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/**
 * 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;
    }

    /**
     * A local reference to the document.
     */
    var document = 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) {
            throw new Error('root must be an 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.
        if (doc != (this.root && this.root.ownerDocument || document)) {
            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 || 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 newEntry = item.entry = new IntersectionObserverEntry({
                time: now(),
                target: target,
                boundingClientRect: targetRect,
                rootBounds: crossOriginUpdater && !this.root ? null : rootRect,
                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) {
            rootRect = getBoundingClientRect(this.root);
        } else {
            // Use <html>/<body> instead of window since scroll bars affect size.
            var html = document.documentElement;
            var body = document.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) {
        return containsDeep(this.root || document, target) &&
            (!this.root || this.root.ownerDocument == 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 (parent && parent.nodeType == 11 && parent.host) {
            // If the parent is a shadow root, return the host element.
            return parent.host;
        }

        if (parent && parent.assignedSlot) {
            // If the parent is distributed in a <slot>, return the parent of a slot.
            return parent.assignedSlot.parentNode;
        }

        return parent;
    }

    /**
     * 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;
        }
    }

// Exposes the constructors globally.
    window.IntersectionObserver = IntersectionObserver;
    window.IntersectionObserverEntry = IntersectionObserverEntry;

}());

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