text: (function (global, factory) {
    typeof exports === 'object' && typeof module !== 'undefined' ? module.exports = factory() :
    typeof define === 'function' && define.amd ? define(factory) :
    (global = global || self, global.dialogPolyfill = factory());
  }(this, function () { 'use strict';
  
    // nb. This is for IE10 and lower _only_.
    var supportCustomEvent = window.CustomEvent;
    if (!supportCustomEvent || typeof supportCustomEvent === 'object') {
      supportCustomEvent = function CustomEvent(event, x) {
        x = x || {};
        var ev = document.createEvent('CustomEvent');
        ev.initCustomEvent(event, !!x.bubbles, !!x.cancelable, x.detail || null);
        return ev;
      };
      supportCustomEvent.prototype = window.Event.prototype;
    }
  
    /**
     * Dispatches the passed event to both an "on<type>" handler as well as via the
     * normal dispatch operation. Does not bubble.
     *
     * @param {!EventTarget} target
     * @param {!Event} event
     * @return {boolean}
     */
    function safeDispatchEvent(target, event) {
      var check = 'on' + event.type.toLowerCase();
      if (typeof target[check] === 'function') {
        target[check](event);
      }
      return target.dispatchEvent(event);
    }
  
    /**
     * @param {Element} el to check for stacking context
     * @return {boolean} whether this el or its parents creates a stacking context
     */
    function createsStackingContext(el) {
      while (el && el !== document.body) {
        var s = window.getComputedStyle(el);
        var invalid = function(k, ok) {
          return !(s[k] === undefined || s[k] === ok);
        };
  
        if (s.opacity < 1 ||
            invalid('zIndex', 'auto') ||
            invalid('transform', 'none') ||
            invalid('mixBlendMode', 'normal') ||
            invalid('filter', 'none') ||
            invalid('perspective', 'none') ||
            s['isolation'] === 'isolate' ||
            s.position === 'fixed' ||
            s.webkitOverflowScrolling === 'touch') {
          return true;
        }
        el = el.parentElement;
      }
      return false;
    }
  
    /**
     * Finds the nearest <dialog> from the passed element.
     *
     * @param {Element} el to search from
     * @return {HTMLDialogElement} dialog found
     */
    function findNearestDialog(el) {
      while (el) {
        if (el.localName === 'dialog' || el.classList.contains('dialog')) {
          return /** @type {HTMLDialogElement} */ (el);
        }
        if (el.parentElement) {
          el = el.parentElement;
        } else if (el.parentNode) {
          el = el.parentNode.host;
        } else {
          el = null;
        }
      }
      return null;
    }
  
    /**
     * Blur the specified element, as long as it's not the HTML body element.
     * This works around an IE9/10 bug - blurring the body causes Windows to
     * blur the whole application.
     *
     * @param {Element} el to blur
     */
    function safeBlur(el) {
      // Find the actual focused element when the active element is inside a shadow root
      while (el && el.shadowRoot && el.shadowRoot.activeElement) {
        el = el.shadowRoot.activeElement;
      }
  
      if (el && el.blur && el !== document.body) {
        el.blur();
      }
    }
  
    /**
     * @param {!NodeList} nodeList to search
     * @param {Node} node to find
     * @return {boolean} whether node is inside nodeList
     */
    function inNodeList(nodeList, node) {
      for (var i = 0; i < nodeList.length; ++i) {
        if (nodeList[i] === node) {
          return true;
        }
      }
      return false;
    }
  
    /**
     * @param {HTMLFormElement} el to check
     * @return {boolean} whether this form has method="dialog"
     */
    function isFormMethodDialog(el) {
      if (!el || !el.hasAttribute('method')) {
        return false;
      }
      return el.getAttribute('method').toLowerCase() === 'dialog';
    }
  
