his one may be easily converted to Custom Elements,
 * although they would lose the ability of being associated lazily.
 */
var AssociatedToNode = class {
  constructor(node) {
    /**
     * Node associated to this object.
     */
    this.node = node;

    /**
     * This promise is resolved when the current set of blockers set by event
     * handlers have all been processed.
     */
    this._blockersPromise = Promise.resolve();
  }

  /**
   * Retrieves the instance associated with the given node, constructing a new
   * one if necessary. When the last reference to the node is released, the
   * object instance will be garbage collected as well.
   *
   * @param {DOMNode} node
   *   The node to retrieve or construct the AssociatedToNode instance for.
   * @returns {AssociatedToNode}
   */
  static forNode(node) {
    let associatedToNode = gNodeToObjectMap.get(node);
    if (!associatedToNode) {
      associatedToNode = new this(node);
      gNodeToObjectMap.set(node, associatedToNode);
    }
    return associatedToNode;
  }

  /**
   * A shortcut to the document that the node belongs to.
   *
   * @returns {Document}
   */
  get document() {
    return this.node.ownerDocument;
  }

  /**
   * A shortcut to the window global that the node belongs to.
   *
   * @returns {DOMWindow}
   */
  get window() {
    return this.node.ownerGlobal;
  }

  /**
   * A shortcut to windowUtils.getBoundsWithoutFlushing for the window global
   * associated with the node.
   *
   * This is a pseudo-private method using an `_` because we want it to be
   * used by subclasses. Please don't use outside of this module.
   *
   * @param {DOMNode} element
   *   The element to retrieve the bounds for without flushing layout.
   * @returns {DOMRect}
   *   The bounding rect of the element.
   */
  _getBoundsWithoutFlushing(element) {
    return this.window.windowUtils.getBoundsWithoutFlushing(element);
  }

  /**
   * Dispatches a custom event on this element.
   *
   * @param {string} eventName
   *   Name of the event to dispatch.
   * @param {object|undefined} [detail]
   *   Event detail object. Optional.
   * @param {boolean} cancelable
   *   True if the event can be canceled.
   * @returns {boolean}
   *   True if the event was canceled by an event handler, false otherwise.
   */
  dispatchCustomEvent(eventName, detail, cancelable = false) {
    let event = new this.window.CustomEvent(eventName, {
      detail,
      bubbles: true,
      cancelable,
    });
    this.node.dispatchEvent(event);
    return event.defaultPrevented;
  }

  /**
   * Dispatches a custom event on this element and waits for any blocking
   * promises registered using the "addBlocker" function on the details object.
   * If this function is called again, the event is only dispatched after all
   * the previously registered blockers have returned.
   *
   * The event can be canceled either by resolving any blocking promise to the
   * boolean value "false" or by calling preventDefault on the event. Rejections
   * and exceptions will be reported and will cancel the event.
   *
   * Blocking should be used sporadically because it slows down the interface.
   * Also, non-reentrancy is not strictly guaranteed because a safety timeout of
   * BLOCKERS_TIMEOUT_MS is implemented, after which the event will be canceled.
   * This helps to prevent deadlocks if any of the event handlers does not
   * resolve a blocker promise.
   *
   * Note:
   *   Since there is no use case for dispatching different asynchronous
   *   events in parallel for the same element, this function will also wait
   *   for previous blockers when the event name is different.
   *
   * @param {string} eventName
   *   Name of the custom event to dispatch.
   * @returns {Promise<boolean>}
   *   Resolves to true if the event was canceled by a handler, false otherwise.
   */
  async dispatchAsyncEvent(eventName) {
    // Wait for all the previous blockers before dispatching the event.
    let blockersPromise = this._blockersPromise.catch(() => {});
    return (this._blockersPromise = blockersPromise.then(async () => {
      let blockers = new Set();
      let cancel = this.dispatchCustomEvent(
        eventName,
        {
          addBlocker(promise) {
            // Any exception in the blocker will cancel the operation.
            blockers.add(
              promise.catch(ex => {
                console.error(ex);
                return true;
              })
            );
          },
        },
        true
      );
      if (blockers.size) {
        let timeoutPromise = new Promise((resolve, reject) => {
          this.window.setTimeout(reject, BLOCKERS_TIMEOUT_MS);
        });
        try {
          let results = await Promise.race([
            Promise.all(blockers),
            timeoutPromise,
          ]);
          cancel = cancel || results.some(result => result === false);
        } catch (ex) {
          console.error(
            new Error(`One of the blockers for ${eventName} timed out.`)
          );
          return true;
        }
      }
      return cancel;
    }));
  }
};

/**
 * This is associated to <panelmultiview> elements.
 */
export var PanelMultiView = class extends AssociatedToNode {
  /**
   * Tries to open the specified <panel> and displays the main view specified
   * with the "mainViewId" attribute on the <panelmultiview> node it contains.
   *
   * If the panel does not contain a <panelmultiview>, it is opened directly.
   * This allows consumers like page actions to accept different panel types.
   *
   * See the non-static openPopup method for details.
   *
   * @static
   * @memberof PanelMultiView
   * @param {DOMNode} panelNode
   *   The <panel> node that is to be opened.
   * @param {...*} args
   *   Additional arguments to be forwarded to the openPopup method of the
   *   panel.
   * @returns {Promise<boolean, Exception>|boolean}
   *   Returns a Promise that resolves to `true` if the panel successfully
   *   opened, or rejects if the panel cannot open. May also return `true`
   *   immediately if the panel does not contain a <panelmultiview>.
   */
  static async openPopup(panelNode, ...args) {
    let panelMultiViewNode = panelNode.querySelector("panelmultiview");
    if (panelMultiViewNode) {
      return this.forNode(panelMultiViewNode).openPopup(...args);
    }
    panelNode.openPopup(...args);
    return true;
  }

  /**
   * Closes the specified <panel> which contains a <panelmultiview> node.
   *
   * If the panel does not contain a <panelmultiview>, it is closed directly.
   * This allows consumers like page actions to accept different panel types.
   *
   * See the non-static hidePopup method for details.
   *
   * @static
   * @memberof PanelMultiView
   * @param {DOMNode} panelNode
   *   The <panel> node.
   * @param {boolean} [animate=false]
   *   Whether to show a fade animation.
   */
  static hidePopup(panelNode, animate = false) {
    let panelMultiViewNode = panelNode.querySelector("panelmultiview");
    if (panelMultiViewNode) {
      this.forNode(panelMultiViewNode).hidePopup(animate);
    } else {
      panelNode.hidePopup(animate);
    }
  }

  /**
   * Removes the specified <panel> from the document, ensuring that any
   * <panelmultiview> node it contains is destroyed properly.
   *
   * If the viewCacheId attribute is present on the <panelmultiview> element,
   * imported subviews will be moved out again to the element it specifies, so
   * that the panel element can be removed safely.
   *
   * If the panel does not contain a <panelmultiview>, it is removed directly.
   * This allows consumers like page actions to accept different panel types.
   *
   * @param {DOMNode} panelNode
   *   The <panel> to remove.
   */
  static removePopup(panelNode) {
    try {
      let panelMultiViewNode = panelNode.querySelector("panelmultiview");
      if (panelMultiViewNode) {
        let panelMultiView = this.forNode(panelMultiViewNode);
        panelMultiView._moveOutKids();
        panelMultiView.disconnect();
      }
    } finally {
      // Make sure to remove the panel element even if disconnecting fails.
      panelNode.remove();
    }
  }

