Adding upstream version 110.0.1.upstream/110.0.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 6286 insertions, 0 deletions

diff --git a/browser/components/customizableui/CustomizableUI.jsm b/browser/components/customizableui/CustomizableUI.jsm
new file mode 100644
index 0000000000..d43758e275
--- /dev/null
+++ b/browser/components/customizableui/CustomizableUI.jsm
@@ -0,0 +1,6286 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+var EXPORTED_SYMBOLS = ["CustomizableUI"];
+
+const { XPCOMUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/XPCOMUtils.sys.mjs"
+);
+const { AppConstants } = ChromeUtils.importESModule(
+  "resource://gre/modules/AppConstants.sys.mjs"
+);
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  PrivateBrowsingUtils: "resource://gre/modules/PrivateBrowsingUtils.sys.mjs",
+  ShortcutUtils: "resource://gre/modules/ShortcutUtils.sys.mjs",
+});
+
+XPCOMUtils.defineLazyModuleGetters(lazy, {
+  AddonManager: "resource://gre/modules/AddonManager.jsm",
+  AddonManagerPrivate: "resource://gre/modules/AddonManager.jsm",
+  SearchWidgetTracker: "resource:///modules/SearchWidgetTracker.jsm",
+  CustomizableWidgets: "resource:///modules/CustomizableWidgets.jsm",
+  PanelMultiView: "resource:///modules/PanelMultiView.jsm",
+  BrowserUsageTelemetry: "resource:///modules/BrowserUsageTelemetry.jsm",
+  HomePage: "resource:///modules/HomePage.jsm",
+});
+
+XPCOMUtils.defineLazyGetter(lazy, "gWidgetsBundle", function() {
+  const kUrl =
+    "chrome://browser/locale/customizableui/customizableWidgets.properties";
+  return Services.strings.createBundle(kUrl);
+});
+
+const kDefaultThemeID = "default-theme@mozilla.org";
+
+const kSpecialWidgetPfx = "customizableui-special-";
+
+const kPrefCustomizationState = "browser.uiCustomization.state";
+const kPrefCustomizationAutoAdd = "browser.uiCustomization.autoAdd";
+const kPrefCustomizationDebug = "browser.uiCustomization.debug";
+const kPrefDrawInTitlebar = "browser.tabs.inTitlebar";
+const kPrefUIDensity = "browser.uidensity";
+const kPrefAutoTouchMode = "browser.touchmode.auto";
+const kPrefAutoHideDownloadsButton = "browser.download.autohideButton";
+const kPrefProtonToolbarVersion = "browser.proton.toolbar.version";
+const kPrefHomeButtonUsed = "browser.engagement.home-button.has-used";
+const kPrefLibraryButtonUsed = "browser.engagement.library-button.has-used";
+const kPrefSidebarButtonUsed = "browser.engagement.sidebar-button.has-used";
+const kPrefUnifiedExtensionsEnabled = "extensions.unifiedExtensions.enabled";
+
+const kExpectedWindowURL = AppConstants.BROWSER_CHROME_URL;
+
+var gDefaultTheme;
+var gSelectedTheme;
+
+/**
+ * The keys are the handlers that are fired when the event type (the value)
+ * is fired on the subview. A widget that provides a subview has the option
+ * of providing onViewShowing and onViewHiding event handlers.
+ */
+const kSubviewEvents = ["ViewShowing", "ViewHiding"];
+
+/**
+ * The current version. We can use this to auto-add new default widgets as necessary.
+ * (would be const but isn't because of testing purposes)
+ */
+var kVersion = 18;
+
+/**
+ * Buttons removed from built-ins by version they were removed. kVersion must be
+ * bumped any time a new id is added to this. Use the button id as key, and
+ * version the button is removed in as the value.  e.g. "pocket-button": 5
+ */
+var ObsoleteBuiltinButtons = {
+  "feed-button": 15,
+};
+
+/**
+ * gPalette is a map of every widget that CustomizableUI.jsm knows about, keyed
+ * on their IDs.
+ */
+var gPalette = new Map();
+
+/**
+ * gAreas maps area IDs to Sets of properties about those areas. An area is a
+ * place where a widget can be put.
+ */
+var gAreas = new Map();
+
+/**
+ * gPlacements maps area IDs to Arrays of widget IDs, indicating that the widgets
+ * are placed within that area (either directly in the area node, or in the
+ * customizationTarget of the node).
+ */
+var gPlacements = new Map();
+
+/**
+ * gFuturePlacements represent placements that will happen for areas that have
+ * not yet loaded (due to lazy-loading). This can occur when add-ons register
+ * widgets.
+ */
+var gFuturePlacements = new Map();
+
+var gSupportedWidgetTypes = new Set([
+  // A button that does a command.
+  "button",
+
+  // A button that opens a view in a panel (or in a subview of the panel).
+  "view",
+
+  // A combination of the above, which looks different depending on whether it's
+  // located in the toolbar or in the panel: When located in the toolbar, shown
+  // as a combined item of a button and a dropmarker button. The button triggers
+  // the command and the dropmarker button opens the view. When located in the
+  // panel, shown as one item which opens the view, and the button command
+  // cannot be triggered separately.
+  "button-and-view",
+
+  // A custom widget that defines its own markup.
+  "custom",
+]);
+
+/**
+ * gPanelsForWindow is a list of known panels in a window which we may need to close
+ * should command events fire which target them.
+ */
+var gPanelsForWindow = new WeakMap();
+
+/**
+ * gSeenWidgets remembers which widgets the user has seen for the first time
+ * before. This way, if a new widget is created, and the user has not seen it
+ * before, it can be put in its default location. Otherwise, it remains in the
+ * palette.
+ */
+var gSeenWidgets = new Set();
+
+/**
+ * gDirtyAreaCache is a set of area IDs for areas where items have been added,
+ * moved or removed at least once. This set is persisted, and is used to
+ * optimize building of toolbars in the default case where no toolbars should
+ * be "dirty".
+ */
+var gDirtyAreaCache = new Set();
+
+/**
+ * gPendingBuildAreas is a map from area IDs to map from build nodes to their
+ * existing children at the time of node registration, that are waiting
+ * for the area to be registered
+ */
+var gPendingBuildAreas = new Map();
+
+var gSavedState = null;
+var gRestoring = false;
+var gDirty = false;
+var gInBatchStack = 0;
+var gResetting = false;
+var gUndoResetting = false;
+
+/**
+ * gBuildAreas maps area IDs to actual area nodes within browser windows.
+ */
+var gBuildAreas = new Map();
+
+/**
+ * gBuildWindows is a map of windows that have registered build areas, mapped
+ * to a Set of known toolboxes in that window.
+ */
+var gBuildWindows = new Map();
+
+var gNewElementCount = 0;
+var gGroupWrapperCache = new Map();
+var gSingleWrapperCache = new WeakMap();
+var gListeners = new Set();
+
+var gUIStateBeforeReset = {
+  uiCustomizationState: null,
+  drawInTitlebar: null,
+  currentTheme: null,
+  uiDensity: null,
+  autoTouchMode: null,
+};
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  lazy,
+  "gDebuggingEnabled",
+  kPrefCustomizationDebug,
+  false,
+  (pref, oldVal, newVal) => {
+    if (typeof lazy.log != "undefined") {
+      lazy.log.maxLogLevel = newVal ? "all" : "log";
+    }
+  }
+);
+
+XPCOMUtils.defineLazyPreferenceGetter(
+  lazy,
+  "gUnifiedExtensionsEnabled",
+  kPrefUnifiedExtensionsEnabled,
+  false
+);
+
+XPCOMUtils.defineLazyGetter(lazy, "log", () => {
+  let { ConsoleAPI } = ChromeUtils.importESModule(
+    "resource://gre/modules/Console.sys.mjs"
+  );
+  let consoleOptions = {
+    maxLogLevel: lazy.gDebuggingEnabled ? "all" : "log",
+    prefix: "CustomizableUI",
+  };
+  return new ConsoleAPI(consoleOptions);
+});
+
+var CustomizableUIInternal = {
+  initialize() {
+    lazy.log.debug("Initializing");
+
+    lazy.AddonManagerPrivate.databaseReady.then(async () => {
+      lazy.AddonManager.addAddonListener(this);
+
+      let addons = await lazy.AddonManager.getAddonsByTypes(["theme"]);
+      gDefaultTheme = addons.find(addon => addon.id == kDefaultThemeID);
+      gSelectedTheme = addons.find(addon => addon.isActive) || gDefaultTheme;
+    });
+
+    this.addListener(this);
+    this._defineBuiltInWidgets();
+    this.loadSavedState();
+    this._updateForNewVersion();
+    this._updateForNewProtonVersion();
+    this._updateForUnifiedExtensions();
+    this._markObsoleteBuiltinButtonsSeen();
+
+    this.registerArea(
+      CustomizableUI.AREA_FIXED_OVERFLOW_PANEL,
+      {
+        type: CustomizableUI.TYPE_PANEL,
+        defaultPlacements: [],
+        anchor: "nav-bar-overflow-button",
+      },
+      true
+    );
+
+    this.registerArea(
+      CustomizableUI.AREA_ADDONS,
+      {
+        type: CustomizableUI.TYPE_PANEL,
+        defaultPlacements: [],
+        anchor: "unified-extensions-button",
+      },
+      false
+    );
+
+    let navbarPlacements = [
+      "back-button",
+      "forward-button",
+      "stop-reload-button",
+      Services.policies.isAllowed("removeHomeButtonByDefault")
+        ? null
+        : "home-button",
+      "spring",
+      "urlbar-container",
+      "spring",
+      "save-to-pocket-button",
+      "downloads-button",
+      AppConstants.MOZ_DEV_EDITION ? "developer-button" : null,
+      "fxa-toolbar-menu-button",
+    ].filter(name => name);
+
+    this.registerArea(
+      CustomizableUI.AREA_NAVBAR,
+      {
+        type: CustomizableUI.TYPE_TOOLBAR,
+        overflowable: true,
+        defaultPlacements: navbarPlacements,
+        defaultCollapsed: false,
+      },
+      true
+    );
+
+    if (AppConstants.MENUBAR_CAN_AUTOHIDE) {
+      this.registerArea(
+        CustomizableUI.AREA_MENUBAR,
+        {
+          type: CustomizableUI.TYPE_TOOLBAR,
+          defaultPlacements: ["menubar-items"],
+          defaultCollapsed: true,
+        },
+        true
+      );
+    }
+
+    this.registerArea(
+      CustomizableUI.AREA_TABSTRIP,
+      {
+        type: CustomizableUI.TYPE_TOOLBAR,
+        defaultPlacements: [
+          "firefox-view-button",
+          "tabbrowser-tabs",
+          "new-tab-button",
+          "alltabs-button",
+        ],
+        defaultCollapsed: null,
+      },
+      true
+    );
+    this.registerArea(
+      CustomizableUI.AREA_BOOKMARKS,
+      {
+        type: CustomizableUI.TYPE_TOOLBAR,
+        defaultPlacements: ["personal-bookmarks"],
+        defaultCollapsed: "newtab",
+      },
+      true
+    );
+
+    lazy.SearchWidgetTracker.init();
+
+    Services.obs.addObserver(this, "browser-set-toolbar-visibility");
+  },
+
+  onEnabled(addon) {
+    if (addon.type == "theme") {
+      gSelectedTheme = addon;
+    }
+  },
+
+  get _builtinAreas() {
+    return new Set([
+      ...this._builtinToolbars,
+      CustomizableUI.AREA_FIXED_OVERFLOW_PANEL,
+      CustomizableUI.AREA_ADDONS,
+    ]);
+  },
+
+  get _builtinToolbars() {
+    let toolbars = new Set([
+      CustomizableUI.AREA_NAVBAR,
+      CustomizableUI.AREA_BOOKMARKS,
+      CustomizableUI.AREA_TABSTRIP,
+    ]);
+    if (AppConstants.platform != "macosx") {
+      toolbars.add(CustomizableUI.AREA_MENUBAR);
+    }
+    return toolbars;
+  },
+
+  _defineBuiltInWidgets() {
+    for (let widgetDefinition of lazy.CustomizableWidgets) {
+      this.createBuiltinWidget(widgetDefinition);
+    }
+  },
+
+  // eslint-disable-next-line complexity
+  _updateForNewVersion() {
+    // We should still enter even if gSavedState.currentVersion >= kVersion
+    // because the per-widget pref facility is independent of versioning.