    /**
     * @param {!DocumentFragment|!Element} hostElement
     * @return {?Element}
     */
    function findFocusableElementWithin(hostElement) {
      // Note that this is 'any focusable area'. This list is probably not exhaustive, but the
      // alternative involves stepping through and trying to focus everything.
      var opts = ['button', 'input', 'keygen', 'select', 'textarea'];
      var query = opts.map(function(el) {
        return el + ':not([disabled])';
      });
      // TODO(samthor): tabindex values that are not numeric are not focusable.
      query.push('[tabindex]:not([disabled]):not([tabindex=""])');  // tabindex != "", not disabled
      var target = hostElement.querySelector(query.join(', '));
  
      if (!target && 'attachShadow' in Element.prototype) {
        // If we haven't found a focusable target, see if the host element contains an element
        // which has a shadowRoot.
        // Recursively search for the first focusable item in shadow roots.
        var elems = hostElement.querySelectorAll('*');
        for (var i = 0; i < elems.length; i++) {
          if (elems[i].tagName && elems[i].shadowRoot) {
            target = findFocusableElementWithin(elems[i].shadowRoot);
            if (target) {
              break;
            }
          }
        }
      }
      return target;
    }
  
    /**
     * Determines if an element is attached to the DOM.
     * @param {Element} element to check
     * @return {boolean} whether the element is in DOM
     */
    function isConnected(element) {
      return element.isConnected || document.body.contains(element);
    }
  
    /**
     * @param {!Event} event
     * @return {?Element}
     */
    function findFormSubmitter(event) {
      if (event.submitter) {
        return event.submitter;
      }
  
      var form = event.target;
      if (!(form instanceof HTMLFormElement)) {
        return null;
      }
  
      var submitter = dialogPolyfill.formSubmitter;
      if (!submitter) {
        var target = event.target;
        var root = ('getRootNode' in target && target.getRootNode() || document);
        submitter = root.activeElement;
      }
  
      if (!submitter || submitter.form !== form) {
        return null;
      }
      return submitter;
    }
  
    /**
     * @param {!Event} event
     */
    function maybeHandleSubmit(event) {
      if (event.defaultPrevented) {
        return;
      }
      var form = /** @type {!HTMLFormElement} */ (event.target);
  
      // We'd have a value if we clicked on an imagemap.
      var value = dialogPolyfill.imagemapUseValue;
      var submitter = findFormSubmitter(event);
      if (value === null && submitter) {
        value = submitter.value;
      }
  
      // There should always be a dialog as this handler is added specifically on them, but check just
      // in case.
      var dialog = findNearestDialog(form);
      if (!dialog) {
        return;
      }
  
      // Prefer formmethod on the button.
      var formmethod = submitter && submitter.getAttribute('formmethod') || form.getAttribute('method');
      if (formmethod !== 'dialog') {
        return;
      }
      event.preventDefault();
  
      if (value != null) {
        // nb. we explicitly check against null/undefined
        dialog.close(value);
      } else {
        dialog.close();
      }
    }
  
    /**
     * @param {!HTMLDialogElement} dialog to upgrade
     * @constructor
     */
    function dialogPolyfillInfo(dialog) {
      this.dialog_ = dialog;
      this.replacedStyleTop_ = false;
      this.openAsModal_ = false;
  
      // Set a11y role. Browsers that support dialog implicitly know this already.
      if (!dialog.hasAttribute('role')) {
        dialog.setAttribute('role', 'dialog');
      }
  
      dialog.show = this.show.bind(this);
      dialog.showModal = this.showModal.bind(this);
      dialog.close = this.close.bind(this);
  
      dialog.addEventListener('submit', maybeHandleSubmit, false);
  
      if (!('returnValue' in dialog)) {
        dialog.returnValue = '';
      }
  
      if ('MutationObserver' in window) {
        var mo = new MutationObserver(this.maybeHideModal.bind(this));
        mo.observe(dialog, {attributes: true, attributeFilter: ['open']});
      } else {
        // IE10 and below support. Note that DOMNodeRemoved etc fire _before_ removal. They also
        // seem to fire even if the element was removed as part of a parent removal. Use the removed
        // events to force downgrade (useful if removed/immediately added).
        var removed = false;
        var cb = function() {
          removed ? this.downgradeModal() : this.maybeHideModal();
          removed = false;
        }.bind(this);
        var timeout;
        var delayModel = function(ev) {
          if (ev.target !== dialog) { return; }  // not for a child element
          var cand = 'DOMNodeRemoved';
          removed |= (ev.type.substr(0, cand.length) === cand);
          window.clearTimeout(timeout);
          timeout = window.setTimeout(cb, 0);
        };
        ['DOMAttrModified', 'DOMNodeRemoved', 'DOMNodeRemovedFromDocument'].forEach(function(name) {
          dialog.addEventListener(name, delayModel);
        });
      }
      // Note that the DOM is observed inside DialogManager while any dialog
      // is being displayed as a modal, to catch modal removal from the DOM.
  