  /**
   * Returns the element with the given id.
   * For nodes that are lazily loaded and not yet in the DOM, the node should
   * be retrieved from the view cache template.
   *
   * @param {Document} doc
   *   The document to retrieve the node for.
   * @param {string} id
   *   The ID of the element to retrieve from the DOM (or the
   *   appMenu-viewCache).
   * @returns {DOMNode|null}
   *   The found DOMNode or null if no node was found with that ID.
   */
  static getViewNode(doc, id) {
    let viewCacheTemplate = doc.getElementById("appMenu-viewCache");

    return (
      doc.getElementById(id) ||
      viewCacheTemplate?.content.querySelector("#" + id)
    );
  }

  /**
   * Ensures that when the specified window is closed all the <panelmultiview>
   * node it contains are destroyed properly.
   *
   * @param {DOMWindow} window
   *   The window to add the unload handler to.
   */
  static ensureUnloadHandlerRegistered(window) {
    if (gWindowsWithUnloadHandler.has(window)) {
      return;
    }

    window.addEventListener(
      "unload",
      () => {
        for (let panelMultiViewNode of window.document.querySelectorAll(
          "panelmultiview"
        )) {
          this.forNode(panelMultiViewNode).disconnect();
        }
      },
      { once: true }
    );

    gWindowsWithUnloadHandler.add(window);
  }

  /**
   * Returns the parent element of the <panelmultiview>, which should be a
   * <panel>.
   */
  get #panel() {
    return this.node.parentNode;
  }

  /**
   * Sets the `transitioning` attribute of the <panelmultiview>.
   *
   * @param {boolean} val
   *   If true, sets the attribute to `"true"`. If false, removes the attribute.
   */
  set #transitioning(val) {
    if (val) {
      this.node.setAttribute("transitioning", "true");
    } else {
      this.node.removeAttribute("transitioning");
    }
  }

  constructor(node) {
    super(node);
    this._openPopupPromise = Promise.resolve(false);
  }

  /**
   * Binds this PanelMultiView class to the underlying <panelmultiview> element.
   * Also creates the appropriate <panelmultview> child elements, like the
   * viewcontainer and the viewstack. Sets up popup event handlers for the
   * panel.
   */
  connect() {
    this.connected = true;

    PanelMultiView.ensureUnloadHandlerRegistered(this.window);

    let viewContainer = (this._viewContainer =
      this.document.createXULElement("box"));
    viewContainer.classList.add("panel-viewcontainer");

    let viewStack = (this._viewStack = this.document.createXULElement("box"));
    viewStack.classList.add("panel-viewstack");
    viewContainer.append(viewStack);

    let offscreenViewContainer = this.document.createXULElement("box");
    offscreenViewContainer.classList.add("panel-viewcontainer", "offscreen");

    let offscreenViewStack = (this._offscreenViewStack =
      this.document.createXULElement("box"));
    offscreenViewStack.classList.add("panel-viewstack");
    offscreenViewContainer.append(offscreenViewStack);

    this.node.prepend(offscreenViewContainer);
    this.node.prepend(viewContainer);

    this.openViews = [];

    this.#panel.addEventListener("popupshowing", this);
    this.#panel.addEventListener("popuphidden", this);
    this.#panel.addEventListener("popupshown", this);

    // Proxy these public properties and methods, as used elsewhere by various
    // parts of the browser, to this instance.
    ["goBack", "showSubView"].forEach(method => {
      Object.defineProperty(this.node, method, {
        enumerable: true,
        value: (...args) => this[method](...args),
      });
    });
  }

  /**
   * Disconnects this PanelMultiView instance from a <panelmultiview> element.
   * This does not remove any of the nodes created in `connect`, but does clean
   * up the event listeners that were set up.
   */
  disconnect() {
    // Guard against re-entrancy.
    if (!this.node || !this.connected) {
      return;
    }

    this.#panel.removeEventListener("mousemove", this);
    this.#panel.removeEventListener("popupshowing", this);
    this.#panel.removeEventListener("popupshown", this);
    this.#panel.removeEventListener("popuphidden", this);
    this.document.documentElement.removeEventListener("keydown", this, true);
    this.node =
      this._openPopupPromise =
      this._openPopupCancelCallback =
      this._viewContainer =
      this._viewStack =
      this._transitionDetails =
        null;
  }

  /**
   * Tries to open the panel associated with this PanelMultiView, and displays
   * the main view specified with the "mainViewId" attribute.
   *
   * The hidePopup method can be called while the operation is in progress to
   * prevent the panel from being displayed. View events may also cancel the
   * operation, so there is no guarantee that the panel will become visible.
   *
   * The "popuphidden" event will be fired either when the operation is canceled
   * or when the popup is closed later. This event can be used for example to
   * reset the "open" state of the anchor or tear down temporary panels.
   *
   * If this method is called again before the panel is shown, the result
   * depends on the operation currently in progress. If the operation was not
   * canceled, the panel is opened using the arguments from the previous call,
   * and this call is ignored. If the operation was canceled, it will be
   * retried again using the arguments from this call.
   *
   * It's not necessary for the <panelmultiview> binding to be connected when
   * this method is called, but the containing panel must have its display
   * turned on, for example it shouldn't have the "hidden" attribute.
   *
   * @instance
   * @memberof PanelMultiView#
   * @param {DOMNode} anchor
   *   The node to anchor the popup to.
   * @param {StringOrOpenPopupOptions} options
   *   Either options to use or a string position. This is forwarded to
   *   the openPopup method of the panel.
   * @param {...*} args
   *   Additional arguments to be forwarded to the openPopup method of the
   *   panel.
   * @returns {Promise<boolean, Exception>}
   *   Resolves with true as soon as the request to display the panel has been
   *   sent, or with false if the operation was canceled. The state of
   *   the panel at this point is not guaranteed. It may be still
   *   showing, completely shown, or completely hidden.
   *
   *   Rejects if an exception is thrown at any point in the process before the
   *   request to display the panel is sent.
   */
  async openPopup(anchor, options, ...args) {
    // Set up the function that allows hidePopup or a second call to showPopup
    // to cancel the specific panel opening operation that we're starting below.
    // This function must be synchronous, meaning we can't use Promise.race,
    // because hidePopup wants to dispatch the "popuphidden" event synchronously
    // even if the panel has not been opened yet.
    let canCancel = true;
    let cancelCallback = (this._openPopupCancelCallback = () => {
      // If the cancel callback is called and the panel hasn't been prepared
      // yet, cancel showing it. Setting canCancel to false will prevent the
      // popup from opening. If the panel has opened by the time the cancel
      // callback is called, canCancel will be false already, and we will not
      // fire the "popuphidden" event.
      if (canCancel && this.node) {
        canCancel = false;
        this.dispatchCustomEvent("popuphidden");
      }
      if (cancelCallback == this._openPopupCancelCallback) {
        // If still current, let go of the cancel callback since it will capture
        // the entire scope and tie it to the main window.
        delete this._openPopupCancelCallback;
      }
    });

    // Create a promise that is resolved with the result of the last call to
    // this method, where errors indicate that the panel was not opened.
    let openPopupPromise = this._openPopupPromise.catch(() => {
      return false;
    });