+    if (!gSavedState) {
+      // Flip all the prefs so we don't try to re-introduce later:
+      for (let [, widget] of gPalette) {
+        if (widget.defaultArea && widget._introducedInVersion === "pref") {
+          let prefId = "browser.toolbarbuttons.introduced." + widget.id;
+          Services.prefs.setBoolPref(prefId, true);
+        }
+      }
+      return;
+    }
+
+    let currentVersion = gSavedState.currentVersion;
+    for (let [id, widget] of gPalette) {
+      if (widget.defaultArea) {
+        let shouldAdd = false;
+        let shouldSetPref = false;
+        let prefId = "browser.toolbarbuttons.introduced." + widget.id;
+        if (widget._introducedInVersion === "pref") {
+          try {
+            shouldAdd = !Services.prefs.getBoolPref(prefId);
+          } catch (ex) {
+            // Pref doesn't exist:
+            shouldAdd = true;
+          }
+          shouldSetPref = shouldAdd;
+        } else if (widget._introducedInVersion > currentVersion) {
+          shouldAdd = true;
+        }
+
+        if (shouldAdd) {
+          let futurePlacements = gFuturePlacements.get(widget.defaultArea);
+          if (futurePlacements) {
+            futurePlacements.add(id);
+          } else {
+            gFuturePlacements.set(widget.defaultArea, new Set([id]));
+          }
+          if (shouldSetPref) {
+            Services.prefs.setBoolPref(prefId, true);
+          }
+        }
+      }
+    }
+
+    // Nothing to migrate now if we don't have placements.
+    if (!gSavedState.placements) {
+      return;
+    }
+
+    if (
+      currentVersion < 7 &&
+      gSavedState.placements[CustomizableUI.AREA_NAVBAR]
+    ) {
+      let placements = gSavedState.placements[CustomizableUI.AREA_NAVBAR];
+      let newPlacements = [
+        "back-button",
+        "forward-button",
+        "stop-reload-button",
+        "home-button",
+      ];
+      for (let button of placements) {
+        if (!newPlacements.includes(button)) {
+          newPlacements.push(button);
+        }
+      }
+
+      if (!newPlacements.includes("sidebar-button")) {
+        newPlacements.push("sidebar-button");
+      }
+
+      gSavedState.placements[CustomizableUI.AREA_NAVBAR] = newPlacements;
+    }
+
+    if (currentVersion < 8 && gSavedState.placements["PanelUI-contents"]) {
+      let savedPanelPlacements = gSavedState.placements["PanelUI-contents"];
+      delete gSavedState.placements["PanelUI-contents"];
+      let defaultPlacements = [
+        "edit-controls",
+        "zoom-controls",
+        "new-window-button",
+        "privatebrowsing-button",
+        "save-page-button",
+        "print-button",
+        "history-panelmenu",
+        "fullscreen-button",
+        "find-button",
+        "preferences-button",
+        "add-ons-button",
+        "sync-button",
+      ];
+
+      if (!AppConstants.MOZ_DEV_EDITION) {
+        defaultPlacements.splice(-1, 0, "developer-button");
+      }
+
+      let showCharacterEncoding = Services.prefs.getComplexValue(
+        "browser.menu.showCharacterEncoding",
+        Ci.nsIPrefLocalizedString
+      ).data;
+      if (showCharacterEncoding == "true") {
+        defaultPlacements.push("characterencoding-button");
+      }
+
+      savedPanelPlacements = savedPanelPlacements.filter(
+        id => !defaultPlacements.includes(id)
+      );
+
+      if (savedPanelPlacements.length) {
+        gSavedState.placements[
+          CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+        ] = savedPanelPlacements;
+      }
+    }
+
+    if (currentVersion < 9 && gSavedState.placements["nav-bar"]) {
+      let placements = gSavedState.placements["nav-bar"];
+      if (placements.includes("urlbar-container")) {
+        let urlbarIndex = placements.indexOf("urlbar-container");
+        let secondSpringIndex = urlbarIndex + 1;
+        // Insert if there isn't already a spring before the urlbar
+        if (
+          urlbarIndex == 0 ||
+          !placements[urlbarIndex - 1].startsWith(kSpecialWidgetPfx + "spring")
+        ) {
+          placements.splice(urlbarIndex, 0, "spring");
+          // The url bar is now 1 index later, so increment the insertion point for
+          // the second spring.
+          secondSpringIndex++;
+        }
+        // If the search container is present, insert after the search container
+        // instead of after the url bar
+        let searchContainerIndex = placements.indexOf("search-container");
+        if (searchContainerIndex != -1) {
+          secondSpringIndex = searchContainerIndex + 1;
+        }
+        if (
+          secondSpringIndex == placements.length ||
+          !placements[secondSpringIndex].startsWith(
+            kSpecialWidgetPfx + "spring"
+          )
+        ) {
+          placements.splice(secondSpringIndex, 0, "spring");
+        }
+      }
+
+      // Finally, replace the bookmarks menu button with the library one if present
+      if (placements.includes("bookmarks-menu-button")) {
+        let bmbIndex = placements.indexOf("bookmarks-menu-button");
+        placements.splice(bmbIndex, 1);
+        let downloadButtonIndex = placements.indexOf("downloads-button");
+        let libraryIndex =
+          downloadButtonIndex == -1 ? bmbIndex : downloadButtonIndex + 1;
+        placements.splice(libraryIndex, 0, "library-button");
+      }
+    }
+
+    if (currentVersion < 10) {
+      for (let placements of Object.values(gSavedState.placements)) {
+        if (placements.includes("webcompat-reporter-button")) {
+          placements.splice(placements.indexOf("webcompat-reporter-button"), 1);
+          break;
+        }
+      }
+    }
+
+    // Move the downloads button to the default position in the navbar if it's
+    // not there already.
+    if (currentVersion < 11) {
+      let navbarPlacements = gSavedState.placements[CustomizableUI.AREA_NAVBAR];
+      // First remove from wherever it currently lives, if anywhere:
+      for (let placements of Object.values(gSavedState.placements)) {
+        let existingIndex = placements.indexOf("downloads-button");
+        if (existingIndex != -1) {
+          placements.splice(existingIndex, 1);
+          break; // It can only be in 1 place, so no point looking elsewhere.
+        }
+      }
+
+      // Now put the button in the navbar in the correct spot:
+      if (navbarPlacements) {
+        let insertionPoint = navbarPlacements.indexOf("urlbar-container");
+        // Deliberately iterate to 1 past the end of the array to insert at the
+        // end if need be.
