text: /**
   * This work is licensed under the W3C Software and Document License
   * (http://www.w3.org/Consortium/Legal/2015/copyright-software-and-document).
   */
  
  // Convenience function for converting NodeLists.
  /** @type {typeof Array.prototype.slice} */
  const slice = Array.prototype.slice;
  
  /**
   * IE has a non-standard name for "matches".
   * @type {typeof Element.prototype.matches}
   */
  const matches =
      Element.prototype.matches || Element.prototype.msMatchesSelector;
  
  /** @type {string} */
  const _focusableElementsString = ['a[href]',
                                    'area[href]',
                                    'input:not([disabled])',
                                    'select:not([disabled])',
                                    'textarea:not([disabled])',
                                    'button:not([disabled])',
                                    'iframe',
                                    'object',
                                    'embed',
                                    '[contenteditable]'].join(',');
  
  /**
   * `InertRoot` manages a single inert subtree, i.e. a DOM subtree whose root element has an `inert`
   * attribute.
   *
   * Its main functions are:
   *
   * - to create and maintain a set of managed `InertNode`s, including when mutations occur in the
   *   subtree. The `makeSubtreeUnfocusable()` method handles collecting `InertNode`s via registering
   *   each focusable node in the subtree with the singleton `InertManager` which manages all known
   *   focusable nodes within inert subtrees. `InertManager` ensures that a single `InertNode`
   *   instance exists for each focusable node which has at least one inert root as an ancestor.
   *
   * - to notify all managed `InertNode`s when this subtree stops being inert (i.e. when the `inert`
   *   attribute is removed from the root node). This is handled in the destructor, which calls the
   *   `deregister` method on `InertManager` for each managed inert node.
   */
  class InertRoot {
    /**
     * @param {!Element} rootElement The Element at the root of the inert subtree.
     * @param {!InertManager} inertManager The global singleton InertManager object.
     */
    constructor(rootElement, inertManager) {
      /** @type {!InertManager} */
      this._inertManager = inertManager;
  
      /** @type {!Element} */
      this._rootElement = rootElement;
  
      /**
       * @type {!Set<!InertNode>}
       * All managed focusable nodes in this InertRoot's subtree.
       */
      this._managedNodes = new Set();
  
      // Make the subtree hidden from assistive technology
      if (this._rootElement.hasAttribute('aria-hidden')) {
        /** @type {?string} */
        this._savedAriaHidden = this._rootElement.getAttribute('aria-hidden');
      } else {
        this._savedAriaHidden = null;
      }
      this._rootElement.setAttribute('aria-hidden', 'true');
  
      // Make all focusable elements in the subtree unfocusable and add them to _managedNodes
      this._makeSubtreeUnfocusable(this._rootElement);
  
      // Watch for:
      // - any additions in the subtree: make them unfocusable too
      // - any removals from the subtree: remove them from this inert root's managed nodes
      // - attribute changes: if `tabindex` is added, or removed from an intrinsically focusable
      //   element, make that node a managed node.
      this._observer = new MutationObserver(this._onMutation.bind(this));
      this._observer.observe(this._rootElement, {attributes: true, childList: true, subtree: true});
    }
  
    /**
     * Call this whenever this object is about to become obsolete.  This unwinds all of the state
     * stored in this object and updates the state of all of the managed nodes.
     */
    destructor() {
      this._observer.disconnect();
  
      if (this._rootElement) {
        if (this._savedAriaHidden !== null) {
          this._rootElement.setAttribute('aria-hidden', this._savedAriaHidden);
        } else {
          this._rootElement.removeAttribute('aria-hidden');
        }
      }
  
      this._managedNodes.forEach(function(inertNode) {
        this._unmanageNode(inertNode.node);
      }, this);
  
