exists.
+ * @param {IntersectionObserverEntry} newEntry The current entry for a
+ *    particular target element.
+ * @return {boolean} Returns true if a any threshold has been crossed.
+ * @private
+ */
+IntersectionObserver.prototype._hasCrossedThreshold =
+    function(oldEntry, newEntry) {
+
+  // To make comparing easier, an entry that has a ratio of 0
+  // but does not actually intersect is given a value of -1
+  var oldRatio = oldEntry && oldEntry.isIntersecting ?
+      oldEntry.intersectionRatio || 0 : -1;
+  var newRatio = newEntry.isIntersecting ?
+      newEntry.intersectionRatio || 0 : -1;
+
+  // Ignore unchanged ratios
+  if (oldRatio === newRatio) return;
+
+  for (var i = 0; i < this.thresholds.length; i++) {
+    var threshold = this.thresholds[i];
+
+    // Return true if an entry matches a threshold or if the new ratio
+    // and the old ratio are on the opposite sides of a threshold.
+    if (threshold == oldRatio || threshold == newRatio ||
+        threshold < oldRatio !== threshold < newRatio) {
+      return true;
+    }
+  }
+};
+
+
+/**
+ * Returns whether or not the root element is an element and is in the DOM.
+ * @return {boolean} True if the root element is an element and is in the DOM.
+ * @private
+ */
+IntersectionObserver.prototype._rootIsInDom = function() {
+  return !this.root || containsDeep(document, this.root);
+};
+
+
+/**
+ * Returns whether or not the target element is a child of root.
+ * @param {Element} target The target element to check.
+ * @return {boolean} True if the target element is a child of root.
+ * @private
+ */
+IntersectionObserver.prototype._rootContainsTarget = function(target) {
+  var rootDoc =
+    (this.root && (this.root.ownerDocument || this.root)) || document;
+  return (
+    containsDeep(rootDoc, target) &&
+    (!this.root || rootDoc == target.ownerDocument)
+  );
+};
+
+
+/**
+ * Adds the instance to the global IntersectionObserver registry if it isn't
+ * already present.
+ * @private
+ */
+IntersectionObserver.prototype._registerInstance = function() {
+  if (registry.indexOf(this) < 0) {
+    registry.push(this);
+  }
+};
+
+
+/**
+ * Removes the instance from the global IntersectionObserver registry.
+ * @private
+ */
+IntersectionObserver.prototype._unregisterInstance = function() {
+  var index = registry.indexOf(this);
+  if (index != -1) registry.splice(index, 1);
+};
+
+
+/**
+ * Returns the result of the performance.now() method or null in browsers
+ * that don't support the API.
+ * @return {number} The elapsed time since the page was requested.
+ */
+function now() {
+  return window.performance && performance.now && performance.now();
+}
+
+
+/**
+ * Throttles a function and delays its execution, so it's only called at most
+ * once within a given time period.
+ * @param {Function} fn The function to throttle.
+ * @param {number} timeout The amount of time that must pass before the
+ *     function can be called again.
+ * @return {Function} The throttled function.
+ */
+function throttle(fn, timeout) {
+  var timer = null;
+  return function () {
+    if (!timer) {
+      timer = setTimeout(function() {
+        fn();
+        timer = null;
+      }, timeout);
+    }
+  };
+}
+
+
+/**
+ * Adds an event handler to a DOM node ensuring cross-browser compatibility.
+ * @param {Node} node The DOM node to add the event handler to.
+ * @param {string} event The event name.
+ * @param {Function} fn The event handler to add.
+ * @param {boolean} opt_useCapture Optionally adds the even to the capture
+ *     phase. Note: this only works in modern browsers.
+ */
+function addEvent(node, event, fn, opt_useCapture) {
+  if (typeof node.addEventListener == 'function') {
+    node.addEventListener(event, fn, opt_useCapture || false);
+  }
+  else if (typeof node.attachEvent == 'function') {
+    node.attachEvent('on' + event, fn);
+  }
+}
+
+
+/**
+ * Removes a previously added event handler from a DOM node.
+ * @param {Node} node The DOM node to remove the event handler from.
+ * @param {string} event The event name.
+ * @param {Function} fn The event handler to remove.
+ * @param {boolean} opt_useCapture If the event handler was added with this
+ *     flag set to true, it should be set to true here in order to remove it.
+ */
+function removeEvent(node, event, fn, opt_useCapture) {
+  if (typeof node.removeEventListener == 'function') {
+    node.removeEventListener(event, fn, opt_useCapture || false);
+  }
+  else if (typeof node.detatchEvent == 'function') {
+    node.detatchEvent('on' + event, fn);
+  }
+}
+
+
+/**
+ * Returns the intersection between two rect objects.
+ * @param {Object} rect1 The first rect.
+ * @param {Object} rect2 The second rect.
+ * @return {?Object|?ClientRect} The intersection rect or undefined if no
+ *     intersection is found.
+ */
+function computeRectIntersection(rect1, rect2) {
+  var top = Math.max(rect1.top, rect2.top);
+  var bottom = Math.min(rect1.bottom, rect2.bottom);
+  var left = Math.max(rect1.left, rect2.left);
+  var right = Math.min(rect1.right, rect2.right);
+  var width = right - left;
+  var height = bottom - top;
+
+  return (width >= 0 && height >= 0) && {
+    top: top,
+    bottom: bottom,
+    left: left,
+    right: right,
+    width: width,
+    height: height
+  } || null;
+}
+
+
+/**
+ * Shims the native getBoundingClientRect for compatibility with older IE.