+        while (++insertionPoint < navbarPlacements.length) {
+          let widget = navbarPlacements[insertionPoint];
+          // If we find a non-searchbar, non-spacer node, break out of the loop:
+          if (
+            widget != "search-container" &&
+            !this.matchingSpecials(widget, "spring")
+          ) {
+            break;
+          }
+        }
+        // We either found the right spot, or reached the end of the
+        // placements, so insert here:
+        navbarPlacements.splice(insertionPoint, 0, "downloads-button");
+      }
+    }
+
+    if (currentVersion < 12) {
+      const removedButtons = [
+        "loop-call-button",
+        "loop-button-throttled",
+        "pocket-button",
+      ];
+      for (let placements of Object.values(gSavedState.placements)) {
+        for (let button of removedButtons) {
+          let buttonIndex = placements.indexOf(button);
+          if (buttonIndex != -1) {
+            placements.splice(buttonIndex, 1);
+          }
+        }
+      }
+    }
+
+    // Remove the old placements from the now-gone Nightly-only
+    // "New non-e10s window" button.
+    if (currentVersion < 13) {
+      for (let placements of Object.values(gSavedState.placements)) {
+        let buttonIndex = placements.indexOf("e10s-button");
+        if (buttonIndex != -1) {
+          placements.splice(buttonIndex, 1);
+        }
+      }
+    }
+
+    // Remove unsupported custom toolbar saved placements
+    if (currentVersion < 14) {
+      for (let area in gSavedState.placements) {
+        if (!this._builtinAreas.has(area)) {
+          delete gSavedState.placements[area];
+        }
+      }
+    }
+
+    // Add the FxA toolbar menu as the right most button item
+    if (currentVersion < 16) {
+      let navbarPlacements = gSavedState.placements[CustomizableUI.AREA_NAVBAR];
+      // Place the menu item as the first item to the left of the hamburger menu
+      if (navbarPlacements) {
+        navbarPlacements.push("fxa-toolbar-menu-button");
+      }
+    }
+
+    // Add the save to Pocket button left of downloads button.
+    if (currentVersion < 17) {
+      let navbarPlacements = gSavedState.placements[CustomizableUI.AREA_NAVBAR];
+      let persistedPageActionsPref = Services.prefs.getCharPref(
+        "browser.pageActions.persistedActions",
+        ""
+      );
+      let pocketPreviouslyInUrl = true;
+      try {
+        let persistedPageActionsData = JSON.parse(persistedPageActionsPref);
+        // If Pocket was previously not in the url bar, let's not put it in the toolbar.
+        // It'll still be an option to add from the customization page.
+        pocketPreviouslyInUrl = persistedPageActionsData.idsInUrlbar.includes(
+          "pocket"
+        );
+      } catch (e) {}
+      if (navbarPlacements && pocketPreviouslyInUrl) {
+        // Pocket's new home is next to the downloads button, or the next best spot.
+        let newPosition =
+          navbarPlacements.indexOf("downloads-button") ??
+          navbarPlacements.indexOf("fxa-toolbar-menu-button") ??
+          navbarPlacements.length;
+
+        navbarPlacements.splice(newPosition, 0, "save-to-pocket-button");
+      }
+    }
+
+    // Add firefox-view if not present
+    if (currentVersion < 18) {
+      let tabstripPlacements =
+        gSavedState.placements[CustomizableUI.AREA_TABSTRIP];
+      if (
+        tabstripPlacements &&
+        !tabstripPlacements.includes("firefox-view-button")
+      ) {
+        tabstripPlacements.unshift("firefox-view-button");
+      }
+    }
+  },
+
+  _updateForNewProtonVersion() {
+    const VERSION = 3;
+    let currentVersion = Services.prefs.getIntPref(
+      kPrefProtonToolbarVersion,
+      0
+    );
+    if (currentVersion >= VERSION) {
+      return;
+    }
+
+    let placements = gSavedState?.placements?.[CustomizableUI.AREA_NAVBAR];
+
+    if (!placements) {
+      // The profile was created with this version, so no need to migrate.
+      Services.prefs.setIntPref(kPrefProtonToolbarVersion, VERSION);
+      return;
+    }
+
+    // Remove the home button if it hasn't been used and is set to about:home
+    if (currentVersion < 1) {
+      let homePage = lazy.HomePage.get();
+      if (
+        placements.includes("home-button") &&
+        !Services.prefs.getBoolPref(kPrefHomeButtonUsed) &&
+        (homePage == "about:home" || homePage == "about:blank") &&
+        Services.policies.isAllowed("removeHomeButtonByDefault")
+      ) {
+        placements.splice(placements.indexOf("home-button"), 1);
+      }
+    }
+
+    // Remove the library button if it hasn't been used
+    if (currentVersion < 2) {
+      if (
+        placements.includes("library-button") &&
+        !Services.prefs.getBoolPref(kPrefLibraryButtonUsed)
+      ) {
+        placements.splice(placements.indexOf("library-button"), 1);
+      }
+    }
+
+    // Remove the library button if it hasn't been used
+    if (currentVersion < 3) {
+      if (
+        placements.includes("sidebar-button") &&
+        !Services.prefs.getBoolPref(kPrefSidebarButtonUsed)
+      ) {
+        placements.splice(placements.indexOf("sidebar-button"), 1);
+      }
+    }
+
+    Services.prefs.setIntPref(kPrefProtonToolbarVersion, VERSION);
+  },
+
+  _updateForUnifiedExtensions() {
+    if (!gSavedState?.placements) {
+      return;
+    }
+
+    let overflowPlacements =
+      gSavedState.placements[CustomizableUI.AREA_FIXED_OVERFLOW_PANEL] || [];
+    // The most likely case is that there are no AREA_ADDONS placements, in which case the
+    // array won't exist.
+    let addonsPlacements =
+      gSavedState.placements[CustomizableUI.AREA_ADDONS] || [];
+
+    if (lazy.gUnifiedExtensionsEnabled) {
+      // Migration algorithm for transitioning to Unified Extensions:
+      //
+      // 1. Create two arrays, one for extension widgets, one for built-in widgets.
+      // 2. Iterate all items in the overflow panel, and push them into the
+      //    appropriate array based on whether or not its an extension widget.
+      // 3. Overwrite the overflow panel placements with the built-in widgets array.
+      // 4. Prepend the extension widgets to the addonsPlacements array. Note that this
+      //    does not overwrite this array as a precaution because it's possible
+      //    (though pretty unlikely) that some widgets are already there.
+      //
+      // For extension widgets that were in the palette, they will be appended to the
+      // addons area when they're created within createWidget.
+      let extWidgets = [];
+      let builtInWidgets = [];
+      for (let widgetId of overflowPlacements) {
+        if (CustomizableUI.isWebExtensionWidget(widgetId)) {
+          extWidgets.push(widgetId);
+        } else {
+          builtInWidgets.push(widgetId);
+        }
+      }
+      gSavedState.placements[
+        CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+      ] = builtInWidgets;
+      gSavedState.placements[CustomizableUI.AREA_ADDONS] = [
+        ...extWidgets,
+        ...addonsPlacements,
+      ];
+    } else {
+      // This is an emergency backstop in case things go sideways and we need to
+      // temporarily flip back the Unified Extensions pref if it had already been
+      // enabled. We will do simplest thing and just empty the AREA_ADDONS placements,
+      // and append them to the bottom of the overflow panel, and then blow away
+      // the AREA_ADDONS placements.
+      gSavedState.placements[CustomizableUI.AREA_FIXED_OVERFLOW_PANEL] = [
+        ...overflowPlacements,
+        ...addonsPlacements,
+      ];
+      delete gSavedState.placements[CustomizableUI.AREA_ADDONS];
+    }
+  },
+
+  /**
+   * _markObsoleteBuiltinButtonsSeen
+   * when upgrading, ensure obsoleted buttons are in seen state.
+   */
+  _markObsoleteBuiltinButtonsSeen() {
+    if (!gSavedState) {
+      return;
+    }
+    let currentVersion = gSavedState.currentVersion;
+    if (currentVersion >= kVersion) {
+      return;
+    }
+    // we're upgrading, update state if necessary
+    for (let id in ObsoleteBuiltinButtons) {
+      let version = ObsoleteBuiltinButtons[id];
+      if (version == kVersion) {
+        gSeenWidgets.add(id);
+        gDirty = true;
+      }
+    }
+  },
+
+  _placeNewDefaultWidgetsInArea(aArea) {
+    let futurePlacedWidgets = gFuturePlacements.get(aArea);
+    let savedPlacements =
+      gSavedState && gSavedState.placements && gSavedState.placements[aArea];
+    let defaultPlacements = gAreas.get(aArea).get("defaultPlacements");
+    if (
+      !savedPlacements ||
+      !savedPlacements.length ||
+      !futurePlacedWidgets ||
+      !defaultPlacements ||
+      !defaultPlacements.length
+    ) {
+      return;
+    }
+    let defaultWidgetIndex = -1;
+
+    for (let widgetId of futurePlacedWidgets) {
+      let widget = gPalette.get(widgetId);
+      if (
+        !widget ||
+        widget.source !== CustomizableUI.SOURCE_BUILTIN ||
+        !widget.defaultArea ||
+        !widget._introducedInVersion ||
+        savedPlacements.includes(widget.id)
+      ) {
+        continue;
+      }
+      defaultWidgetIndex = defaultPlacements.indexOf(widget.id);
+      if (defaultWidgetIndex === -1) {
+        continue;
+      }
+      // Now we know that this widget should be here by default, was newly introduced,
+      // and we have a saved state to insert into, and a default state to work off of.
+      // Try introducing after widgets that come before it in the default placements:
+      for (let i = defaultWidgetIndex; i >= 0; i--) {
+        // Special case: if the defaults list this widget as coming first, insert at the beginning:
+        if (i === 0 && i === defaultWidgetIndex) {
+          savedPlacements.splice(0, 0, widget.id);
+          // Before you ask, yes, deleting things inside a let x of y loop where y is a Set is
+          // safe, and we won't skip any items.