    // Make the preparation done before showing the panel non-reentrant. The
    // promise created here will be resolved only after the panel preparation is
    // completed, even if a cancellation request is received in the meantime.
    return (this._openPopupPromise = openPopupPromise.then(async wasShown => {
      // The panel may have been destroyed in the meantime.
      if (!this.node) {
        return false;
      }
      // If the panel has been already opened there is nothing more to do. We
      // check the actual state of the panel rather than setting some state in
      // our handler of the "popuphidden" event because this has a lower chance
      // of locking indefinitely if events aren't raised in the expected order.
      if (wasShown && ["open", "showing"].includes(this.#panel.state)) {
        if (cancelCallback == this._openPopupCancelCallback) {
          // If still current, let go of the cancel callback since it will
          // capture the entire scope and tie it to the main window.
          delete this._openPopupCancelCallback;
        }
        return true;
      }
      try {
        if (!this.connected) {
          this.connect();
        }
        // Allow any of the ViewShowing handlers to prevent showing the main view.
        if (!(await this.#showMainView())) {
          cancelCallback();
        }
      } catch (ex) {
        cancelCallback();
        throw ex;
      }
      // If a cancellation request was received there is nothing more to do.
      if (!canCancel || !this.node) {
        return false;
      }
      // We have to set canCancel to false before opening the popup because the
      // hidePopup method of PanelMultiView can be re-entered by event handlers.
      // If the openPopup call fails, however, we still have to dispatch the
      // "popuphidden" event even if canCancel was set to false.
      try {
        canCancel = false;
        this.#panel.openPopup(anchor, options, ...args);
        if (cancelCallback == this._openPopupCancelCallback) {
          // If still current, let go of the cancel callback since it will
          // capture the entire scope and tie it to the main window.
          delete this._openPopupCancelCallback;
        }
        // Set an attribute on the popup to let consumers style popup elements -
        // for example, the anchor arrow is styled to match the color of the header
        // in the Protections Panel main view.
        this.#panel.setAttribute("mainviewshowing", true);

        // On Windows, if another popup is hiding while we call openPopup, the
        // call won't fail but the popup won't open. In this case, we have to
        // dispatch an artificial "popuphidden" event to reset our state.
        if (this.#panel.state == "closed" && this.openViews.length) {
          this.dispatchCustomEvent("popuphidden");
          return false;
        }

        if (
          options &&
          typeof options == "object" &&
          options.triggerEvent &&
          (options.triggerEvent.type == "keypress" ||
            options.triggerEvent.type == "keydown" ||
            options.triggerEvent?.inputSource ==
              MouseEvent.MOZ_SOURCE_KEYBOARD) &&
          this.openViews.length
        ) {
          // This was opened via the keyboard, so focus the first item.
          this.openViews[0].focusWhenActive = true;
        }

        return true;
      } catch (ex) {
        this.dispatchCustomEvent("popuphidden");
        throw ex;
      }
    }));
  }

  /**
   * Closes the panel associated with this PanelMultiView.
   *
   * If the openPopup method was called but the panel has not been displayed
   * yet, the operation is canceled and the panel will not be displayed, but the
   * "popuphidden" event is fired synchronously anyways.
   *
   * This means that by the time this method returns all the operations handled
   * by the "popuphidden" event are completed, for example resetting the "open"
   * state of the anchor, and the panel is already invisible.
   *
   * Note:
   *   The value of animate could be changed to true by default, in both
   *   this and the static method above. (see bug 1769813)
   *
   * @instance
   * @memberof PanelMultiView#
   * @param {boolean} [animate=false]
   *   Whether to show a fade animation. Optional.
   */
  hidePopup(animate = false) {
    if (!this.node || !this.connected) {
      return;
    }

    // If we have already reached the #panel.openPopup call in the openPopup
    // method, we can call hidePopup. Otherwise, we have to cancel the latest
    // request to open the panel, which will have no effect if the request has
    // been canceled already.
    if (["open", "showing"].includes(this.#panel.state)) {
      this.#panel.hidePopup(animate);
    } else {
      this._openPopupCancelCallback?.();
    }

    // We close all the views synchronously, so that they are ready to be opened
    // in other PanelMultiView instances. The "popuphidden" handler may also
    // call this function, but the second time openViews will be empty.
    this.closeAllViews();
  }

  /**
   * Move any child subviews into the element defined by "viewCacheId" to make
   * sure they will not be removed together with the <panelmultiview> element.
   *
   * This is "pseudo-private" with the underscore so that the static
   * removePopup method can call it.
   */
  _moveOutKids() {
    // this.node may have been set to null by a call to disconnect().
    let viewCacheId = this.node?.getAttribute("viewCacheId");
    if (!viewCacheId) {
      return;
    }

    // Node.children and Node.children is live to DOM changes like the
    // ones we're about to do, so iterate over a static copy:
    let subviews = Array.from(this._viewStack.children);
    let viewCache = this.document.getElementById("appMenu-viewCache");
    for (let subview of subviews) {
      viewCache.appendChild(subview);
    }
  }

  /**
   * Slides in the specified view as a subview. This returns synchronously,
   * but may eventually log an error if showing the subview fails for some
   * reason.
   *
   * @param {string|DOMNode} viewIdOrNode
   *   DOM element or string ID of the <panelview> to display.
   * @param {DOMNode} anchor
   *   DOM element that triggered the subview, which will be highlighted
   *   and whose "label" attribute will be used for the title of the
   *   subview when a "title" attribute is not specified.
   */
  showSubView(viewIdOrNode, anchor) {
    this.#showSubView(viewIdOrNode, anchor).catch(console.error);
  }

  /**
   * The asynchronous private helper method for showSubView that does most of
   * the heavy lifting.
   *
   * @param {string|DOMNode} viewIdOrNode
   *   DOM element or string ID of the <panelview> to display.
   * @param {DOMNode} anchor
   *   DOM element that triggered the subview, which will be highlighted
   *   and whose "label" attribute will be used for the title of the
   *   subview when a "title" attribute is not specified.
   * @returns {Promise<undefined>}
   *   Returns a Promise that resolves when attempting to show the subview
   *   completes.
   */
  async #showSubView(viewIdOrNode, anchor) {
    let viewNode =
      typeof viewIdOrNode == "string"
        ? PanelMultiView.getViewNode(this.document, viewIdOrNode)
        : viewIdOrNode;
    if (!viewNode) {
      console.error(new Error(`Subview ${viewIdOrNode} doesn't exist.`));
      return;
    }

    if (!this.openViews.length) {
      console.error(new Error(`Cannot show a subview in a closed panel.`));
      return;
    }

    let prevPanelView = this.openViews[this.openViews.length - 1];
    let nextPanelView = PanelView.forNode(viewNode);
    if (this.openViews.includes(nextPanelView)) {
      console.error(new Error(`Subview ${viewNode.id} is already open.`));
      return;
    }

    // Do not re-enter the process if navigation is already in progress. Since
    // there is only one active view at any given time, we can do this check
    // safely, even considering that during the navigation process the actual
    // view to which prevPanelView refers will change.
    if (!prevPanelView.active) {
      return;
    }
    // If prevPanelView._doingKeyboardActivation is true, it will be reset to
    // false synchronously. Therefore, we must capture it before we use any
    // "await" statements.
    let doingKeyboardActivation = prevPanelView._doingKeyboardActivation;
    // Marking the view that is about to scrolled out of the visible area as
    // inactive will prevent re-entrancy and also disable keyboard navigation.
    // From this point onwards, "await" statements can be used safely.
    prevPanelView.active = false;

    // Provide visual feedback while navigation is in progress, starting before
    // the transition starts and ending when the previous view is invisible.
    anchor?.setAttribute("open", "true");
    try {
      // If the ViewShowing event cancels the operation we have to re-enable
      // keyboard navigation, but this must be avoided if the panel was closed.
      if (!(await this.#openView(nextPanelView))) {
        if (prevPanelView.isOpenIn(this)) {
          // We don't raise a ViewShown event because nothing actually changed.
          // Technically we should use a different state flag just because there
          // is code that could check the "active" property to determine whether
          // to wait for a ViewShown event later, but this only happens in
          // regression tests and is less likely to be a technique used in
          // production code, where use of ViewShown is less common.
          prevPanelView.active = true;
        }
        return;
      }

      prevPanelView.captureKnownSize();