      Object.defineProperty(dialog, 'open', {
        set: this.setOpen.bind(this),
        get: dialog.hasAttribute.bind(dialog, 'open')
      });
  
      this.backdrop_ = document.createElement('div');
      this.backdrop_.className = 'backdrop';
      this.backdrop_.addEventListener('mouseup'  , this.backdropMouseEvent_.bind(this));
      this.backdrop_.addEventListener('mousedown', this.backdropMouseEvent_.bind(this));
      this.backdrop_.addEventListener('click'    , this.backdropMouseEvent_.bind(this));
    }
  
    dialogPolyfillInfo.prototype = /** @type {HTMLDialogElement.prototype} */ ({
  
      get dialog() {
        return this.dialog_;
      },
  
      /**
       * Maybe remove this dialog from the modal top layer. This is called when
       * a modal dialog may no longer be tenable, e.g., when the dialog is no
       * longer open or is no longer part of the DOM.
       */
      maybeHideModal: function() {
        if (this.dialog_.hasAttribute('open') && isConnected(this.dialog_)) { return; }
        this.downgradeModal();
      },
  
      /**
       * Remove this dialog from the modal top layer, leaving it as a non-modal.
       */
      downgradeModal: function() {
        if (!this.openAsModal_) { return; }
        this.openAsModal_ = false;
        this.dialog_.style.zIndex = '';
  
        // This won't match the native <dialog> exactly because if the user set top on a centered
        // polyfill dialog, that top gets thrown away when the dialog is closed. Not sure it's
        // possible to polyfill this perfectly.
        if (this.replacedStyleTop_) {
          this.dialog_.style.top = '';
          this.replacedStyleTop_ = false;
        }
  
        // Clear the backdrop and remove from the manager.
        this.backdrop_.parentNode && this.backdrop_.parentNode.removeChild(this.backdrop_);
        dialogPolyfill.dm.removeDialog(this);
      },
  
      /**
       * @param {boolean} value whether to open or close this dialog
       */
      setOpen: function(value) {
        if (value) {
          this.dialog_.hasAttribute('open') || this.dialog_.setAttribute('open', '');
        } else {
          this.dialog_.removeAttribute('open');
          this.maybeHideModal();  // nb. redundant with MutationObserver
        }
      },
  
      /**
       * Handles mouse events ('mouseup', 'mousedown', 'click') on the fake .backdrop element, redirecting them as if
       * they were on the dialog itself.
       *
       * @param {!Event} e to redirect
       */
      backdropMouseEvent_: function(e) {
        if (!this.dialog_.hasAttribute('tabindex')) {
          // Clicking on the backdrop should move the implicit cursor, even if dialog cannot be
          // focused. Create a fake thing to focus on. If the backdrop was _before_ the dialog, this
          // would not be needed - clicks would move the implicit cursor there.
          var fake = document.createElement('div');
          this.dialog_.insertBefore(fake, this.dialog_.firstChild);
          fake.tabIndex = -1;
          fake.focus();
          this.dialog_.removeChild(fake);
        } else {
          this.dialog_.focus();
        }
  
        var redirectedEvent = document.createEvent('MouseEvents');
        redirectedEvent.initMouseEvent(e.type, e.bubbles, e.cancelable, window,
            e.detail, e.screenX, e.screenY, e.clientX, e.clientY, e.ctrlKey,
            e.altKey, e.shiftKey, e.metaKey, e.button, e.relatedTarget);
        this.dialog_.dispatchEvent(redirectedEvent);
        e.stopPropagation();
      },
  
      /**
       * Focuses on the first focusable element within the dialog. This will always blur the current
       * focus, even if nothing within the dialog is found.
       */
      focus_: function() {
        // Find element with `autofocus` attribute, or fall back to the first form/tabindex control.
        var target = this.dialog_.querySelector('[autofocus]:not([disabled])');
        if (!target && this.dialog_.tabIndex >= 0) {
          target = this.dialog_;
        }
        if (!target) {
          target = findFocusableElementWithin(this.dialog_);
        }
        safeBlur(document.activeElement);
        target && target.focus();
      },
  