+          futurePlacedWidgets.delete(widget.id);
+          gDirty = true;
+          break;
+        }
+        // Otherwise, if we're somewhere other than the beginning, check if the previous
+        // widget is in the saved placements.
+        if (i) {
+          let previousWidget = defaultPlacements[i - 1];
+          let previousWidgetIndex = savedPlacements.indexOf(previousWidget);
+          if (previousWidgetIndex != -1) {
+            savedPlacements.splice(previousWidgetIndex + 1, 0, widget.id);
+            futurePlacedWidgets.delete(widget.id);
+            gDirty = true;
+            break;
+          }
+        }
+      }
+      // The loop above either inserts the item or doesn't - either way, we can get away
+      // with doing nothing else now; if the item remains in gFuturePlacements, we'll
+      // add it at the end in restoreStateForArea.
+    }
+    this.saveState();
+  },
+
+  getCustomizationTarget(aElement) {
+    if (!aElement) {
+      return null;
+    }
+
+    if (
+      !aElement._customizationTarget &&
+      aElement.hasAttribute("customizable")
+    ) {
+      let id = aElement.getAttribute("customizationtarget");
+      if (id) {
+        aElement._customizationTarget = aElement.ownerDocument.getElementById(
+          id
+        );
+      }
+
+      if (!aElement._customizationTarget) {
+        aElement._customizationTarget = aElement;
+      }
+    }
+
+    return aElement._customizationTarget;
+  },
+
+  wrapWidget(aWidgetId) {
+    if (gGroupWrapperCache.has(aWidgetId)) {
+      return gGroupWrapperCache.get(aWidgetId);
+    }
+
+    let provider = this.getWidgetProvider(aWidgetId);
+    if (!provider) {
+      return null;
+    }
+
+    if (provider == CustomizableUI.PROVIDER_API) {
+      let widget = gPalette.get(aWidgetId);
+      if (!widget.wrapper) {
+        widget.wrapper = new WidgetGroupWrapper(widget);
+        gGroupWrapperCache.set(aWidgetId, widget.wrapper);
+      }
+      return widget.wrapper;
+    }
+
+    // PROVIDER_SPECIAL gets treated the same as PROVIDER_XUL.
+    // XXXgijs: this causes bugs in code that depends on widgetWrapper.provider
+    // giving an accurate answer... filed as bug 1379821
+    let wrapper = new XULWidgetGroupWrapper(aWidgetId);
+    gGroupWrapperCache.set(aWidgetId, wrapper);
+    return wrapper;
+  },
+
+  registerArea(aName, aProperties, aInternalCaller) {
+    if (typeof aName != "string" || !/^[a-z0-9-_]{1,}$/i.test(aName)) {
+      throw new Error("Invalid area name");
+    }
+
+    let areaIsKnown = gAreas.has(aName);
+    let props = areaIsKnown ? gAreas.get(aName) : new Map();
+    const kImmutableProperties = new Set(["type", "overflowable"]);
+    for (let key in aProperties) {
+      if (
+        areaIsKnown &&
+        kImmutableProperties.has(key) &&
+        props.get(key) != aProperties[key]
+      ) {
+        throw new Error("An area cannot change the property for '" + key + "'");
+      }
+      props.set(key, aProperties[key]);
+    }
+    // Default to a toolbar:
+    if (!props.has("type")) {
+      props.set("type", CustomizableUI.TYPE_TOOLBAR);
+    }
+    if (props.get("type") == CustomizableUI.TYPE_TOOLBAR) {
+      // Check aProperties instead of props because this check is only interested
+      // in the passed arguments, not the state of a potentially pre-existing area.
+      if (!aInternalCaller && aProperties.defaultCollapsed) {
+        throw new Error(
+          "defaultCollapsed is only allowed for default toolbars."
+        );
+      }
+      if (!props.has("defaultCollapsed")) {
+        props.set("defaultCollapsed", true);
+      }
+    } else if (props.has("defaultCollapsed")) {
+      throw new Error("defaultCollapsed only applies for TYPE_TOOLBAR areas.");
+    }
+    // Sanity check type:
+    let allTypes = [CustomizableUI.TYPE_TOOLBAR, CustomizableUI.TYPE_PANEL];
+    if (!allTypes.includes(props.get("type"))) {
+      throw new Error("Invalid area type " + props.get("type"));
+    }
+
+    // And to no placements:
+    if (!props.has("defaultPlacements")) {
+      props.set("defaultPlacements", []);
+    }
+    // Sanity check default placements array:
+    if (!Array.isArray(props.get("defaultPlacements"))) {
+      throw new Error("Should provide an array of default placements");
+    }
+
+    if (!areaIsKnown) {
+      gAreas.set(aName, props);
+
+      // Reconcile new default widgets. Have to do this before we start restoring things.
+      this._placeNewDefaultWidgetsInArea(aName);
+
+      if (
+        props.get("type") == CustomizableUI.TYPE_TOOLBAR &&
+        !gPlacements.has(aName)
+      ) {
+        // Guarantee this area exists in gFuturePlacements, to avoid checking it in
+        // various places elsewhere.
+        if (!gFuturePlacements.has(aName)) {
+          gFuturePlacements.set(aName, new Set());
+        }
+      } else {
+        this.restoreStateForArea(aName);
+      }
+
+      // If we have pending build area nodes, register all of them
+      if (gPendingBuildAreas.has(aName)) {
+        let pendingNodes = gPendingBuildAreas.get(aName);
+        for (let pendingNode of pendingNodes) {
+          this.registerToolbarNode(pendingNode);
+        }
+        gPendingBuildAreas.delete(aName);
+      }
+    }
+  },
+
+  unregisterArea(aName, aDestroyPlacements) {
+    if (typeof aName != "string" || !/^[a-z0-9-_]{1,}$/i.test(aName)) {
+      throw new Error("Invalid area name");
+    }
+    if (!gAreas.has(aName) && !gPlacements.has(aName)) {
+      throw new Error("Area not registered");
+    }
+
+    // Move all the widgets out
+    this.beginBatchUpdate();
+    try {
+      let placements = gPlacements.get(aName);
+      if (placements) {
+        // Need to clone this array so removeWidgetFromArea doesn't modify it
+        placements = [...placements];
+        placements.forEach(this.removeWidgetFromArea, this);
+      }
+
+      // Delete all remaining traces.
+      gAreas.delete(aName);
+      // Only destroy placements when necessary:
+      if (aDestroyPlacements) {
+        gPlacements.delete(aName);
+      } else {
+        // Otherwise we need to re-set them, as removeFromArea will have emptied
+        // them out:
+        gPlacements.set(aName, placements);
+      }
+      gFuturePlacements.delete(aName);
+      let existingAreaNodes = gBuildAreas.get(aName);
+      if (existingAreaNodes) {
+        for (let areaNode of existingAreaNodes) {
+          this.notifyListeners(
+            "onAreaNodeUnregistered",
+            aName,
+            this.getCustomizationTarget(areaNode),
+            CustomizableUI.REASON_AREA_UNREGISTERED
+          );
+        }
+      }
+      gBuildAreas.delete(aName);
+    } finally {
+      this.endBatchUpdate(true);
+    }
+  },
+
+  registerToolbarNode(aToolbar) {
+    let area = aToolbar.id;
+    if (gBuildAreas.has(area) && gBuildAreas.get(area).has(aToolbar)) {
+      return;
+    }
+    let areaProperties = gAreas.get(area);
+
+    // If this area is not registered, try to do it automatically:
+    if (!areaProperties) {
+      if (!gPendingBuildAreas.has(area)) {
+        gPendingBuildAreas.set(area, []);
+      }
+      gPendingBuildAreas.get(area).push(aToolbar);
+      return;
+    }
+
+    this.beginBatchUpdate();
+    try {
+      let placements = gPlacements.get(area);
+      if (
+        !placements &&
+        areaProperties.get("type") == CustomizableUI.TYPE_TOOLBAR
+      ) {
+        this.restoreStateForArea(area);
+        placements = gPlacements.get(area);
+      }
+
+      // For toolbars that need it, mark as dirty.
+      let defaultPlacements = areaProperties.get("defaultPlacements");
+      if (
+        !this._builtinToolbars.has(area) ||
+        placements.length != defaultPlacements.length ||
+        !placements.every((id, i) => id == defaultPlacements[i])
+      ) {
+        gDirtyAreaCache.add(area);
+      }
+
+      if (areaProperties.get("overflowable")) {
+        aToolbar.overflowable = new OverflowableToolbar(aToolbar);
+      }
+
+      this.registerBuildArea(area, aToolbar);
+
+      // We only build the toolbar if it's been marked as "dirty". Dirty means
+      // one of the following things:
+      // 1) Items have been added, moved or removed from this toolbar before.
+      // 2) The number of children of the toolbar does not match the length of
+      //    the placements array for that area.
+      //
+      // This notion of being "dirty" is stored in a cache which is persisted
+      // in the saved state.
+      //
+      // Secondly, if the list of placements contains an API-provided widget,
+      // we need to call `buildArea` or it won't be built and put in the toolbar.
+      if (
+        gDirtyAreaCache.has(area) ||
+        placements.some(id => gPalette.has(id))
+      ) {
+        this.buildArea(area, placements, aToolbar);
+      } else {
+        // We must have a builtin toolbar that's in the default state. We need
+        // to only make sure that all the special nodes are correct.