      // The main view of a panel can be a subview in another one. Make sure to
      // reset all the properties that may be set on a subview.
      nextPanelView.mainview = false;
      // The header may be set by a Fluent message with a title attribute
      // that has changed immediately before showing the panelview,
      // and so is not reflected in the DOM yet.
      let title;
      const l10nId = viewNode.getAttribute("data-l10n-id");
      if (l10nId) {
        const l10nArgs = viewNode.getAttribute("data-l10n-args");
        const args = l10nArgs ? JSON.parse(l10nArgs) : undefined;
        const [msg] = await viewNode.ownerDocument.l10n.formatMessages([
          { id: l10nId, args },
        ]);
        title = msg.attributes.find(a => a.name === "title")?.value;
      }
      // If not set by Fluent, the header may change based on how the subview was opened.
      title ??= viewNode.getAttribute("title") || anchor?.getAttribute("label");
      nextPanelView.headerText = title;
      // The constrained width of subviews may also vary between panels.
      nextPanelView.minMaxWidth = prevPanelView.knownWidth;
      let lockPanelVertical =
        this.openViews[0].node.getAttribute("lockpanelvertical") == "true";
      nextPanelView.minMaxHeight = lockPanelVertical
        ? prevPanelView.knownHeight
        : 0;

      if (anchor) {
        viewNode.classList.add("PanelUI-subView");
      }

      await this.#transitionViews(prevPanelView.node, viewNode, false);
    } finally {
      anchor?.removeAttribute("open");
    }

    nextPanelView.focusWhenActive = doingKeyboardActivation;
    this.#activateView(nextPanelView);
  }

  /**
   * Navigates backwards by sliding out the most recent subview.
   */
  goBack() {
    this.#goBack().catch(console.error);
  }

  /**
   * The asynchronous helper method for goBack that does most of the heavy
   * lifting.
   *
   * @returns {Promise<undefined>}
   *   Resolves when attempting to go back completes.
   */
  async #goBack() {
    if (this.openViews.length < 2) {
      // This may be called by keyboard navigation or external code when only
      // the main view is open.
      return;
    }

    let prevPanelView = this.openViews[this.openViews.length - 1];
    let nextPanelView = this.openViews[this.openViews.length - 2];

    // Like in the showSubView method, do not re-enter navigation while it is
    // in progress, and make the view inactive immediately. From this point
    // onwards, "await" statements can be used safely.
    if (!prevPanelView.active) {
      return;
    }
    prevPanelView.active = false;

    prevPanelView.captureKnownSize();
    await this.#transitionViews(prevPanelView.node, nextPanelView.node, true);

    this.#closeLatestView();

    this.#activateView(nextPanelView);
  }

  /**
   * Prepares the main view before showing the panel.
   *
   * @returns {boolean}
   *   Returns true if showing the main view succeeds.
   */
  async #showMainView() {
    let nextPanelView = PanelView.forNode(
      PanelMultiView.getViewNode(
        this.document,
        this.node.getAttribute("mainViewId")
      )
    );

    // If the view is already open in another panel, close the panel first.
    let oldPanelMultiViewNode = nextPanelView.node.panelMultiView;
    if (oldPanelMultiViewNode) {
      PanelMultiView.forNode(oldPanelMultiViewNode).hidePopup();
      // Wait for a layout flush after hiding the popup, otherwise the view may
      // not be displayed correctly for some time after the new panel is opened.
      // This is filed as bug 1441015.
      await this.window.promiseDocumentFlushed(() => {});
    }

    if (!(await this.#openView(nextPanelView))) {
      return false;
    }

    // The main view of a panel can be a subview in another one. Make sure to
    // reset all the properties that may be set on a subview.
    nextPanelView.mainview = true;
    nextPanelView.headerText = "";
    nextPanelView.minMaxWidth = 0;
    nextPanelView.minMaxHeight = 0;

    // Ensure the view will be visible once the panel is opened.
    nextPanelView.visible = true;

    return true;
  }

  /**
   * Opens the specified PanelView and dispatches the ViewShowing event, which
   * can be used to populate the subview or cancel the operation.
   *
   * This also clears all the attributes and styles that may be left by a
   * transition that was interrupted.
   *
   * @param {DOMNode} panelView
   *   The <panelview> element to show.
   * @returns {Promise<boolean>}
   *   Resolves with true if the view was opened, false otherwise.
   */
  async #openView(panelView) {
    if (panelView.node.parentNode != this._viewStack) {
      this._viewStack.appendChild(panelView.node);
    }

    panelView.node.panelMultiView = this.node;
    this.openViews.push(panelView);

    // Panels could contain out-pf-process <browser> elements, that need to be
    // supported with a remote attribute on the panel in order to display properly.
    // See bug https://bugzilla.mozilla.org/show_bug.cgi?id=1365660
    if (panelView.node.getAttribute("remote") == "true") {
      this.#panel.setAttribute("remote", "true");
    }

    let canceled = await panelView.dispatchAsyncEvent("ViewShowing");

    // The panel can be hidden while we are processing the ViewShowing event.
    // This results in all the views being closed synchronously, and at this
    // point the ViewHiding event has already been dispatched for all of them.
    if (!this.openViews.length) {
      return false;
    }

    // Check if the event requested cancellation but the panel is still open.
    if (canceled) {
      // Handlers for ViewShowing can't know if a different handler requested
      // cancellation, so this will dispatch a ViewHiding event to give a chance
      // to clean up.
      this.#closeLatestView();
      return false;
    }

    // Clean up all the attributes and styles related to transitions. We do this
    // here rather than when the view is closed because we are likely to make
    // other DOM modifications soon, which isn't the case when closing.
    let { style } = panelView.node;
    style.removeProperty("outline");
    style.removeProperty("width");

    return true;
  }

  /**
   * Activates the specified view and raises the ViewShown event, unless the
   * view was closed in the meantime.
   *
   * @param {DOMNode} panelView
   *   The <panelview> to be activated.
   */
  #activateView(panelView) {
    if (panelView.isOpenIn(this)) {
      panelView.active = true;
      if (panelView.focusWhenActive) {
        panelView.focusFirstNavigableElement(false, true);
        panelView.focusWhenActive = false;
      }
      panelView.dispatchCustomEvent("ViewShown");
    }
  }

  /**
   * Closes the most recent PanelView and raises the ViewHiding event.
   *
   * Note:
   *   The ViewHiding event is not cancelable and should probably be renamed
   *   to ViewHidden or ViewClosed instead, see bug 1438507.
   */
  #closeLatestView() {
    let panelView = this.openViews.pop();
    panelView.clearNavigation();
    panelView.dispatchCustomEvent("ViewHiding");
    panelView.node.panelMultiView = null;
    // Views become invisible synchronously when they are closed, and they won't
    // become visible again until they are opened. When this is called at the
    // end of backwards navigation, the view is already invisible.
    panelView.visible = false;
  }

  /**
   * Closes all the views that are currently open.
   */
  closeAllViews() {
    // Raise ViewHiding events for open views in reverse order.
    while (this.openViews.length) {
      this.#closeLatestView();
    }
  }

  /**
   * Apply a transition to 'slide' from the currently active view to the next
   * one.
   * Sliding the next subview in means that the previous panelview stays where
   * it is and the active panelview slides in from the left in LTR mode, right
   * in RTL mode.
   *
   * @param {DOMNode} previousViewNode
   *   The panelview node that is currently displayed, but is about to be
   *   transitioned away. This must be already inactive at this point.
   * @param {DOMNode} viewNode
   *   The panelview node that will becode the active view after the transition
   *   has finished.
   * @param {boolean} reverse
   *   Whether we're navigation back to a previous view or forward to a next
   *   view.
   */
  async #transitionViews(previousViewNode, viewNode, reverse) {
    const { window } = this;

    let nextPanelView = PanelView.forNode(viewNode);
    let prevPanelView = PanelView.forNode(previousViewNode);

    let details = (this._transitionDetails = {
      phase: TRANSITION_PHASES.START,
    });