      /**
       * Sets the zIndex for the backdrop and dialog.
       *
       * @param {number} dialogZ
       * @param {number} backdropZ
       */
      updateZIndex: function(dialogZ, backdropZ) {
        if (dialogZ < backdropZ) {
          throw new Error('dialogZ should never be < backdropZ');
        }
        this.dialog_.style.zIndex = dialogZ;
        this.backdrop_.style.zIndex = backdropZ;
      },
  
      /**
       * Shows the dialog. If the dialog is already open, this does nothing.
       */
      show: function() {
        if (!this.dialog_.open) {
          this.setOpen(true);
          this.focus_();
        }
      },
  
      /**
       * Show this dialog modally.
       */
      showModal: function() {
        if (this.dialog_.hasAttribute('open')) {
          throw new Error('Failed to execute \'showModal\' on dialog: The element is already open, and therefore cannot be opened modally.');
        }
        if (!isConnected(this.dialog_)) {
          throw new Error('Failed to execute \'showModal\' on dialog: The element is not in a Document.');
        }
        if (!dialogPolyfill.dm.pushDialog(this)) {
          throw new Error('Failed to execute \'showModal\' on dialog: There are too many open modal dialogs.');
        }
  
        if (createsStackingContext(this.dialog_.parentElement)) {
          console.warn('A dialog is being shown inside a stacking context. ' +
              'This may cause it to be unusable. For more information, see this link: ' +
              'https://github.com/GoogleChrome/dialog-polyfill/#stacking-context');
        }
  
        this.setOpen(true);
        this.openAsModal_ = true;
  
        // Optionally center vertically, relative to the current viewport.
        if (dialogPolyfill.needsCentering(this.dialog_)) {
          dialogPolyfill.reposition(this.dialog_);
          this.replacedStyleTop_ = true;
        } else {
          this.replacedStyleTop_ = false;
        }
  
        // Insert backdrop.
        this.dialog_.parentNode.insertBefore(this.backdrop_, this.dialog_.nextSibling);
  
        // Focus on whatever inside the dialog.
        this.focus_();
      },
  
      /**
       * Closes this HTMLDialogElement. This is optional vs clearing the open
       * attribute, however this fires a 'close' event.
       *
       * @param {string=} opt_returnValue to use as the returnValue
       */
      close: function(opt_returnValue) {
        if (!this.dialog_.hasAttribute('open')) {
          throw new Error('Failed to execute \'close\' on dialog: The element does not have an \'open\' attribute, and therefore cannot be closed.');
        }
        this.setOpen(false);
  
        // Leave returnValue untouched in case it was set directly on the element
        if (opt_returnValue !== undefined) {
          this.dialog_.returnValue = opt_returnValue;
        }
  
        // Triggering "close" event for any attached listeners on the <dialog>.
        var closeEvent = new supportCustomEvent('close', {
          bubbles: false,
          cancelable: false
        });
        safeDispatchEvent(this.dialog_, closeEvent);
      }
  
    });
  
    var dialogPolyfill = {};
  
    dialogPolyfill.reposition = function(element) {
      var scrollTop = document.body.scrollTop || document.documentElement.scrollTop;
      var topValue = scrollTop + (window.innerHeight - element.offsetHeight) / 2;
      element.style.top = Math.max(scrollTop, topValue) + 'px';
    };
  
    dialogPolyfill.isInlinePositionSetByStylesheet = function(element) {
      for (var i = 0; i < document.styleSheets.length; ++i) {
        var styleSheet = document.styleSheets[i];
        var cssRules = null;
        // Some browsers throw on cssRules.
        try {
          cssRules = styleSheet.cssRules;
        } catch (e) {}
        if (!cssRules) { continue; }
        for (var j = 0; j < cssRules.length; ++j) {
          var rule = cssRules[j];
          var selectedNodes = null;
          // Ignore errors on invalid selector texts.
          try {
            selectedNodes = document.querySelectorAll(rule.selectorText);
          } catch(e) {}
          if (!selectedNodes || !inNodeList(selectedNodes, element)) {
            continue;
          }
          var cssTop = rule.style.getPropertyValue('top');
          var cssBottom = rule.style.getPropertyValue('bottom');
          if ((cssTop && cssTop !== 'auto') || (cssBottom && cssBottom !== 'auto')) {
            return true;
          }
        }
      }
      return false;
    };
  
    dialogPolyfill.needsCentering = function(dialog) {
      var computedStyle = window.getComputedStyle(dialog);
      if (computedStyle.position !== 'absolute') {
        return false;
      }
  