      // Note we cast the nulls to the ANY type here because:
      // 1) We want the class properties to be declared as non-null, or else we
      //    need even more casts throughout this code. All bets are off if an
      //    instance has been destroyed and a method is called.
      // 2) We don't want to cast "this", because we want type-aware optimizations
      //    to know which properties we're setting.
      this._observer = /** @type {?} */ (null);
      this._rootElement = /** @type {?} */ (null);
      this._managedNodes = /** @type {?} */ (null);
      this._inertManager = /** @type {?} */ (null);
    }
  
    /**
     * @return {!Set<!InertNode>} A copy of this InertRoot's managed nodes set.
     */
    get managedNodes() {
      return new Set(this._managedNodes);
    }
  
    /** @return {boolean} */
    get hasSavedAriaHidden() {
      return this._savedAriaHidden !== null;
    }
  
    /** @param {?string} ariaHidden */
    set savedAriaHidden(ariaHidden) {
      this._savedAriaHidden = ariaHidden;
    }
  
    /** @return {?string} */
    get savedAriaHidden() {
      return this._savedAriaHidden;
    }
  
    /**
     * @param {!Node} startNode
     */
    _makeSubtreeUnfocusable(startNode) {
      composedTreeWalk(startNode, (node) => this._visitNode(node));
  
      let activeElement = document.activeElement;
  
      if (!document.body.contains(startNode)) {
        // startNode may be in shadow DOM, so find its nearest shadowRoot to get the activeElement.
        let node = startNode;
        /** @type {!ShadowRoot|undefined} */
        let root = undefined;
        while (node) {
          if (node.nodeType === Node.DOCUMENT_FRAGMENT_NODE) {
            root = /** @type {!ShadowRoot} */ (node);
            break;
          }
          node = node.parentNode;
        }
        if (root) {
          activeElement = root.activeElement;
        }
      }
      if (startNode.contains(activeElement)) {
        activeElement.blur();
        // In IE11, if an element is already focused, and then set to tabindex=-1
        // calling blur() will not actually move the focus.
        // To work around this we call focus() on the body instead.
        if (activeElement === document.activeElement) {
          document.body.focus();
        }
      }
    }
  
    /**
     * @param {!Node} node
     */
    _visitNode(node) {
      if (node.nodeType !== Node.ELEMENT_NODE) {
        return;
      }
      const element = /** @type {!Element} */ (node);
  
      // If a descendant inert root becomes un-inert, its descendants will still be inert because of
      // this inert root, so all of its managed nodes need to be adopted by this InertRoot.
      if (element !== this._rootElement && element.hasAttribute('inert')) {
        this._adoptInertRoot(element);
      }
  
      if (matches.call(element, _focusableElementsString) || element.hasAttribute('tabindex')) {
        this._manageNode(element);
      }
    }
  
    /**
     * Register the given node with this InertRoot and with InertManager.
     * @param {!Node} node
     */
    _manageNode(node) {
      const inertNode = this._inertManager.register(node, this);
      this._managedNodes.add(inertNode);
    }
  
    /**
     * Unregister the given node with this InertRoot and with InertManager.
     * @param {!Node} node
     */
    _unmanageNode(node) {
      const inertNode = this._inertManager.deregister(node, this);
      if (inertNode) {
        this._managedNodes.delete(inertNode);
      }
    }
  
    /**
     * Unregister the entire subtree starting at `startNode`.
     * @param {!Node} startNode
     */
    _unmanageSubtree(startNode) {
      composedTreeWalk(startNode, (node) => this._unmanageNode(node));
    }
  
    /**
     * If a descendant node is found with an `inert` attribute, adopt its managed nodes.
     * @param {!Element} node
     */
    _adoptInertRoot(node) {
      let inertSubroot = this._inertManager.getInertRoot(node);
  
      // During initialisation this inert root may not have been registered yet,
      // so register it now if need be.
      if (!inertSubroot) {
        this._inertManager.setInert(node, true);
        inertSubroot = this._inertManager.getInertRoot(node);
      }
  
      inertSubroot.managedNodes.forEach(function(savedInertNode) {
        this._manageNode(savedInertNode.node);
      }, this);
    }
  