    // Set the viewContainer dimensions to make sure only the current view is
    // visible.
    let olderView = reverse ? nextPanelView : prevPanelView;
    this._viewContainer.style.minHeight = olderView.knownHeight + "px";
    this._viewContainer.style.height = prevPanelView.knownHeight + "px";
    this._viewContainer.style.width = prevPanelView.knownWidth + "px";
    // Lock the dimensions of the window that hosts the popup panel.
    let rect = this._getBoundsWithoutFlushing(this.#panel);
    this.#panel.style.width = rect.width + "px";
    this.#panel.style.height = rect.height + "px";

    let viewRect;
    if (reverse) {
      // Use the cached size when going back to a previous view, but not when
      // reopening a subview, because its contents may have changed.
      viewRect = {
        width: nextPanelView.knownWidth,
        height: nextPanelView.knownHeight,
      };
      nextPanelView.visible = true;
    } else if (viewNode.customRectGetter) {
      // We use a customRectGetter for WebExtensions panels, because they need
      // to query the size from an embedded browser. The presence of this
      // getter also provides an indication that the view node shouldn't be
      // moved around, otherwise the state of the browser would get disrupted.
      let width = prevPanelView.knownWidth;
      let height = prevPanelView.knownHeight;
      viewRect = Object.assign({ height, width }, viewNode.customRectGetter());
      nextPanelView.visible = true;
      // Until the header is visible, it has 0 height.
      // Wait for layout before measuring it
      let header = viewNode.firstElementChild;
      if (header && header.classList.contains("panel-header")) {
        viewRect.height += await window.promiseDocumentFlushed(() => {
          return this._getBoundsWithoutFlushing(header).height;
        });
      }
      // Bail out if the panel was closed in the meantime.
      if (!nextPanelView.isOpenIn(this)) {
        return;
      }
    } else {
      this._offscreenViewStack.style.minHeight = olderView.knownHeight + "px";
      this._offscreenViewStack.appendChild(viewNode);
      nextPanelView.visible = true;

      viewRect = await window.promiseDocumentFlushed(() => {
        return this._getBoundsWithoutFlushing(viewNode);
      });
      // Bail out if the panel was closed in the meantime.
      if (!nextPanelView.isOpenIn(this)) {
        return;
      }

      // Place back the view after all the other views that are already open in
      // order for the transition to work as expected.
      this._viewStack.appendChild(viewNode);

      this._offscreenViewStack.style.removeProperty("min-height");
    }

    this.#transitioning = true;
    details.phase = TRANSITION_PHASES.PREPARE;

    // The 'magic' part: build up the amount of pixels to move right or left.
    let moveToLeft =
      (this.window.RTL_UI && !reverse) || (!this.window.RTL_UI && reverse);
    let deltaX = prevPanelView.knownWidth;
    let deepestNode = reverse ? previousViewNode : viewNode;

    // With a transition when navigating backwards - user hits the 'back'
    // button - we need to make sure that the views are positioned in a way
    // that a translateX() unveils the previous view from the right direction.
    if (reverse) {
      this._viewStack.style.marginInlineStart = "-" + deltaX + "px";
    }

    // Set the transition style and listen for its end to clean up and make sure
    // the box sizing becomes dynamic again.
    // Somehow, putting these properties in PanelUI.css doesn't work for newly
    // shown nodes in a XUL parent node.
    this._viewStack.style.transition =
      "transform var(--animation-easing-function)" +
      " var(--panelui-subview-transition-duration)";
    this._viewStack.style.willChange = "transform";
    // Use an outline instead of a border so that the size is not affected.
    deepestNode.style.outline = "1px solid var(--panel-separator-color)";

    // Now that all the elements are in place for the start of the transition,
    // give the layout code a chance to set the initial values.
    await window.promiseDocumentFlushed(() => {});
    // Bail out if the panel was closed in the meantime.
    if (!nextPanelView.isOpenIn(this)) {
      return;
    }

    // Now set the viewContainer dimensions to that of the new view, which
    // kicks of the height animation.
    this._viewContainer.style.height = viewRect.height + "px";
    this._viewContainer.style.width = viewRect.width + "px";
    this.#panel.style.removeProperty("width");
    this.#panel.style.removeProperty("height");
    // We're setting the width property to prevent flickering during the
    // sliding animation with smaller views.
    viewNode.style.width = viewRect.width + "px";

    // Kick off the transition!
    details.phase = TRANSITION_PHASES.TRANSITION;

    // If we're going to show the main view, we can remove the
    // min-height property on the view container. It's also time
    // to set the mainviewshowing attribute on the popup.
    if (viewNode.getAttribute("mainview")) {
      this._viewContainer.style.removeProperty("min-height");
      this.#panel.setAttribute("mainviewshowing", true);
    } else {
      this.#panel.removeAttribute("mainviewshowing");
    }

    // Avoid transforming element if the user has prefers-reduced-motion set
    if (
      this.window.matchMedia("(prefers-reduced-motion: no-preference)")
        .matches &&
      !viewNode.getAttribute("no-panelview-transition")
    ) {
      this._viewStack.style.transform =
        "translateX(" + (moveToLeft ? "" : "-") + deltaX + "px)";

      await new Promise(resolve => {
        details.resolve = resolve;
        this._viewContainer.addEventListener(
          "transitionend",
          (details.listener = ev => {
            // It's quite common that `height` on the view container doesn't need
            // to transition, so we make sure to do all the work on the transform
            // transition-end, because that is guaranteed to happen.
            if (
              ev.target != this._viewStack ||
              ev.propertyName != "transform"
            ) {
              return;
            }
            this._viewContainer.removeEventListener(
              "transitionend",
              details.listener
            );
            delete details.listener;
            resolve();
          })
        );
        this._viewContainer.addEventListener(
          "transitioncancel",
          (details.cancelListener = ev => {
            if (ev.target != this._viewStack) {
              return;
            }
            this._viewContainer.removeEventListener(
              "transitioncancel",
              details.cancelListener
            );
            delete details.cancelListener;
            resolve();
          })
        );
      });
    }

    // Bail out if the panel was closed during the transition.
    if (!nextPanelView.isOpenIn(this)) {
      return;
    }
    prevPanelView.visible = false;

    // This will complete the operation by removing any transition properties.
    nextPanelView.node.style.removeProperty("width");
    deepestNode.style.removeProperty("outline");
    this.#cleanupTransitionPhase();
    // Ensure the newly-visible view has been through a layout flush before we
    // attempt to focus anything in it.
    // See https://firefox-source-docs.mozilla.org/performance/bestpractices.html#detecting-and-avoiding-synchronous-reflow
    // for more information.
    await this.window.promiseDocumentFlushed(() => {});
    nextPanelView.focusSelectedElement();
  }

  /**
   * Attempt to clean up the attributes and properties set by `#transitionViews`
   * above. Which attributes and properties depends on the phase the transition
   * was left from.
   */
  #cleanupTransitionPhase() {
    if (!this._transitionDetails) {
      return;
    }

    let { phase, resolve, listener, cancelListener } = this._transitionDetails;
    this._transitionDetails = null;

    if (phase >= TRANSITION_PHASES.START) {
      this.#panel.removeAttribute("width");
      this.#panel.removeAttribute("height");
      this._viewContainer.style.removeProperty("height");
      this._viewContainer.style.removeProperty("width");
    }
    if (phase >= TRANSITION_PHASES.PREPARE) {
      this.#transitioning = false;
      this._viewStack.style.removeProperty("margin-inline-start");
      this._viewStack.style.removeProperty("transition");
    }
    if (phase >= TRANSITION_PHASES.TRANSITION) {
      this._viewStack.style.removeProperty("transform");
      if (listener) {
        this._viewContainer.removeEventListener("transitionend", listener);
      }
      if (cancelListener) {
        this._viewContainer.removeEventListener(
          "transitioncancel",
          cancelListener
        );
      }
      if (resolve) {
        resolve();
      }
    }
  }