+ * @param {Element} el The element whose bounding rect to get.
+ * @return {DOMRect|ClientRect} The (possibly shimmed) rect of the element.
+ */
+function getBoundingClientRect(el) {
+  var rect;
+
+  try {
+    rect = el.getBoundingClientRect();
+  } catch (err) {
+    // Ignore Windows 7 IE11 "Unspecified error"
+    // https://github.com/w3c/IntersectionObserver/pull/205
+  }
+
+  if (!rect) return getEmptyRect();
+
+  // Older IE
+  if (!(rect.width && rect.height)) {
+    rect = {
+      top: rect.top,
+      right: rect.right,
+      bottom: rect.bottom,
+      left: rect.left,
+      width: rect.right - rect.left,
+      height: rect.bottom - rect.top
+    };
+  }
+  return rect;
+}
+
+
+/**
+ * Returns an empty rect object. An empty rect is returned when an element
+ * is not in the DOM.
+ * @return {ClientRect} The empty rect.
+ */
+function getEmptyRect() {
+  return {
+    top: 0,
+    bottom: 0,
+    left: 0,
+    right: 0,
+    width: 0,
+    height: 0
+  };
+}
+
+
+/**
+ * Ensure that the result has all of the necessary fields of the DOMRect.
+ * Specifically this ensures that `x` and `y` fields are set.
+ *
+ * @param {?DOMRect|?ClientRect} rect
+ * @return {?DOMRect}
+ */
+function ensureDOMRect(rect) {
+  // A `DOMRect` object has `x` and `y` fields.
+  if (!rect || 'x' in rect) {
+    return rect;
+  }
+  // A IE's `ClientRect` type does not have `x` and `y`. The same is the case
+  // for internally calculated Rect objects. For the purposes of
+  // `IntersectionObserver`, it's sufficient to simply mirror `left` and `top`
+  // for these fields.
+  return {
+    top: rect.top,
+    y: rect.top,
+    bottom: rect.bottom,
+    left: rect.left,
+    x: rect.left,
+    right: rect.right,
+    width: rect.width,
+    height: rect.height
+  };
+}
+
+
+/**
+ * Inverts the intersection and bounding rect from the parent (frame) BCR to
+ * the local BCR space.
+ * @param {DOMRect|ClientRect} parentBoundingRect The parent's bound client rect.
+ * @param {DOMRect|ClientRect} parentIntersectionRect The parent's own intersection rect.
+ * @return {ClientRect} The local root bounding rect for the parent's children.
+ */
+function convertFromParentRect(parentBoundingRect, parentIntersectionRect) {
+  var top = parentIntersectionRect.top - parentBoundingRect.top;
+  var left = parentIntersectionRect.left - parentBoundingRect.left;
+  return {
+    top: top,
+    left: left,
+    height: parentIntersectionRect.height,
+    width: parentIntersectionRect.width,
+    bottom: top + parentIntersectionRect.height,
+    right: left + parentIntersectionRect.width
+  };
+}
+
+
+/**
+ * Checks to see if a parent element contains a child element (including inside
+ * shadow DOM).
+ * @param {Node} parent The parent element.
+ * @param {Node} child The child element.
+ * @return {boolean} True if the parent node contains the child node.
+ */
+function containsDeep(parent, child) {
+  var node = child;
+  while (node) {
+    if (node == parent) return true;
+
+    node = getParentNode(node);
+  }
+  return false;
+}
+
+
+/**
+ * Gets the parent node of an element or its host element if the parent node
+ * is a shadow root.
+ * @param {Node} node The node whose parent to get.
+ * @return {Node|null} The parent node or null if no parent exists.
+ */
+function getParentNode(node) {
+  var parent = node.parentNode;
+
+  if (node.nodeType == /* DOCUMENT */ 9 && node != document) {
+    // If this node is a document node, look for the embedding frame.
+    return getFrameElement(node);
+  }
+
+  // If the parent has element that is assigned through shadow root slot
+  if (parent && parent.assignedSlot) {
+    parent = parent.assignedSlot.parentNode
+  }
+
+  if (parent && parent.nodeType == 11 && parent.host) {
+    // If the parent is a shadow root, return the host element.
+    return parent.host;
+  }
+
+  return parent;
+}
+
+/**
+ * Returns true if `node` is a Document.
+ * @param {!Node} node
+ * @returns {boolean}
+ */
+function isDoc(node) {
+  return node && node.nodeType === 9;
+}
+
+
+// Exposes the constructors globally.
+window.IntersectionObserver = IntersectionObserver;
+window.IntersectionObserverEntry = IntersectionObserverEntry;
+
+}());
diff --git a/resources/src/skip-intersectionObserver.js b/resources/src/skip-intersectionObserver.js
new file mode 100644
index 000000000000..e2ba34e5884f
--- /dev/null
+++ b/resources/src/skip-intersectionObserver.js
@@ -0,0 +1 @@
+return 'IntersectionObserver' in window;
\ No newline at end of file