    /**
     * Callback used when mutation observer detects subtree additions, removals, or attribute changes.
     * @param {!Array<!MutationRecord>} records
     * @param {!MutationObserver} self
     */
    _onMutation(records, self) {
      records.forEach(function(record) {
        const target = /** @type {!Element} */ (record.target);
        if (record.type === 'childList') {
          // Manage added nodes
          slice.call(record.addedNodes).forEach(function(node) {
            this._makeSubtreeUnfocusable(node);
          }, this);
  
          // Un-manage removed nodes
          slice.call(record.removedNodes).forEach(function(node) {
            this._unmanageSubtree(node);
          }, this);
        } else if (record.type === 'attributes') {
          if (record.attributeName === 'tabindex') {
            // Re-initialise inert node if tabindex changes
            this._manageNode(target);
          } else if (target !== this._rootElement &&
                     record.attributeName === 'inert' &&
                     target.hasAttribute('inert')) {
            // If a new inert root is added, adopt its managed nodes and make sure it knows about the
            // already managed nodes from this inert subroot.
            this._adoptInertRoot(target);
            const inertSubroot = this._inertManager.getInertRoot(target);
            this._managedNodes.forEach(function(managedNode) {
              if (target.contains(managedNode.node)) {
                inertSubroot._manageNode(managedNode.node);
              }
            });
          }
        }
      }, this);
    }
  }
  
  /**
   * `InertNode` initialises and manages a single inert node.
   * A node is inert if it is a descendant of one or more inert root elements.
   *
   * On construction, `InertNode` saves the existing `tabindex` value for the node, if any, and
   * either removes the `tabindex` attribute or sets it to `-1`, depending on whether the element
   * is intrinsically focusable or not.
   *
   * `InertNode` maintains a set of `InertRoot`s which are descendants of this `InertNode`. When an
   * `InertRoot` is destroyed, and calls `InertManager.deregister()`, the `InertManager` notifies the
   * `InertNode` via `removeInertRoot()`, which in turn destroys the `InertNode` if no `InertRoot`s
   * remain in the set. On destruction, `InertNode` reinstates the stored `tabindex` if one exists,
   * or removes the `tabindex` attribute if the element is intrinsically focusable.
   */
  class InertNode {
    /**
     * @param {!Node} node A focusable element to be made inert.
     * @param {!InertRoot} inertRoot The inert root element associated with this inert node.
     */
    constructor(node, inertRoot) {
      /** @type {!Node} */
      this._node = node;
  
      /** @type {boolean} */
      this._overrodeFocusMethod = false;
  
      /**
       * @type {!Set<!InertRoot>} The set of descendant inert roots.
       *    If and only if this set becomes empty, this node is no longer inert.
       */
      this._inertRoots = new Set([inertRoot]);
  
      /** @type {?number} */
      this._savedTabIndex = null;
  
      /** @type {boolean} */
      this._destroyed = false;
  
      // Save any prior tabindex info and make this node untabbable
      this.ensureUntabbable();
    }
  
    /**
     * Call this whenever this object is about to become obsolete.
     * This makes the managed node focusable again and deletes all of the previously stored state.
     */
    destructor() {
      this._throwIfDestroyed();
  
      if (this._node && this._node.nodeType === Node.ELEMENT_NODE) {
        const element = /** @type {!Element} */ (this._node);
        if (this._savedTabIndex !== null) {
          element.setAttribute('tabindex', this._savedTabIndex);
        } else {
          element.removeAttribute('tabindex');
        }
  
        // Use `delete` to restore native focus method.
        if (this._overrodeFocusMethod) {
          delete element.focus;
        }
      }
  
      // See note in InertRoot.destructor for why we cast these nulls to ANY.
      this._node = /** @type {?} */ (null);
      this._inertRoots = /** @type {?} */ (null);
      this._destroyed = true;
    }
  