  /**
   * Centralized event handler for the associated <panelmultiview>.
   *
   * @param {Event} aEvent
   *   The event being handled.
   */
  handleEvent(aEvent) {
    // Only process actual popup events from the panel or events we generate
    // ourselves, but not from menus being shown from within the panel.
    if (
      aEvent.type.startsWith("popup") &&
      aEvent.target != this.#panel &&
      aEvent.target != this.node
    ) {
      return;
    }
    switch (aEvent.type) {
      case "keydown": {
        // Since we start listening for the "keydown" event when the popup is
        // already showing and stop listening when the panel is hidden, we
        // always have at least one view open.
        let currentView = this.openViews[this.openViews.length - 1];
        currentView.keyNavigation(aEvent);
        break;
      }
      case "mousemove": {
        this.openViews.forEach(panelView => {
          if (!panelView.ignoreMouseMove) {
            panelView.clearNavigation();
          }
        });
        break;
      }
      case "popupshowing": {
        this._viewContainer.setAttribute("panelopen", "true");
        if (!this.node.hasAttribute("disablekeynav")) {
          // We add the keydown handler on the root so that it handles key
          // presses when a panel appears but doesn't get focus, as happens
          // when a button to open a panel is clicked with the mouse.
          // However, this means the listener is on an ancestor of the panel,
          // which means that handlers such as ToolbarKeyboardNavigator are
          // deeper in the tree. Therefore, this must be a capturing listener
          // so we get the event first.
          this.document.documentElement.addEventListener("keydown", this, true);
          this.#panel.addEventListener("mousemove", this);
        }
        break;
      }
      case "popupshown": {
        // The main view is always open and visible when the panel is first
        // shown, so we can check the height of the description elements it
        // contains and notify consumers using the ViewShown event. In order to
        // minimize flicker we need to allow synchronous reflows, and we still
        // make sure the ViewShown event is dispatched synchronously.
        let mainPanelView = this.openViews[0];
        this.#activateView(mainPanelView);
        break;
      }
      case "popuphidden": {
        // WebExtensions consumers can hide the popup from viewshowing, or
        // mid-transition, which disrupts our state:
        this.#transitioning = false;
        this._viewContainer.removeAttribute("panelopen");
        this.#cleanupTransitionPhase();
        this.document.documentElement.removeEventListener(
          "keydown",
          this,
          true
        );
        this.#panel.removeEventListener("mousemove", this);
        this.closeAllViews();

        // Clear the main view size caches. The dimensions could be different
        // when the popup is opened again, e.g. through touch mode sizing.
        this._viewContainer.style.removeProperty("min-height");
        this._viewStack.style.removeProperty("max-height");
        this._viewContainer.style.removeProperty("width");
        this._viewContainer.style.removeProperty("height");

        this.dispatchCustomEvent("PanelMultiViewHidden");
        break;
      }
    }
  }
};

/**
 * This is associated to <panelview> elements.
 */
export var PanelView = class extends AssociatedToNode {
  constructor(node) {
    super(node);

    /**
     * Indicates whether the view is active. When this is false, consumers can
     * wait for the ViewShown event to know when the view becomes active.
     */
    this.active = false;

    /**
     * Specifies whether the view should be focused when active. When this
     * is true, the first navigable element in the view will be focused
     * when the view becomes active. This should be set to true when the view
     * is activated from the keyboard. It will be set to false once the view
     * is active.
     */
    this.focusWhenActive = false;
  }

  /**
   * Indicates whether the view is open in the specified PanelMultiView object.
   *
   * @param {PanelMultiView} panelMultiView
   *   The PanelMultiView instance to check.
   * @returns {boolean}
   *   True if the PanelView is open in the specified PanelMultiView object.
   */
  isOpenIn(panelMultiView) {
    return this.node.panelMultiView == panelMultiView.node;
  }

  /**
   * The "mainview" attribute is set before the panel is opened when this view
   * is displayed as the main view, and is removed before the <panelview> is
   * displayed as a subview. The same view element can be displayed as a main
   * view and as a subview at different times.
   *
   * @param {boolean} value
   *   True to set the `"mainview"` attribute to `true`, otherwise removes the
   *   attribute.
   */
  set mainview(value) {
    if (value) {
      this.node.setAttribute("mainview", true);
    } else {
      this.node.removeAttribute("mainview");
    }
  }

  /**
   * Determines whether the view is visible. Setting this to false also resets
   * the "active" property.
   *
   * @param {boolean} value
   *   True to set the `"visible"` attribute to `true`, otherwise removes the
   *   attribute, and sets active and focusWhenActive to `false`.
   */
  set visible(value) {
    if (value) {
      this.node.setAttribute("visible", true);
    } else {
      this.node.removeAttribute("visible");
      this.active = false;
      this.focusWhenActive = false;
    }
  }

  /**
   * Constrains the width of this view using the "min-width" and "max-width"
   * styles. Setting this to zero removes the constraints.
   *
   * @param {number} value
   *   Sets the min and max width of the element to ${value}px.
   */
  set minMaxWidth(value) {
    let style = this.node.style;
    if (value) {
      style.minWidth = style.maxWidth = value + "px";
    } else {
      style.removeProperty("min-width");
      style.removeProperty("max-width");
    }
  }

  /**
   * Constrains the height of this view using the "min-height" and "max-height"
   * styles. Setting this to zero removes the constraints.
   *
   * @param {number} value
   *   Sets the min and max height of the element to ${value}px.
   */
  set minMaxHeight(value) {
    let style = this.node.style;
    if (value) {
      style.minHeight = style.maxHeight = value + "px";
    } else {
      style.removeProperty("min-height");
      style.removeProperty("max-height");
    }
  }

  /**
   * Adds a header with the given title, or removes it if the title is empty.
   *
   * If an element matching `.panel-header` is found in the PanelView, then
   * this method will attempt to set the textContent of the first `h1 > span`
   * underneath that `.panel-header` to `value`.
   *
   * Otherwise, this will attempt to insert a header element and a separator
   * beneath it, with the text of the header set to `value`.
   *
   * If `value` is null, then these elements are cleared and removed.
   *
   * @param {string} value
   *   The header to set.
   */
  set headerText(value) {
    let ensureHeaderSeparator = headerNode => {
      if (headerNode.nextSibling.tagName != "toolbarseparator") {
        let separator = this.document.createXULElement("toolbarseparator");
        this.node.insertBefore(separator, headerNode.nextSibling);
      }
    };

    // If the header already exists, update or remove it as requested.
    let isMainView = this.node.getAttribute("mainview");
    let header = this.node.querySelector(".panel-header");
    if (header) {
      let headerBackButton = header.querySelector(".subviewbutton-back");
      if (isMainView) {
        if (headerBackButton) {
          // A back button should not appear in a mainview.
          // This codepath can be reached if a user enters a panelview in
          // the overflow panel (like the Profiler), and then unpins it back to the toolbar.
          headerBackButton.remove();
        }
      }
      if (value) {
        if (
          !isMainView &&
          !headerBackButton &&
          !this.node.getAttribute("no-back-button")
        ) {
          // Add a back button when not in mainview (if it doesn't exist already),
          // also when a panelview specifies it doesn't want a back button,
          // like the Report Broken Site (sent) panelview.
          header.prepend(this.createHeaderBackButton());
        }
        // Set the header title based on the value given.
        header.querySelector(".panel-header > h1 > span").textContent = value;
        ensureHeaderSeparator(header);
      } else if (
        !this.node.getAttribute("has-custom-header") &&
        !this.node.getAttribute("mainview-with-header")
      ) {
        // No value supplied, and the panelview doesn't have a certain requirement
        // for any kind of header, so remove it and the following toolbarseparator.
        if (header.nextSibling.tagName == "toolbarseparator") {
          header.nextSibling.remove();
        }
        header.remove();
        return;
      }
      // Either the header exists and has been adjusted accordingly by now,
      // or it doesn't (or shouldn't) exist. Bail out to not create a duplicate header.
      return;
    }