+        let specials = placements.filter(p => this.isSpecialWidget(p));
+        if (specials.length) {
+          this.updateSpecialsForBuiltinToolbar(aToolbar, specials);
+        }
+      }
+      this.notifyListeners(
+        "onAreaNodeRegistered",
+        area,
+        this.getCustomizationTarget(aToolbar)
+      );
+    } finally {
+      this.endBatchUpdate();
+    }
+  },
+
+  updateSpecialsForBuiltinToolbar(aToolbar, aSpecialIDs) {
+    // Nodes are going to be in the correct order, so we can do this straightforwardly:
+    let { children } = this.getCustomizationTarget(aToolbar);
+    for (let kid of children) {
+      if (
+        this.matchingSpecials(aSpecialIDs[0], kid) &&
+        kid.getAttribute("skipintoolbarset") != "true"
+      ) {
+        kid.id = aSpecialIDs.shift();
+      }
+      if (!aSpecialIDs.length) {
+        return;
+      }
+    }
+  },
+
+  buildArea(aArea, aPlacements, aAreaNode) {
+    let document = aAreaNode.ownerDocument;
+    let window = document.defaultView;
+    let inPrivateWindow = lazy.PrivateBrowsingUtils.isWindowPrivate(window);
+    let container = this.getCustomizationTarget(aAreaNode);
+    let areaIsPanel =
+      gAreas.get(aArea).get("type") == CustomizableUI.TYPE_PANEL;
+
+    if (!container) {
+      throw new Error(
+        "Expected area " + aArea + " to have a customizationTarget attribute."
+      );
+    }
+
+    // Restore nav-bar visibility since it may have been hidden
+    // through a migration path (bug 938980) or an add-on.
+    if (aArea == CustomizableUI.AREA_NAVBAR) {
+      aAreaNode.collapsed = false;
+    }
+
+    this.beginBatchUpdate();
+
+    try {
+      let currentNode = container.firstElementChild;
+      let placementsToRemove = new Set();
+      for (let id of aPlacements) {
+        while (
+          currentNode &&
+          currentNode.getAttribute("skipintoolbarset") == "true"
+        ) {
+          currentNode = currentNode.nextElementSibling;
+        }
+
+        // Fix ids for specials and continue, for correctly placed specials.
+        if (
+          currentNode &&
+          (!currentNode.id || CustomizableUI.isSpecialWidget(currentNode)) &&
+          this.matchingSpecials(id, currentNode)
+        ) {
+          currentNode.id = id;
+        }
+        if (currentNode && currentNode.id == id) {
+          currentNode = currentNode.nextElementSibling;
+          continue;
+        }
+
+        if (this.isSpecialWidget(id) && areaIsPanel) {
+          placementsToRemove.add(id);
+          continue;
+        }
+
+        let [provider, node] = this.getWidgetNode(id, window);
+        if (!node) {
+          lazy.log.debug("Unknown widget: " + id);
+          continue;
+        }
+
+        let widget = null;
+        // If the placements have items in them which are (now) no longer removable,
+        // we shouldn't be moving them:
+        if (provider == CustomizableUI.PROVIDER_API) {
+          widget = gPalette.get(id);
+          if (!widget.removable && aArea != widget.defaultArea) {
+            placementsToRemove.add(id);
+            continue;
+          }
+        } else if (
+          provider == CustomizableUI.PROVIDER_XUL &&
+          node.parentNode != container &&
+          !this.isWidgetRemovable(node)
+        ) {
+          placementsToRemove.add(id);
+          continue;
+        } // Special widgets are always removable, so no need to check them
+
+        if (inPrivateWindow && widget && !widget.showInPrivateBrowsing) {
+          continue;
+        }
+
+        this.ensureButtonContextMenu(node, aAreaNode);
+
+        // This needs updating in case we're resetting / undoing a reset.
+        if (widget) {
+          widget.currentArea = aArea;
+        }
+        this.insertWidgetBefore(node, currentNode, container, aArea);
+        if (gResetting) {
+          this.notifyListeners("onWidgetReset", node, container);
+        } else if (gUndoResetting) {
+          this.notifyListeners("onWidgetUndoMove", node, container);
+        }
+      }
+
+      if (currentNode) {
+        let palette = window.gNavToolbox ? window.gNavToolbox.palette : null;
+        let limit = currentNode.previousElementSibling;
+        let node = container.lastElementChild;
+        while (node && node != limit) {
+          let previousSibling = node.previousElementSibling;
+          // Nodes opt-in to removability. If they're removable, and we haven't
+          // seen them in the placements array, then we toss them into the palette
+          // if one exists. If no palette exists, we just remove the node. If the
+          // node is not removable, we leave it where it is. However, we can only
+          // safely touch elements that have an ID - both because we depend on
+          // IDs (or are specials), and because such elements are not intended to
+          // be widgets (eg, titlebar-spacer elements).
+          if (
+            (node.id || this.isSpecialWidget(node)) &&
+            node.getAttribute("skipintoolbarset") != "true"
+          ) {
+            if (this.isWidgetRemovable(node)) {
+              if (node.id && (gResetting || gUndoResetting)) {
+                let widget = gPalette.get(node.id);
+                if (widget) {
+                  widget.currentArea = null;
+                }
+              }
+              if (palette && !this.isSpecialWidget(node.id)) {
+                palette.appendChild(node);
+                this.removeLocationAttributes(node);
+              } else {
+                container.removeChild(node);
+              }
+            } else {
+              node.setAttribute("removable", false);
+              lazy.log.debug(
+                "Adding non-removable widget to placements of " +
+                  aArea +
+                  ": " +
+                  node.id
+              );
+              gPlacements.get(aArea).push(node.id);
+              gDirty = true;
+            }
+          }
+          node = previousSibling;
+        }
+      }
+
+      // If there are placements in here which aren't removable from their original area,
+      // we remove them from this area's placement array. They will (have) be(en) added
+      // to their original area's placements array in the block above this one.
+      if (placementsToRemove.size) {
+        let placementAry = gPlacements.get(aArea);
+        for (let id of placementsToRemove) {
+          let index = placementAry.indexOf(id);
+          placementAry.splice(index, 1);
+        }
+      }
+
+      if (gResetting) {
+        this.notifyListeners("onAreaReset", aArea, container);
+      }
+    } finally {
+      this.endBatchUpdate();
+    }
+  },
+
+  addPanelCloseListeners(aPanel) {
+    Services.els.addSystemEventListener(aPanel, "click", this, false);
+    Services.els.addSystemEventListener(aPanel, "keypress", this, false);
+    let win = aPanel.ownerGlobal;
+    if (!gPanelsForWindow.has(win)) {
+      gPanelsForWindow.set(win, new Set());
+    }
+    gPanelsForWindow.get(win).add(this._getPanelForNode(aPanel));
+  },
+
+  removePanelCloseListeners(aPanel) {
+    Services.els.removeSystemEventListener(aPanel, "click", this, false);
+    Services.els.removeSystemEventListener(aPanel, "keypress", this, false);
+    let win = aPanel.ownerGlobal;
+    let panels = gPanelsForWindow.get(win);
+    if (panels) {
+      panels.delete(this._getPanelForNode(aPanel));
+    }
+  },
+
+  ensureButtonContextMenu(aNode, aAreaNode, forcePanel) {
+    const kPanelItemContextMenu = "customizationPanelItemContextMenu";
+
+    let currentContextMenu =
+      aNode.getAttribute("context") || aNode.getAttribute("contextmenu");
+    let contextMenuForPlace;
+
+    if (
+      lazy.gUnifiedExtensionsEnabled &&
+      CustomizableUI.isWebExtensionWidget(aNode.id) &&
+      (aAreaNode?.id == CustomizableUI.AREA_ADDONS ||
+        aNode.getAttribute("overflowedItem") == "true")
+    ) {
+      contextMenuForPlace = null;
+    } else {
+      contextMenuForPlace =
+        forcePanel || "panel" == CustomizableUI.getPlaceForItem(aAreaNode)
+          ? kPanelItemContextMenu
+          : null;
+    }
+    if (contextMenuForPlace && !currentContextMenu) {
+      aNode.setAttribute("context", contextMenuForPlace);
+    } else if (
+      currentContextMenu == kPanelItemContextMenu &&
+      contextMenuForPlace != kPanelItemContextMenu
+    ) {
+      aNode.removeAttribute("context");
+      aNode.removeAttribute("contextmenu");
+    }
+  },
+
+  getWidgetProvider(aWidgetId) {
+    if (this.isSpecialWidget(aWidgetId)) {
+      return CustomizableUI.PROVIDER_SPECIAL;
+    }
+    if (gPalette.has(aWidgetId)) {
+      return CustomizableUI.PROVIDER_API;
+    }
+    // If this was an API widget that was destroyed, return null:
+    if (gSeenWidgets.has(aWidgetId)) {
+      return null;
+    }
+
+    // We fall back to the XUL provider, but we don't know for sure (at this
+    // point) whether it exists there either. So the API is technically lying.
+    // Ideally, it would be able to return an error value (or throw an
+    // exception) if it really didn't exist. Our code calling this function
+    // handles that fine, but this is a public API.
+    return CustomizableUI.PROVIDER_XUL;
+  },
+
+  getWidgetNode(aWidgetId, aWindow) {
+    let document = aWindow.document;
+
+    if (this.isSpecialWidget(aWidgetId)) {
+      let widgetNode =
+        document.getElementById(aWidgetId) ||
+        this.createSpecialWidget(aWidgetId, document);
+      return [CustomizableUI.PROVIDER_SPECIAL, widgetNode];
+    }
+
+    let widget = gPalette.get(aWidgetId);
+    if (widget) {
+      // If we have an instance of this widget already, just use that.
+      if (widget.instances.has(document)) {
+        lazy.log.debug(
+          "An instance of widget " +
+            aWidgetId +
+            " already exists in this " +
+            "document. Reusing."