    /**
     * @type {boolean} Whether this object is obsolete because the managed node is no longer inert.
     * If the object has been destroyed, any attempt to access it will cause an exception.
     */
    get destroyed() {
      return /** @type {!InertNode} */ (this)._destroyed;
    }
  
    /**
     * Throw if user tries to access destroyed InertNode.
     */
    _throwIfDestroyed() {
      if (this.destroyed) {
        throw new Error('Trying to access destroyed InertNode');
      }
    }
  
    /** @return {boolean} */
    get hasSavedTabIndex() {
      return this._savedTabIndex !== null;
    }
  
    /** @return {!Node} */
    get node() {
      this._throwIfDestroyed();
      return this._node;
    }
  
    /** @param {?number} tabIndex */
    set savedTabIndex(tabIndex) {
      this._throwIfDestroyed();
      this._savedTabIndex = tabIndex;
    }
  
    /** @return {?number} */
    get savedTabIndex() {
      this._throwIfDestroyed();
      return this._savedTabIndex;
    }
  
    /** Save the existing tabindex value and make the node untabbable and unfocusable */
    ensureUntabbable() {
      if (this.node.nodeType !== Node.ELEMENT_NODE) {
        return;
      }
      const element = /** @type {!Element} */ (this.node);
      if (matches.call(element, _focusableElementsString)) {
        if (/** @type {!HTMLElement} */ (element).tabIndex === -1 &&
            this.hasSavedTabIndex) {
          return;
        }
  
        if (element.hasAttribute('tabindex')) {
          this._savedTabIndex = /** @type {!HTMLElement} */ (element).tabIndex;
        }
        element.setAttribute('tabindex', '-1');
        if (element.nodeType === Node.ELEMENT_NODE) {
          element.focus = function() {};
          this._overrodeFocusMethod = true;
        }
      } else if (element.hasAttribute('tabindex')) {
        this._savedTabIndex = /** @type {!HTMLElement} */ (element).tabIndex;
        element.removeAttribute('tabindex');
      }
    }
  
    /**
     * Add another inert root to this inert node's set of managing inert roots.
     * @param {!InertRoot} inertRoot
     */
    addInertRoot(inertRoot) {
      this._throwIfDestroyed();
      this._inertRoots.add(inertRoot);
    }
  
    /**
     * Remove the given inert root from this inert node's set of managing inert roots.
     * If the set of managing inert roots becomes empty, this node is no longer inert,
     * so the object should be destroyed.
     * @param {!InertRoot} inertRoot
     */
    removeInertRoot(inertRoot) {
      this._throwIfDestroyed();
      this._inertRoots.delete(inertRoot);
      if (this._inertRoots.size === 0) {
        this.destructor();
      }
    }
  }
  
  /**
   * InertManager is a per-document singleton object which manages all inert roots and nodes.
   *
   * When an element becomes an inert root by having an `inert` attribute set and/or its `inert`
   * property set to `true`, the `setInert` method creates an `InertRoot` object for the element.
   * The `InertRoot` in turn registers itself as managing all of the element's focusable descendant
   * nodes via the `register()` method. The `InertManager` ensures that a single `InertNode` instance
   * is created for each such node, via the `_managedNodes` map.
   */
  class InertManager {
    /**
     * @param {!Document} document
     */
    constructor(document) {
      if (!document) {
        throw new Error('Missing required argument; InertManager needs to wrap a document.');
      }
  
      /** @type {!Document} */
      this._document = document;
  
      /**
       * All managed nodes known to this InertManager. In a map to allow looking up by Node.
       * @type {!Map<!Node, !InertNode>}
       */
      this._managedNodes = new Map();
  
      /**
       * All inert roots known to this InertManager. In a map to allow looking up by Node.
       * @type {!Map<!Node, !InertRoot>}
       */
      this._inertRoots = new Map();
  
      /**
       * Observer for mutations on `document.body`.
       * @type {!MutationObserver}
       */
      this._observer = new MutationObserver(this._watchForInert.bind(this));
  
      // Add inert style.
      addInertStyle(document.head || document.body || document.documentElement);
  