      // We must determine whether the top/bottom specified value is non-auto.  In
      // WebKit/Blink, checking computedStyle.top == 'auto' is sufficient, but
      // Firefox returns the used value. So we do this crazy thing instead: check
      // the inline style and then go through CSS rules.
      if ((dialog.style.top !== 'auto' && dialog.style.top !== '') ||
          (dialog.style.bottom !== 'auto' && dialog.style.bottom !== '')) {
        return false;
      }
      return !dialogPolyfill.isInlinePositionSetByStylesheet(dialog);
    };
  
    /**
     * @param {!Element} element to force upgrade
     */
    dialogPolyfill.forceRegisterDialog = function(element) {
      if (element.localName === 'dialog' && (window.HTMLDialogElement || element.showModal)) {
        console.warn('This browser already supports <dialog>, the polyfill ' +
            'may not work correctly', element);
      }
      if (element.localName !== 'dialog' && !element.classList.contains('dialog')) {
        throw new Error('Failed to register dialog: The element is not a dialog.');
      }
      new dialogPolyfillInfo(/** @type {!HTMLDialogElement} */ (element));
    };
  
    /**
     * @param {!Element} element to upgrade, if necessary
     */
    dialogPolyfill.registerDialog = function(element) {
      if (!element.showModal) {
        dialogPolyfill.forceRegisterDialog(element);
      }
    };
  
    /**
     * @constructor
     */
    dialogPolyfill.DialogManager = function() {
      /** @type {!Array<!dialogPolyfillInfo>} */
      this.pendingDialogStack = [];
  
      var checkDOM = this.checkDOM_.bind(this);
  
      // The overlay is used to simulate how a modal dialog blocks the document.
      // The blocking dialog is positioned on top of the overlay, and the rest of
      // the dialogs on the pending dialog stack are positioned below it. In the
      // actual implementation, the modal dialog stacking is controlled by the
      // top layer, where z-index has no effect.
      this.overlay = document.createElement('div');
      this.overlay.className = '_dialog_overlay';
      this.overlay.addEventListener('click', function(e) {
        this.forwardTab_ = undefined;
        e.stopPropagation();
        checkDOM([]);  // sanity-check DOM
      }.bind(this));
  
      this.handleKey_ = this.handleKey_.bind(this);
      this.handleFocus_ = this.handleFocus_.bind(this);
  
      this.zIndexLow_ = 100000;
      this.zIndexHigh_ = 100000 + 150;
  
      this.forwardTab_ = undefined;
  
      if ('MutationObserver' in window) {
        this.mo_ = new MutationObserver(function(records) {
          var removed = [];
          records.forEach(function(rec) {
            for (var i = 0, c; c = rec.removedNodes[i]; ++i) {
              if (!(c instanceof Element)) {
                continue;
              } else if (c.localName === 'dialog' || c.classList.contains('dialog')) {
                removed.push(c);
              }
              removed = removed.concat(c.querySelectorAll('dialog, .dialog'));
            }
          });
          removed.length && checkDOM(removed);
        });
      }
    };
  
    /**
     * Called on the first modal dialog being shown. Adds the overlay and related
     * handlers.
     */
    dialogPolyfill.DialogManager.prototype.blockDocument = function() {
      document.documentElement.addEventListener('focus', this.handleFocus_, true);
      document.addEventListener('keydown', this.handleKey_);
      this.mo_ && this.mo_.observe(document, {childList: true, subtree: true});
    };
  
    /**
     * Called on the first modal dialog being removed, i.e., when no more modal
     * dialogs are visible.
     */
    dialogPolyfill.DialogManager.prototype.unblockDocument = function() {
      document.documentElement.removeEventListener('focus', this.handleFocus_, true);
      document.removeEventListener('keydown', this.handleKey_);
      this.mo_ && this.mo_.disconnect();
    };
  