+        );
+        return [CustomizableUI.PROVIDER_API, widget.instances.get(document)];
+      }
+
+      return [CustomizableUI.PROVIDER_API, this.buildWidget(document, widget)];
+    }
+
+    lazy.log.debug("Searching for " + aWidgetId + " in toolbox.");
+    let node = this.findWidgetInWindow(aWidgetId, aWindow);
+    if (node) {
+      return [CustomizableUI.PROVIDER_XUL, node];
+    }
+
+    lazy.log.debug("No node for " + aWidgetId + " found.");
+    return [null, null];
+  },
+
+  registerPanelNode(aNode, aArea) {
+    if (gBuildAreas.has(aArea) && gBuildAreas.get(aArea).has(aNode)) {
+      return;
+    }
+
+    aNode._customizationTarget = aNode;
+    this.addPanelCloseListeners(this._getPanelForNode(aNode));
+
+    let placements = gPlacements.get(aArea);
+    this.buildArea(aArea, placements, aNode);
+    this.notifyListeners("onAreaNodeRegistered", aArea, aNode);
+
+    for (let child of aNode.children) {
+      if (child.localName != "toolbarbutton") {
+        if (child.localName == "toolbaritem") {
+          this.ensureButtonContextMenu(child, aNode, true);
+        }
+        continue;
+      }
+      this.ensureButtonContextMenu(child, aNode, true);
+    }
+
+    this.registerBuildArea(aArea, aNode);
+  },
+
+  onWidgetAdded(aWidgetId, aArea, aPosition) {
+    this.insertNode(aWidgetId, aArea, aPosition, true);
+
+    if (!gResetting) {
+      this._clearPreviousUIState();
+    }
+  },
+
+  onWidgetRemoved(aWidgetId, aArea) {
+    let areaNodes = gBuildAreas.get(aArea);
+    if (!areaNodes) {
+      return;
+    }
+
+    let area = gAreas.get(aArea);
+    let isToolbar = area.get("type") == CustomizableUI.TYPE_TOOLBAR;
+    let isOverflowable = isToolbar && area.get("overflowable");
+    let showInPrivateBrowsing = gPalette.has(aWidgetId)
+      ? gPalette.get(aWidgetId).showInPrivateBrowsing
+      : true;
+
+    for (let areaNode of areaNodes) {
+      let window = areaNode.ownerGlobal;
+      if (
+        !showInPrivateBrowsing &&
+        lazy.PrivateBrowsingUtils.isWindowPrivate(window)
+      ) {
+        continue;
+      }
+
+      let container = this.getCustomizationTarget(areaNode);
+      let widgetNode = window.document.getElementById(aWidgetId);
+      if (widgetNode && isOverflowable) {
+        container = areaNode.overflowable.getContainerFor(widgetNode);
+      }
+
+      if (!widgetNode || !container.contains(widgetNode)) {
+        lazy.log.info(
+          "Widget " + aWidgetId + " not found, unable to remove from " + aArea
+        );
+        continue;
+      }
+
+      this.notifyListeners(
+        "onWidgetBeforeDOMChange",
+        widgetNode,
+        null,
+        container,
+        true
+      );
+
+      // We remove location attributes here to make sure they're gone too when a
+      // widget is removed from a toolbar to the palette. See bug 930950.
+      this.removeLocationAttributes(widgetNode);
+      // We also need to remove the panel context menu if it's there:
+      this.ensureButtonContextMenu(widgetNode);
+      if (gPalette.has(aWidgetId) || this.isSpecialWidget(aWidgetId)) {
+        container.removeChild(widgetNode);
+      } else {
+        window.gNavToolbox.palette.appendChild(widgetNode);
+      }
+      this.notifyListeners(
+        "onWidgetAfterDOMChange",
+        widgetNode,
+        null,
+        container,
+        true
+      );
+
+      let windowCache = gSingleWrapperCache.get(window);
+      if (windowCache) {
+        windowCache.delete(aWidgetId);
+      }
+    }
+    if (!gResetting) {
+      this._clearPreviousUIState();
+    }
+  },
+
+  onWidgetMoved(aWidgetId, aArea, aOldPosition, aNewPosition) {
+    this.insertNode(aWidgetId, aArea, aNewPosition);
+    if (!gResetting) {
+      this._clearPreviousUIState();
+    }
+  },
+
+  onCustomizeEnd(aWindow) {
+    this._clearPreviousUIState();
+  },
+
+  registerBuildArea(aArea, aNode) {
+    // We ensure that the window is registered to have its customization data
+    // cleaned up when unloading.
+    let window = aNode.ownerGlobal;
+    if (window.closed) {
+      return;
+    }
+    this.registerBuildWindow(window);
+
+    // Also register this build area's toolbox.
+    if (window.gNavToolbox) {
+      gBuildWindows.get(window).add(window.gNavToolbox);
+    }
+
+    if (!gBuildAreas.has(aArea)) {
+      gBuildAreas.set(aArea, new Set());
+    }
+
+    gBuildAreas.get(aArea).add(aNode);
+
+    // Give a class to all customize targets to be used for styling in Customize Mode
+    let customizableNode = this.getCustomizeTargetForArea(aArea, window);
+    customizableNode.classList.add("customization-target");
+  },
+
+  registerBuildWindow(aWindow) {
+    if (!gBuildWindows.has(aWindow)) {
+      gBuildWindows.set(aWindow, new Set());
+
+      aWindow.addEventListener("unload", this);
+      aWindow.addEventListener("command", this, true);
+
+      this.notifyListeners("onWindowOpened", aWindow);
+    }
+  },
+
+  unregisterBuildWindow(aWindow) {
+    aWindow.removeEventListener("unload", this);
+    aWindow.removeEventListener("command", this, true);
+    gPanelsForWindow.delete(aWindow);
+    gBuildWindows.delete(aWindow);
+    gSingleWrapperCache.delete(aWindow);
+    let document = aWindow.document;
+
+    for (let [areaId, areaNodes] of gBuildAreas) {
+      let areaProperties = gAreas.get(areaId);
+      for (let node of areaNodes) {
+        if (node.ownerDocument == document) {
+          this.notifyListeners(
+            "onAreaNodeUnregistered",
+            areaId,
+            this.getCustomizationTarget(node),
+            CustomizableUI.REASON_WINDOW_CLOSED
+          );
+          if (areaProperties.get("overflowable")) {
+            node.overflowable.uninit();
+            node.overflowable = null;
+          }
+          areaNodes.delete(node);
+        }
+      }
+    }
+
+    for (let [, widget] of gPalette) {
+      widget.instances.delete(document);
+      this.notifyListeners("onWidgetInstanceRemoved", widget.id, document);
+    }
+
+    for (let [, pendingNodes] of gPendingBuildAreas) {
+      for (let i = pendingNodes.length - 1; i >= 0; i--) {
+        if (pendingNodes[i].ownerDocument == document) {
+          pendingNodes.splice(i, 1);
+        }
+      }
+    }
+
+    this.notifyListeners("onWindowClosed", aWindow);
+  },
+
+  setLocationAttributes(aNode, aArea) {
+    let props = gAreas.get(aArea);
+    if (!props) {
+      throw new Error(
+        "Expected area " +
+          aArea +
+          " to have a properties Map " +
+          "associated with it."
+      );
+    }
+
+    aNode.setAttribute("cui-areatype", props.get("type") || "");
+    let anchor = props.get("anchor");
+    if (anchor) {
+      aNode.setAttribute("cui-anchorid", anchor);
+    } else {
+      aNode.removeAttribute("cui-anchorid");
+    }
+  },
+
+  removeLocationAttributes(aNode) {
+    aNode.removeAttribute("cui-areatype");
+    aNode.removeAttribute("cui-anchorid");
+  },
+
+  insertNode(aWidgetId, aArea, aPosition, isNew) {
+    let areaNodes = gBuildAreas.get(aArea);
+    if (!areaNodes) {
+      return;
+    }
+
+    let placements = gPlacements.get(aArea);
+    if (!placements) {
+      lazy.log.error(
+        "Could not find any placements for " + aArea + " when moving a widget."
+      );
+      return;
+    }
+
+    // Go through each of the nodes associated with this area and move the
+    // widget to the requested location.
+    for (let areaNode of areaNodes) {
+      this.insertNodeInWindow(aWidgetId, areaNode, isNew);
+    }
+  },
+
+  insertNodeInWindow(aWidgetId, aAreaNode, isNew) {
+    let window = aAreaNode.ownerGlobal;
+    let showInPrivateBrowsing = gPalette.has(aWidgetId)
+      ? gPalette.get(aWidgetId).showInPrivateBrowsing
+      : true;
+
+    if (
+      !showInPrivateBrowsing &&
+      lazy.PrivateBrowsingUtils.isWindowPrivate(window)
+    ) {
+      return;
+    }
+
+    let [, widgetNode] = this.getWidgetNode(aWidgetId, window);
+    if (!widgetNode) {
+      lazy.log.error("Widget '" + aWidgetId + "' not found, unable to move");
+      return;
+    }
+
+    let areaId = aAreaNode.id;
+    if (isNew) {
+      this.ensureButtonContextMenu(widgetNode, aAreaNode);
+    }
+
+    let [insertionContainer, nextNode] = this.findInsertionPoints(
+      widgetNode,
+      aAreaNode
+    );
+    this.insertWidgetBefore(widgetNode, nextNode, insertionContainer, areaId);
+  },
+
+  findInsertionPoints(aNode, aAreaNode) {
+    let areaId = aAreaNode.id;
+    let props = gAreas.get(areaId);
+
+    // For overflowable toolbars, rely on them (because the work is more complicated):
+    if (
+      props.get("type") == CustomizableUI.TYPE_TOOLBAR &&
+      props.get("overflowable")
+    ) {
+      return aAreaNode.overflowable.findOverflowedInsertionPoints(aNode);
+    }
+
+    let container = this.getCustomizationTarget(aAreaNode);
+    let placements = gPlacements.get(areaId);
+    let nodeIndex = placements.indexOf(aNode.id);
+
+    while (++nodeIndex < placements.length) {
+      let nextNodeId = placements[nodeIndex];
+      // We use aAreaNode here, because if aNode is in a template, its
+      // `ownerDocument` is *not* going to be the browser.xhtml document,
+      // so we cannot rely on it.