      // Wait for document to be loaded.
      if (document.readyState === 'loading') {
        document.addEventListener('DOMContentLoaded', this._onDocumentLoaded.bind(this));
      } else {
        this._onDocumentLoaded();
      }
    }
  
    /**
     * Set whether the given element should be an inert root or not.
     * @param {!Element} root
     * @param {boolean} inert
     */
    setInert(root, inert) {
      if (inert) {
        if (this._inertRoots.has(root)) { // element is already inert
          return;
        }
  
        const inertRoot = new InertRoot(root, this);
        root.setAttribute('inert', '');
        this._inertRoots.set(root, inertRoot);
        // If not contained in the document, it must be in a shadowRoot.
        // Ensure inert styles are added there.
        if (!this._document.body.contains(root)) {
          let parent = root.parentNode;
          while (parent) {
            if (parent.nodeType === 11) {
              addInertStyle(parent);
            }
            parent = parent.parentNode;
          }
        }
      } else {
        if (!this._inertRoots.has(root)) { // element is already non-inert
          return;
        }
  
        const inertRoot = this._inertRoots.get(root);
        inertRoot.destructor();
        this._inertRoots.delete(root);
        root.removeAttribute('inert');
      }
    }
  
    /**
     * Get the InertRoot object corresponding to the given inert root element, if any.
     * @param {!Node} element
     * @return {!InertRoot|undefined}
     */
    getInertRoot(element) {
      return this._inertRoots.get(element);
    }
  
    /**
     * Register the given InertRoot as managing the given node.
     * In the case where the node has a previously existing inert root, this inert root will
     * be added to its set of inert roots.
     * @param {!Node} node
     * @param {!InertRoot} inertRoot
     * @return {!InertNode} inertNode
     */
    register(node, inertRoot) {
      let inertNode = this._managedNodes.get(node);
      if (inertNode !== undefined) { // node was already in an inert subtree
        inertNode.addInertRoot(inertRoot);
      } else {
        inertNode = new InertNode(node, inertRoot);
      }
  
      this._managedNodes.set(node, inertNode);
  
      return inertNode;
    }
  
    /**
     * De-register the given InertRoot as managing the given inert node.
     * Removes the inert root from the InertNode's set of managing inert roots, and remove the inert
     * node from the InertManager's set of managed nodes if it is destroyed.
     * If the node is not currently managed, this is essentially a no-op.
     * @param {!Node} node
     * @param {!InertRoot} inertRoot
     * @return {?InertNode} The potentially destroyed InertNode associated with this node, if any.
     */
    deregister(node, inertRoot) {
      const inertNode = this._managedNodes.get(node);
      if (!inertNode) {
        return null;
      }
  
      inertNode.removeInertRoot(inertRoot);
      if (inertNode.destroyed) {
        this._managedNodes.delete(node);
      }
  
      return inertNode;
    }
  
    /**
     * Callback used when document has finished loading.
     */
    _onDocumentLoaded() {
      // Find all inert roots in document and make them actually inert.
      const inertElements = slice.call(this._document.querySelectorAll('[inert]'));
      inertElements.forEach(function(inertElement) {
        this.setInert(inertElement, true);
      }, this);
  
      // Comment this out to use programmatic API only.
      this._observer.observe(this._document.body, {attributes: true, subtree: true, childList: true});
    }
  
    /**
     * Callback used when mutation observer detects attribute changes.
     * @param {!Array<!MutationRecord>} records
     * @param {!MutationObserver} self
     */
    _watchForInert(records, self) {
      const _this = this;
      records.forEach(function(record) {
        switch (record.type) {
        case 'childList':
          slice.call(record.addedNodes).forEach(function(node) {
            if (node.nodeType !== Node.ELEMENT_NODE) {
              return;
            }
            const inertElements = slice.call(node.querySelectorAll('[inert]'));
            if (matches.call(node, '[inert]')) {
              inertElements.unshift(node);
            }
            inertElements.forEach(function(inertElement) {
              this.setInert(inertElement, true);
            }, _this);
          }, _this);
          break;
        case 'attributes':
          if (record.attributeName !== 'inert') {
            return;
          }
          const target = /** @type {!Element} */ (record.target);
          const inert = target.hasAttribute('inert');
          _this.setInert(target, inert);
          break;
        }
      }, this);
    }
  }
  