    // The header doesn't and shouldn't exist, only create it if needed.
    if (!value) {
      return;
    }

    header = this.document.createXULElement("box");
    header.classList.add("panel-header");

    if (!isMainView) {
      let backButton = this.createHeaderBackButton();
      header.append(backButton);
    }

    let h1 = this.document.createElement("h1");
    let span = this.document.createElement("span");
    span.textContent = value;
    h1.appendChild(span);

    header.append(h1);
    this.node.prepend(header);

    ensureHeaderSeparator(header);
  }

  /**
   * Creates and returns a panel header back toolbarbutton.
   */
  createHeaderBackButton() {
    let backButton = this.document.createXULElement("toolbarbutton");
    backButton.className =
      "subviewbutton subviewbutton-iconic subviewbutton-back";
    backButton.setAttribute("closemenu", "none");
    backButton.setAttribute("tabindex", "0");
    backButton.setAttribute(
      "aria-label",
      lazy.gBundle.GetStringFromName("panel.back")
    );
    backButton.addEventListener("command", () => {
      // The panelmultiview element may change if the view is reused.
      this.node.panelMultiView.goBack();
      backButton.blur();
    });
    return backButton;
  }

  /**
   * Dispatches a custom event on the PanelView, and also makes sure that the
   * correct method is called on CustomizableWidget if applicable.
   *
   * @see AssociatedToNode.dispatchCustomEvent
   * @param {...*} args
   *   Additional arguments to be forwarded to the dispatchCustomEvent method of
   *   AssociatedToNode.
   */
  dispatchCustomEvent(...args) {
    lazy.CustomizableUI.ensureSubviewListeners(this.node);
    return super.dispatchCustomEvent(...args);
  }

  /**
   * Populates the "knownWidth" and "knownHeight" properties with the current
   * dimensions of the view. These may be zero if the view is invisible.
   *
   * These values are relevant during transitions and are retained for backwards
   * navigation if the view is still open but is invisible.
   */
  captureKnownSize() {
    let rect = this._getBoundsWithoutFlushing(this.node);
    this.knownWidth = rect.width;
    this.knownHeight = rect.height;
  }

  /**
   * Determine whether an element can only be navigated to with tab/shift+tab,
   * not the arrow keys.
   *
   * @param {DOMNode} element
   *   The element to check for navigation with tab only.
   * @returns {boolean}
   *   True if the element can only be navigated to with tab/shift+tab.
   */
  #isNavigableWithTabOnly(element) {
    let tag = element.localName;
    return (
      tag == "menulist" ||
      tag == "select" ||
      tag == "radiogroup" ||
      tag == "input" ||
      tag == "textarea" ||
      // Allow tab to reach embedded documents.
      tag == "browser" ||
      tag == "iframe" ||
      // This is currently needed for the unified extensions panel to allow
      // users to use up/down arrow to more quickly move between the extension
      // items. See Bug 1784118
      element.dataset?.navigableWithTabOnly === "true"
    );
  }

  /**
   * Make a TreeWalker for keyboard navigation.
   *
   * @param {boolean} arrowKey
   *   If `true`, elements only navigable with tab are excluded.
   * @returns {TreeWalker}
   *   The created TreeWalker instance.
   */
  #makeNavigableTreeWalker(arrowKey) {
    let filter = node => {
      if (node.disabled) {
        return NodeFilter.FILTER_REJECT;
      }
      let bounds = this._getBoundsWithoutFlushing(node);
      if (bounds.width == 0 || bounds.height == 0) {
        return NodeFilter.FILTER_REJECT;
      }
      let isNavigableWithTabOnly = this.#isNavigableWithTabOnly(node);
      // Early return when the node is navigable with tab only and we are using
      // arrow keys so that nodes like button, toolbarbutton, checkbox, etc.
      // can also be marked as "navigable with tab only", otherwise the next
      // condition will unconditionally make them focusable.
      if (arrowKey && isNavigableWithTabOnly) {
        return NodeFilter.FILTER_REJECT;
      }
      let localName = node.localName.toLowerCase();
      if (
        localName == "button" ||
        localName == "toolbarbutton" ||
        localName == "checkbox" ||
        localName == "a" ||
        localName == "moz-button" ||
        localName == "moz-box-button" ||
        localName == "moz-toggle" ||
        node.classList.contains("text-link") ||
        (!arrowKey && isNavigableWithTabOnly) ||
        node.dataset?.capturesFocus === "true"
      ) {
        // Set the tabindex attribute to make sure the node is focusable.
        // Don't do this for browser and iframe elements because this breaks
        // tabbing behavior. They're already focusable anyway.
        if (
          localName != "browser" &&
          localName != "iframe" &&
          !node.hasAttribute("tabindex") &&
          node.dataset?.capturesFocus !== "true"
        ) {
          node.setAttribute("tabindex", "-1");
        }
        return NodeFilter.FILTER_ACCEPT;
      }
      return NodeFilter.FILTER_SKIP;
    };
    return this.document.createTreeWalker(
      this.node,
      NodeFilter.SHOW_ELEMENT,
      filter
    );
  }

  /**
   * Get a TreeWalker which finds elements navigable with tab/shift+tab.
   *
   * This is currently pseudo private with the underscore because
   * AccessibilityUtils.js appears to be accessing this.
   *
   * @returns {TreeWalker}
   *   The TreeWalker for tab/shift+tab navigable elements.
   */
  #_tabNavigableWalker = null;
  get _tabNavigableWalker() {
    if (!this.#_tabNavigableWalker) {
      this.#_tabNavigableWalker = this.#makeNavigableTreeWalker(false);
    }
    return this.#_tabNavigableWalker;
  }

  /**
   * Get a TreeWalker which finds elements navigable with up/down arrow keys.
   *
   * @returns {TreeWalker}
   *   The TreeWalker for arrow key navigable elements.
   */
  #_arrowNavigableWalker = null;
  get #arrowNavigableWalker() {
    if (!this.#_arrowNavigableWalker) {
      this.#_arrowNavigableWalker = this.#makeNavigableTreeWalker(true);
    }
    return this.#_arrowNavigableWalker;
  }

  /**
   * Element that is currently selected with the keyboard, or null if no element
   * is selected. Since the reference is held weakly, it can become null or
   * undefined at any time.
   *
   * @type {DOMNode|null}
   *   The selected element, or null if no element is selected.
   */
  get selectedElement() {
    return this._selectedElement && this._selectedElement.get();
  }

  set selectedElement(value) {
    if (!value) {
      delete this._selectedElement;
    } else {
      this._selectedElement = Cu.getWeakReference(value);
    }
  }

  /**
   * Focuses and moves keyboard selection to the first navigable element.
   * This is a no-op if there are no navigable elements.
   *
   * @param {boolean} [homeKey=false]
   *   True if this is for the home key.
   * @param {boolean} [skipBack=false]
   *   True if the Back button should be skipped.
   */
  focusFirstNavigableElement(homeKey = false, skipBack = false) {
    // The home key is conceptually similar to the up/down arrow keys.
    let walker = homeKey
      ? this.#arrowNavigableWalker
      : this._tabNavigableWalker;
    walker.currentNode = walker.root;
    this.selectedElement = walker.firstChild();
    if (
      skipBack &&
      walker.currentNode &&
      walker.currentNode.classList.contains("subviewbutton-back") &&
      walker.nextNode()
    ) {
      this.selectedElement = walker.currentNode;
    }
    this.focusSelectedElement(/* byKey */ true);
  }