+      let nextNode = aAreaNode.ownerDocument.getElementById(nextNodeId);
+      // If the next placed widget exists, and is a direct child of the
+      // container, or wrapped in a customize mode wrapper (toolbarpaletteitem)
+      // inside the container, insert beside it.
+      // We have to check the parent to avoid errors when the placement ids
+      // are for nodes that are no longer customizable.
+      if (
+        nextNode &&
+        (nextNode.parentNode == container ||
+          (nextNode.parentNode.localName == "toolbarpaletteitem" &&
+            nextNode.parentNode.parentNode == container))
+      ) {
+        return [container, nextNode];
+      }
+    }
+
+    return [container, null];
+  },
+
+  insertWidgetBefore(aNode, aNextNode, aContainer, aArea) {
+    this.notifyListeners(
+      "onWidgetBeforeDOMChange",
+      aNode,
+      aNextNode,
+      aContainer
+    );
+    this.setLocationAttributes(aNode, aArea);
+    aContainer.insertBefore(aNode, aNextNode);
+    this.notifyListeners(
+      "onWidgetAfterDOMChange",
+      aNode,
+      aNextNode,
+      aContainer
+    );
+  },
+
+  handleEvent(aEvent) {
+    switch (aEvent.type) {
+      case "command":
+        if (!this._originalEventInPanel(aEvent)) {
+          break;
+        }
+        aEvent = aEvent.sourceEvent;
+      // Fall through
+      case "click":
+      case "keypress":
+        this.maybeAutoHidePanel(aEvent);
+        break;
+      case "unload":
+        this.unregisterBuildWindow(aEvent.currentTarget);
+        break;
+    }
+  },
+
+  _originalEventInPanel(aEvent) {
+    let e = aEvent.sourceEvent;
+    if (!e) {
+      return false;
+    }
+    let node = this._getPanelForNode(e.target);
+    if (!node) {
+      return false;
+    }
+    let win = e.view;
+    let panels = gPanelsForWindow.get(win);
+    return !!panels && panels.has(node);
+  },
+
+  _getSpecialIdForNode(aNode) {
+    if (typeof aNode == "object" && aNode.localName) {
+      if (aNode.id) {
+        return aNode.id;
+      }
+      if (aNode.localName.startsWith("toolbar")) {
+        return aNode.localName.substring(7);
+      }
+      return "";
+    }
+    return aNode;
+  },
+
+  isSpecialWidget(aId) {
+    aId = this._getSpecialIdForNode(aId);
+    return (
+      aId.startsWith(kSpecialWidgetPfx) ||
+      aId.startsWith("separator") ||
+      aId.startsWith("spring") ||
+      aId.startsWith("spacer")
+    );
+  },
+
+  matchingSpecials(aId1, aId2) {
+    aId1 = this._getSpecialIdForNode(aId1);
+    aId2 = this._getSpecialIdForNode(aId2);
+
+    return (
+      this.isSpecialWidget(aId1) &&
+      this.isSpecialWidget(aId2) &&
+      aId1.match(/spring|spacer|separator/)[0] ==
+        aId2.match(/spring|spacer|separator/)[0]
+    );
+  },
+
+  ensureSpecialWidgetId(aId) {
+    let nodeType = aId.match(/spring|spacer|separator/)[0];
+    // If the ID we were passed isn't a generated one, generate one now:
+    if (nodeType == aId) {
+      // Ids are differentiated through a unique count suffix.
+      return kSpecialWidgetPfx + aId + ++gNewElementCount;
+    }
+    return aId;
+  },
+
+  createSpecialWidget(aId, aDocument) {
+    let nodeName = "toolbar" + aId.match(/spring|spacer|separator/)[0];
+    let node = aDocument.createXULElement(nodeName);
+    node.className = "chromeclass-toolbar-additional";
+    node.id = this.ensureSpecialWidgetId(aId);
+    return node;
+  },
+
+  /* Find a XUL-provided widget in a window. Don't try to use this
+   * for an API-provided widget or a special widget.
+   */
+  findWidgetInWindow(aId, aWindow) {
+    if (!gBuildWindows.has(aWindow)) {
+      throw new Error("Build window not registered");
+    }
+
+    if (!aId) {
+      lazy.log.error("findWidgetInWindow was passed an empty string.");
+      return null;
+    }
+
+    let document = aWindow.document;
+
+    // look for a node with the same id, as the node may be
+    // in a different toolbar.
+    let node = document.getElementById(aId);
+    if (node) {
+      let parent = node.parentNode;
+      while (
+        parent &&
+        !(
+          this.getCustomizationTarget(parent) ||
+          parent == aWindow.gNavToolbox.palette
+        )
+      ) {
+        parent = parent.parentNode;
+      }
+
+      if (parent) {
+        let nodeInArea =
+          node.parentNode.localName == "toolbarpaletteitem"
+            ? node.parentNode
+            : node;
+        // Check if we're in a customization target, or in the palette:
+        if (
+          (this.getCustomizationTarget(parent) == nodeInArea.parentNode &&
+            gBuildWindows.get(aWindow).has(aWindow.gNavToolbox)) ||
+          aWindow.gNavToolbox.palette == nodeInArea.parentNode
+        ) {
+          // Normalize the removable attribute. For backwards compat, if
+          // the widget is not located in a toolbox palette then absence
+          // of the "removable" attribute means it is not removable.
+          if (!node.hasAttribute("removable")) {
+            // If we first see this in customization mode, it may be in the
+            // customization palette instead of the toolbox palette.
+            node.setAttribute(
+              "removable",
+              !this.getCustomizationTarget(parent)
+            );
+          }
+          return node;
+        }
+      }
+    }
+
+    let toolboxes = gBuildWindows.get(aWindow);
+    for (let toolbox of toolboxes) {
+      if (toolbox.palette) {
+        // Attempt to locate an element with a matching ID within
+        // the palette.
+        let element = toolbox.palette.getElementsByAttribute("id", aId)[0];
+        if (element) {
+          // Normalize the removable attribute. For backwards compat, this
+          // is optional if the widget is located in the toolbox palette,
+          // and defaults to *true*, unlike if it was located elsewhere.
+          if (!element.hasAttribute("removable")) {
+            element.setAttribute("removable", true);
+          }
+          return element;
+        }
+      }
+    }
+    return null;
+  },
+
+  buildWidget(aDocument, aWidget) {
+    if (aDocument.documentURI != kExpectedWindowURL) {
+      throw new Error("buildWidget was called for a non-browser window!");
+    }
+    if (typeof aWidget == "string") {
+      aWidget = gPalette.get(aWidget);
+    }
+    if (!aWidget) {
+      throw new Error("buildWidget was passed a non-widget to build.");
+    }
+    if (
+      !aWidget.showInPrivateBrowsing &&
+      lazy.PrivateBrowsingUtils.isWindowPrivate(aDocument.defaultView)
+    ) {
+      return null;
+    }
+
+    lazy.log.debug("Building " + aWidget.id + " of type " + aWidget.type);
+
+    let node;
+    let button;
+    if (aWidget.type == "custom") {
+      if (aWidget.onBuild) {
+        node = aWidget.onBuild(aDocument);
+      }
+      if (
+        !node ||
+        !aDocument.defaultView.XULElement.isInstance(node) ||
+        (aWidget.viewId && !node.viewButton)
+      ) {
+        lazy.log.error(
+          "Custom widget with id " +
+            aWidget.id +
+            " does not return a valid node"
+        );
+      }
+      // A custom widget can define a viewId for the panel and a viewButton
+      // property for the panel anchor.  With that, it will be treated as a view
+      // type where necessary to hook up the view panel.
+      if (aWidget.viewId) {
+        button = node.viewButton;
+      }
+    }
+    // Button and view widget types, plus custom widgets that have a viewId and thus a button.
+    if (button || aWidget.type != "custom") {
+      if (
+        aWidget.onBeforeCreated &&
+        aWidget.onBeforeCreated(aDocument) === false
+      ) {
+        return null;
+      }
+
+      if (!button) {
+        button = aDocument.createXULElement("toolbarbutton");
+        node = button;
+      }
+      button.classList.add("toolbarbutton-1");
+      button.setAttribute("delegatesanchor", "true");
+
+      let viewbutton = null;
+      if (aWidget.type == "button-and-view") {
+        button.setAttribute("id", aWidget.id + "-button");
+        let dropmarker = aDocument.createXULElement("toolbarbutton");
+        dropmarker.setAttribute("id", aWidget.id + "-dropmarker");
+        dropmarker.setAttribute("delegatesanchor", "true");
+        dropmarker.classList.add(
+          "toolbarbutton-1",
+          "toolbarbutton-combined-buttons-dropmarker"
+        );
+        node = aDocument.createXULElement("toolbaritem");
+        node.classList.add("toolbaritem-combined-buttons");
+        node.append(button, dropmarker);
+        viewbutton = dropmarker;
+      } else if (aWidget.viewId) {
+        // Also set viewbutton for anything with a view
+        viewbutton = button;
+      }
+
+      node.setAttribute("id", aWidget.id);
+      node.setAttribute("widget-id", aWidget.id);
+      node.setAttribute("widget-type", aWidget.type);
+      if (aWidget.disabled) {
+        node.setAttribute("disabled", true);
+      }
+      node.setAttribute("removable", aWidget.removable);
+      node.setAttribute("overflows", aWidget.overflows);
+      if (aWidget.tabSpecific) {
+        node.setAttribute("tabspecific", aWidget.tabSpecific);
+      }
+      if (aWidget.locationSpecific) {
+        node.setAttribute("locationspecific", aWidget.locationSpecific);
+      }
+      if (aWidget.keepBroadcastAttributesWhenCustomizing) {
+        node.setAttribute(
+          "keepbroadcastattributeswhencustomizing",
+          aWidget.keepBroadcastAttributesWhenCustomizing
+        );
+      }
+
+      let shortcut;
+      if (aWidget.shortcutId) {
+        let keyEl = aDocument.getElementById(aWidget.shortcutId);
+        if (keyEl) {
+          shortcut = lazy.ShortcutUtils.prettifyShortcut(keyEl);
+        } else {
+          lazy.log.error(
+            "Key element with id '" +
+              aWidget.shortcutId +
+              "' for widget '" +
+              aWidget.id +
+              "' not found!"