    /**
     * Updates the stacking of all known dialogs.
     */
    dialogPolyfill.DialogManager.prototype.updateStacking = function() {
      var zIndex = this.zIndexHigh_;
  
      for (var i = 0, dpi; dpi = this.pendingDialogStack[i]; ++i) {
        dpi.updateZIndex(--zIndex, --zIndex);
        if (i === 0) {
          this.overlay.style.zIndex = --zIndex;
        }
      }
  
      // Make the overlay a sibling of the dialog itself.
      var last = this.pendingDialogStack[0];
      if (last) {
        var p = last.dialog.parentNode || document.body;
        p.appendChild(this.overlay);
      } else if (this.overlay.parentNode) {
        this.overlay.parentNode.removeChild(this.overlay);
      }
    };
  
    /**
     * @param {Element} candidate to check if contained or is the top-most modal dialog
     * @return {boolean} whether candidate is contained in top dialog
     */
    dialogPolyfill.DialogManager.prototype.containedByTopDialog_ = function(candidate) {
      while (candidate = findNearestDialog(candidate)) {
        for (var i = 0, dpi; dpi = this.pendingDialogStack[i]; ++i) {
          if (dpi.dialog === candidate) {
            return i === 0;  // only valid if top-most
          }
        }
        candidate = candidate.parentElement;
      }
      return false;
    };
  
    dialogPolyfill.DialogManager.prototype.handleFocus_ = function(event) {
      var target = event.composedPath ? event.composedPath()[0] : event.target;
  
      if (this.containedByTopDialog_(target)) { return; }
  
      if (document.activeElement === document.documentElement) { return; }
  
      event.preventDefault();
      event.stopPropagation();
      safeBlur(/** @type {Element} */ (target));
  
      if (this.forwardTab_ === undefined) { return; }  // move focus only from a tab key
  
      var dpi = this.pendingDialogStack[0];
      var dialog = dpi.dialog;
      var position = dialog.compareDocumentPosition(target);
      if (position & Node.DOCUMENT_POSITION_PRECEDING) {
        if (this.forwardTab_) {
          // forward
          dpi.focus_();
        } else if (target !== document.documentElement) {
          // backwards if we're not already focused on <html>
          document.documentElement.focus();
        }
      }
  
      return false;
    };
  
    dialogPolyfill.DialogManager.prototype.handleKey_ = function(event) {
      this.forwardTab_ = undefined;
      if (event.keyCode === 27) {
        event.preventDefault();
        event.stopPropagation();
        var cancelEvent = new supportCustomEvent('cancel', {
          bubbles: false,
          cancelable: true
        });
        var dpi = this.pendingDialogStack[0];
        if (dpi && safeDispatchEvent(dpi.dialog, cancelEvent)) {
          dpi.dialog.close();
        }
      } else if (event.keyCode === 9) {
        this.forwardTab_ = !event.shiftKey;
      }
    };
  
    /**
     * Finds and downgrades any known modal dialogs that are no longer displayed. Dialogs that are
     * removed and immediately readded don't stay modal, they become normal.
     *
     * @param {!Array<!HTMLDialogElement>} removed that have definitely been removed
     */
    dialogPolyfill.DialogManager.prototype.checkDOM_ = function(removed) {
      // This operates on a clone because it may cause it to change. Each change also calls
      // updateStacking, which only actually needs to happen once. But who removes many modal dialogs
      // at a time?!
      var clone = this.pendingDialogStack.slice();
      clone.forEach(function(dpi) {
        if (removed.indexOf(dpi.dialog) !== -1) {
          dpi.downgradeModal();
        } else {
          dpi.maybeHideModal();
        }
      });
    };
  
    /**
     * @param {!dialogPolyfillInfo} dpi
     * @return {boolean} whether the dialog was allowed
     */
    dialogPolyfill.DialogManager.prototype.pushDialog = function(dpi) {
      var allowed = (this.zIndexHigh_ - this.zIndexLow_) / 2 - 1;
      if (this.pendingDialogStack.length >= allowed) {
        return false;
      }
      if (this.pendingDialogStack.unshift(dpi) === 1) {
        this.blockDocument();
      }
      this.updateStacking();
      return true;
    };
  