  /**
   * Focuses and moves keyboard selection to the last navigable element.
   * This is a no-op if there are no navigable elements.
   *
   * @param {boolean} [endKey=false]
   *   True if this is for the end key.
   */
  focusLastNavigableElement(endKey = false) {
    // The end key is conceptually similar to the up/down arrow keys.
    let walker = endKey ? this.#arrowNavigableWalker : this._tabNavigableWalker;
    walker.currentNode = walker.root;
    this.selectedElement = walker.lastChild();
    this.focusSelectedElement(/* byKey */ true);
  }

  /**
   * Based on going up or down, select the previous or next focusable element.
   *
   * @param {boolean} isDown
   *   True if the selection is going down, false if going up.
   * @param {boolean} [arrowKey=false]
   *   True if this is for the up/down arrow keys.
   * @returns {DOMNode} the element we selected.
   */
  moveSelection(isDown, arrowKey = false) {
    let walker = arrowKey
      ? this.#arrowNavigableWalker
      : this._tabNavigableWalker;
    let oldSel = this.selectedElement;
    let newSel;
    if (oldSel) {
      walker.currentNode = oldSel;
      newSel = isDown ? walker.nextNode() : walker.previousNode();
    }
    // If we couldn't find something, select the first or last item:
    if (!newSel) {
      walker.currentNode = walker.root;
      newSel = isDown ? walker.firstChild() : walker.lastChild();
    }
    this.selectedElement = newSel;
    return newSel;
  }

  /**
   * Allow for navigating subview buttons using the arrow keys and the Enter key.
   * The Up and Down keys can be used to navigate the list up and down and the
   * Enter, Right or Left - depending on the text direction - key can be used to
   * simulate a click on the currently selected button.
   * The Right or Left key - depending on the text direction - can be used to
   * navigate to the previous view, functioning as a shortcut for the view's
   * back button.
   * Thus, in LTR mode:
   *  - The Right key functions the same as the Enter key, simulating a click
   *  - The Left key triggers a navigation back to the previous view.
   *
   * Key navigation is only enabled while the view is active, meaning that this
   * method will return early if it is invoked during a sliding transition.
   *
   * @param {KeyEvent} event
   *   The KeyEvent to potentially perform navigation for.
   */
  keyNavigation(event) {
    if (!this.active) {
      return;
    }

    let focus = this.document.activeElement;
    // Make sure the focus is actually inside the panel. (It might not be if
    // the panel was opened with the mouse.) If it isn't, we don't care
    // about it for our purposes.
    // We use Node.compareDocumentPosition because Node.contains doesn't
    // behave as expected for anonymous content; e.g. the input inside a
    // textbox.
    if (
      focus &&
      !(
        this.node.compareDocumentPosition(focus) &
        Node.DOCUMENT_POSITION_CONTAINED_BY
      )
    ) {
      focus = null;
    }

    // Some panels contain embedded documents or need to capture focus events.
    // We can't manage keyboard navigation within those.
    if (
      focus &&
      (focus.tagName == "browser" ||
        focus.tagName == "iframe" ||
        focus.dataset?.capturesFocus === "true")
    ) {
      return;
    }

    let stop = () => {
      event.stopPropagation();
      event.preventDefault();
    };

    // If the focused element is only navigable with tab, it wants the arrow
    // keys, etc. We shouldn't handle any keys except tab and shift+tab.
    // We make a function for this for performance reasons: we only want to
    // check this for keys we potentially care about, not *all* keys.
    let tabOnly = () => {
      // We use the real focus rather than this.selectedElement because focus
      // might have been moved without keyboard navigation (e.g. mouse click)
      // and this.selectedElement is only updated for keyboard navigation.
      return focus && this.#isNavigableWithTabOnly(focus);
    };

    // If a context menu is open, we must let it handle all keys.
    // Normally, this just happens, but because we have a capturing root
    // element keydown listener, our listener takes precedence.
    // Again, we only want to do this check on demand for performance.
    let isContextMenuOpen = () => {
      if (!focus) {
        return false;
      }
      let contextNode = focus.closest("[context]");
      if (!contextNode) {
        return false;
      }
      let context = contextNode.getAttribute("context");
      if (!context) {
        return false;
      }
      let popup = this.document.getElementById(context);
      return popup && popup.state == "open";
    };

    this.ignoreMouseMove = false;

    let keyCode = event.code;
    switch (keyCode) {
      case "ArrowDown":
      case "ArrowUp":
        if (tabOnly()) {
          break;
        }
      // Fall-through...
      case "Tab": {
        if (
          isContextMenuOpen() ||
          // Tab in an open menulist should close it.
          (focus && focus.localName == "menulist" && focus.open)
        ) {
          break;
        }
        stop();
        let isDown =
          keyCode == "ArrowDown" || (keyCode == "Tab" && !event.shiftKey);
        let button = this.moveSelection(isDown, keyCode != "Tab");
        button.focus();
        break;
      }
      case "Home":
        if (tabOnly() || isContextMenuOpen()) {
          break;
        }
        stop();
        this.focusFirstNavigableElement(true);
        break;
      case "End":
        if (tabOnly() || isContextMenuOpen()) {
          break;
        }
        stop();
        this.focusLastNavigableElement(true);
        break;
      case "ArrowLeft":
      case "ArrowRight": {
        if (tabOnly() || isContextMenuOpen()) {
          break;
        }
        stop();
        if (
          (!this.window.RTL_UI && keyCode == "ArrowLeft") ||
          (this.window.RTL_UI && keyCode == "ArrowRight")
        ) {
          this.node.panelMultiView.goBack();
          break;
        }
        // If the current button is _not_ one that points to a subview, pressing
        // the arrow key shouldn't do anything.
        let button = this.selectedElement;
        if (
          !button ||
          !(
            button.classList.contains("subviewbutton-nav") ||
            button.classList.contains("moz-button-subviewbutton-nav")
          )
        ) {
          break;
        }
      }
      // Fall-through...
      case "Space":
      case "NumpadEnter":
      case "Enter": {
        if (tabOnly() || isContextMenuOpen()) {
          break;
        }
        let button = this.selectedElement;
        if (!button || button?.localName == "moz-toggle") {
          break;
        }
        stop();

        this._doingKeyboardActivation = true;
        const details = {
          bubbles: true,
          ctrlKey: event.ctrlKey,
          altKey: event.altKey,
          shiftKey: event.shiftKey,
          metaKey: event.metaKey,
        };
        // The a11y-checks want the target to be accessible. For moz-button the
        // focus is really on the inner button which is accessible, but we check
        // a11y against the event target (moz-button) which fails. Dispatch from
        // the inner button element instead.
        let target = button;
        if (
          button.localName == "moz-button" ||
          button.localName == "moz-box-button"
        ) {
          target = button.buttonEl;
          details.composed = true;
        }
        let dispEvent = new event.target.ownerGlobal.MouseEvent(
          "mousedown",
          details
        );
        target.dispatchEvent(dispEvent);
        // This event will trigger a command event too.
        dispEvent = new event.target.ownerGlobal.PointerEvent("click", details);
        target.dispatchEvent(dispEvent);
        this._doingKeyboardActivation = false;
        break;
      }
    }
  }

  /**
   * Focus the last selected element in the view, if any.
   *
   * @param {boolean} [byKey=false]
   *   True if focus was moved by the user pressing a key. Needed to ensure we
   *   show focus styles in the right cases.
   */
  focusSelectedElement(byKey = false) {
    let selected = this.selectedElement;
    if (selected) {
      let flag = byKey ? Services.focus.FLAG_BYKEY : 0;
      Services.focus.setFocus(selected, flag);
    }
  }

  /**
   * Clear all traces of keyboard navigation happening right now.
   */
  clearNavigation() {
    let selected = this.selectedElement;
    if (selected) {
      selected.blur();
      this.selectedElement = null;
    }
  }
};
PK