  /**
   * Recursively walk the composed tree from |node|.
   * @param {!Node} node
   * @param {(function (!Element))=} callback Callback to be called for each element traversed,
   *     before descending into child nodes.
   * @param {?ShadowRoot=} shadowRootAncestor The nearest ShadowRoot ancestor, if any.
   */
  function composedTreeWalk(node, callback, shadowRootAncestor) {
    if (node.nodeType == Node.ELEMENT_NODE) {
      const element = /** @type {!Element} */ (node);
      if (callback) {
        callback(element);
      }
  
      // Descend into node:
      // If it has a ShadowRoot, ignore all child elements - these will be picked
      // up by the <content> or <shadow> elements. Descend straight into the
      // ShadowRoot.
      const shadowRoot = /** @type {!HTMLElement} */ (element).shadowRoot;
      if (shadowRoot) {
        composedTreeWalk(shadowRoot, callback, shadowRoot);
        return;
      }
  
      // If it is a <content> element, descend into distributed elements - these
      // are elements from outside the shadow root which are rendered inside the
      // shadow DOM.
      if (element.localName == 'content') {
        const content = /** @type {!HTMLContentElement} */ (element);
        // Verifies if ShadowDom v0 is supported.
        const distributedNodes = content.getDistributedNodes ?
          content.getDistributedNodes() : [];
        for (let i = 0; i < distributedNodes.length; i++) {
          composedTreeWalk(distributedNodes[i], callback, shadowRootAncestor);
        }
        return;
      }
  
      // If it is a <slot> element, descend into assigned nodes - these
      // are elements from outside the shadow root which are rendered inside the
      // shadow DOM.
      if (element.localName == 'slot') {
        const slot = /** @type {!HTMLSlotElement} */ (element);
        // Verify if ShadowDom v1 is supported.
        const distributedNodes = slot.assignedNodes ?
          slot.assignedNodes({flatten: true}) : [];
        for (let i = 0; i < distributedNodes.length; i++) {
          composedTreeWalk(distributedNodes[i], callback, shadowRootAncestor);
        }
        return;
      }
    }
  
    // If it is neither the parent of a ShadowRoot, a <content> element, a <slot>
    // element, nor a <shadow> element recurse normally.
    let child = node.firstChild;
    while (child != null) {
      composedTreeWalk(child, callback, shadowRootAncestor);
      child = child.nextSibling;
    }
  }
  
  /**
   * Adds a style element to the node containing the inert specific styles
   * @param {!Node} node
   */
  function addInertStyle(node) {
    if (node.querySelector('style#inert-style')) {
      return;
    }
    const style = document.createElement('style');
    style.setAttribute('id', 'inert-style');
    style.textContent = '\n'+
                        '[inert] {\n' +
                        '  pointer-events: none;\n' +
                        '  cursor: default;\n' +
                        '}\n' +
                        '\n' +
                        '[inert], [inert] * {\n' +
                        '  user-select: none;\n' +
                        '  -webkit-user-select: none;\n' +
                        '  -moz-user-select: none;\n' +
                        '  -ms-user-select: none;\n' +
                        '}\n';
    node.appendChild(style);
  }
  
  /** @type {!InertManager} */
  const inertManager = new InertManager(document);
  
  if (!Element.prototype.hasOwnProperty('inert')) {
    Object.defineProperty(Element.prototype, 'inert', {
      enumerable: true,
      /** @this {!Element} */
      get: function() {
        return this.hasAttribute('inert');
      },
      /** @this {!Element} */
      set: function(inert) {
        inertManager.setInert(this, inert);
      },
    });
  }
  