    /**
     * @param {!dialogPolyfillInfo} dpi
     */
    dialogPolyfill.DialogManager.prototype.removeDialog = function(dpi) {
      var index = this.pendingDialogStack.indexOf(dpi);
      if (index === -1) { return; }
  
      this.pendingDialogStack.splice(index, 1);
      if (this.pendingDialogStack.length === 0) {
        this.unblockDocument();
      }
      this.updateStacking();
    };
  
    dialogPolyfill.dm = new dialogPolyfill.DialogManager();
    dialogPolyfill.formSubmitter = null;
    dialogPolyfill.imagemapUseValue = null;
  
    /**
     * Installs global handlers, such as click listers and native method overrides. These are needed
     * even if a no dialog is registered, as they deal with <form method="dialog">.
     */
    if (window.HTMLDialogElement === undefined) {
  
      /**
       * If HTMLFormElement translates method="DIALOG" into 'get', then replace the descriptor with
       * one that returns the correct value.
       */
      var testForm = document.createElement('form');
      testForm.setAttribute('method', 'dialog');
      if (testForm.method !== 'dialog') {
        var methodDescriptor = Object.getOwnPropertyDescriptor(HTMLFormElement.prototype, 'method');
        if (methodDescriptor) {
          // nb. Some older iOS and older PhantomJS fail to return the descriptor. Don't do anything
          // and don't bother to update the element.
          var realGet = methodDescriptor.get;
          methodDescriptor.get = function() {
            if (isFormMethodDialog(this)) {
              return 'dialog';
            }
            return realGet.call(this);
          };
          var realSet = methodDescriptor.set;
          /** @this {HTMLElement} */
          methodDescriptor.set = function(v) {
            if (typeof v === 'string' && v.toLowerCase() === 'dialog') {
              return this.setAttribute('method', v);
            }
            return realSet.call(this, v);
          };
          Object.defineProperty(HTMLFormElement.prototype, 'method', methodDescriptor);
        }
      }
  
      /**
       * Global 'click' handler, to capture the <input type="submit"> or <button> element which has
       * submitted a <form method="dialog">. Needed as Safari and others don't report this inside
       * document.activeElement.
       */
      document.addEventListener('click', function(ev) {
        dialogPolyfill.formSubmitter = null;
        dialogPolyfill.imagemapUseValue = null;
        if (ev.defaultPrevented) { return; }  // e.g. a submit which prevents default submission
  
        var target = /** @type {Element} */ (ev.target);
        if ('composedPath' in ev) {
          var path = ev.composedPath();
          target = path.shift() || target;
        }
        if (!target || !isFormMethodDialog(target.form)) { return; }
  
        var valid = (target.type === 'submit' && ['button', 'input'].indexOf(target.localName) > -1);
        if (!valid) {
          if (!(target.localName === 'input' && target.type === 'image')) { return; }
          // this is a <input type="image">, which can submit forms
          dialogPolyfill.imagemapUseValue = ev.offsetX + ',' + ev.offsetY;
        }
  
        var dialog = findNearestDialog(target);
        if (!dialog) { return; }
  
        dialogPolyfill.formSubmitter = target;
  
      }, false);
  
      /**
       * Global 'submit' handler. This handles submits of `method="dialog"` which are invalid, i.e.,
       * outside a dialog. They get prevented.
       */
      document.addEventListener('submit', function(ev) {
        var form = ev.target;
        var dialog = findNearestDialog(form);
        if (dialog) {
          return;  // ignore, handle there
        }
  
        var submitter = findFormSubmitter(ev);
        var formmethod = submitter && submitter.getAttribute('formmethod') || form.getAttribute('method');
        if (formmethod === 'dialog') {
          ev.preventDefault();
        }
      });
  
      /**
       * Replace the native HTMLFormElement.submit() method, as it won't fire the
       * submit event and give us a chance to respond.
       */
      var nativeFormSubmit = HTMLFormElement.prototype.submit;
      var replacementFormSubmit = function () {
        if (!isFormMethodDialog(this)) {
          return nativeFormSubmit.call(this);
        }
        var dialog = findNearestDialog(this);
        dialog && dialog.close();
      };
      HTMLFormElement.prototype.submit = replacementFormSubmit;
    }
  
    return dialogPolyfill;
  
  }));
  