+          );
+        }
+      }
+
+      if (aWidget.l10nId) {
+        node.setAttribute("data-l10n-id", aWidget.l10nId);
+        if (button != node) {
+          // This is probably a "button-and-view" widget, such as the Profiler
+          // button. In that case, "node" is the "toolbaritem" container, and
+          // "button" the main button (see above).
+          // In this case, the values on the "node" is used in the Customize
+          // view, as well as the tooltips over both buttons; the values on the
+          // "button" are used in the overflow menu.
+          button.setAttribute("data-l10n-id", aWidget.l10nId);
+        }
+
+        if (shortcut) {
+          node.setAttribute("data-l10n-args", JSON.stringify({ shortcut }));
+          if (button != node) {
+            // This is probably a "button-and-view" widget.
+            button.setAttribute("data-l10n-args", JSON.stringify({ shortcut }));
+          }
+        }
+      } else {
+        node.setAttribute("label", this.getLocalizedProperty(aWidget, "label"));
+        if (button != node) {
+          // This is probably a "button-and-view" widget.
+          button.setAttribute("label", node.getAttribute("label"));
+        }
+
+        let tooltip = this.getLocalizedProperty(
+          aWidget,
+          "tooltiptext",
+          shortcut ? [shortcut] : []
+        );
+        if (tooltip) {
+          node.setAttribute("tooltiptext", tooltip);
+          if (button != node) {
+            // This is probably a "button-and-view" widget.
+            button.setAttribute("tooltiptext", tooltip);
+          }
+        }
+      }
+
+      let commandHandler = this.handleWidgetCommand.bind(this, aWidget, node);
+      node.addEventListener("command", commandHandler);
+      let clickHandler = this.handleWidgetClick.bind(this, aWidget, node);
+      node.addEventListener("click", clickHandler);
+
+      node.classList.add("chromeclass-toolbar-additional");
+
+      // If the widget has a view, register a keypress handler because opening
+      // a view with the keyboard has slightly different focus handling than
+      // opening a view with the mouse. (When opened with the keyboard, the
+      // first item in the view should be focused after opening.)
+      if (viewbutton) {
+        lazy.log.debug(
+          "Widget " +
+            aWidget.id +
+            " has a view. Auto-registering event handlers."
+        );
+
+        if (aWidget.source == CustomizableUI.SOURCE_BUILTIN) {
+          node.classList.add("subviewbutton-nav");
+        }
+      }
+
+      if (aWidget.onCreated) {
+        aWidget.onCreated(node);
+      }
+    }
+
+    aWidget.instances.set(aDocument, node);
+    return node;
+  },
+
+  ensureSubviewListeners(viewNode) {
+    if (viewNode._addedEventListeners) {
+      return;
+    }
+    let viewId = viewNode.id;
+    let widget = [...gPalette.values()].find(w => w.viewId == viewId);
+    if (!widget) {
+      return;
+    }
+    for (let eventName of kSubviewEvents) {
+      let handler = "on" + eventName;
+      if (typeof widget[handler] == "function") {
+        viewNode.addEventListener(eventName, widget[handler]);
+      }
+    }
+    viewNode._addedEventListeners = true;
+    lazy.log.debug(
+      "Widget " + widget.id + " showing and hiding event handlers set."
+    );
+  },
+
+  getLocalizedProperty(aWidget, aProp, aFormatArgs, aDef) {
+    const kReqStringProps = ["label"];
+
+    if (typeof aWidget == "string") {
+      aWidget = gPalette.get(aWidget);
+    }
+    if (!aWidget) {
+      throw new Error(
+        "getLocalizedProperty was passed a non-widget to work with."
+      );
+    }
+    let def, name;
+    // Let widgets pass their own string identifiers or strings, so that
+    // we can use strings which aren't the default (in case string ids change)
+    // and so that non-builtin-widgets can also provide labels, tooltips, etc.
+    if (aWidget[aProp] != null) {
+      name = aWidget[aProp];
+      // By using this as the default, if a widget provides a full string rather
+      // than a string ID for localization, we will fall back to that string
+      // and return that.
+      def = aDef || name;
+    } else {
+      name = aWidget.id + "." + aProp;
+      def = aDef || "";
+    }
+    if (aWidget.localized === false) {
+      return def;
+    }
+    try {
+      if (Array.isArray(aFormatArgs) && aFormatArgs.length) {
+        return (
+          lazy.gWidgetsBundle.formatStringFromName(name, aFormatArgs) || def
+        );
+      }
+      return lazy.gWidgetsBundle.GetStringFromName(name) || def;
+    } catch (ex) {
+      // If an empty string was explicitly passed, treat it as an actual
+      // value rather than a missing property.
+      if (!def && (name != "" || kReqStringProps.includes(aProp))) {
+        lazy.log.error("Could not localize property '" + name + "'.");
+      }
+    }
+    return def;
+  },
+
+  addShortcut(aShortcutNode, aTargetNode = aShortcutNode) {
+    // Detect if we've already been here before.
+    if (aTargetNode.hasAttribute("shortcut")) {
+      return;
+    }
+
+    // Use ownerGlobal.document to ensure we get the right doc even for
+    // elements in template tags.
+    let { document } = aShortcutNode.ownerGlobal;
+    let shortcutId = aShortcutNode.getAttribute("key");
+    let shortcut;
+    if (shortcutId) {
+      shortcut = document.getElementById(shortcutId);
+    } else {
+      let commandId = aShortcutNode.getAttribute("command");
+      if (commandId) {
+        shortcut = lazy.ShortcutUtils.findShortcut(
+          document.getElementById(commandId)
+        );
+      }
+    }
+    if (!shortcut) {
+      return;
+    }
+
+    aTargetNode.setAttribute(
+      "shortcut",
+      lazy.ShortcutUtils.prettifyShortcut(shortcut)
+    );
+  },
+
+  doWidgetCommand(aWidget, aNode, aEvent) {
+    if (aWidget.onCommand) {
+      try {
+        aWidget.onCommand.call(null, aEvent);
+      } catch (e) {
+        lazy.log.error(e);
+      }
+    } else {
+      // XXXunf Need to think this through more, and formalize.
+      Services.obs.notifyObservers(
+        aNode,
+        "customizedui-widget-command",
+        aWidget.id
+      );
+    }
+  },
+
+  showWidgetView(aWidget, aNode, aEvent) {
+    let ownerWindow = aNode.ownerGlobal;
+    let area = this.getPlacementOfWidget(aNode.id).area;
+    let areaType = CustomizableUI.getAreaType(area);
+    let anchor = aNode;
+
+    if (
+      aWidget.disallowSubView &&
+      (areaType == CustomizableUI.TYPE_PANEL ||
+        aNode.hasAttribute("overflowedItem"))
+    ) {
+      // Close the containing panel (e.g. overflow), PanelUI will reopen.
+      let wrapper = this.wrapWidget(aWidget.id).forWindow(ownerWindow);
+      if (wrapper?.anchor) {
+        this.hidePanelForNode(aNode);
+        anchor = wrapper.anchor;
+      }
+    } else if (areaType != CustomizableUI.TYPE_PANEL) {
+      let wrapper = this.wrapWidget(aWidget.id).forWindow(ownerWindow);
+
+      let hasMultiView = !!aNode.closest("panelmultiview");
+      if (!hasMultiView && wrapper?.anchor) {
+        this.hidePanelForNode(aNode);
+        anchor = wrapper.anchor;
+      }
+    }
+    ownerWindow.PanelUI.showSubView(aWidget.viewId, anchor, aEvent);
+  },
+
+  handleWidgetCommand(aWidget, aNode, aEvent) {
+    // Note that aEvent can be a keypress event for widgets of type "view".
+    lazy.log.debug("handleWidgetCommand");
+
+    let action;
+    if (aWidget.onBeforeCommand) {
+      try {
+        action = aWidget.onBeforeCommand.call(null, aEvent, aNode);
+      } catch (e) {
+        lazy.log.error(e);
+      }
+    }
+
+    if (aWidget.type == "button" || action == "command") {
+      this.doWidgetCommand(aWidget, aNode, aEvent);
+    } else if (aWidget.type == "view" || action == "view") {
+      this.showWidgetView(aWidget, aNode, aEvent);
+    } else if (aWidget.type == "button-and-view") {
+      // Do the command if we're in the toolbar and the button was clicked.
+      // Otherwise, including when we have currently overflowed out of the
+      // toolbar, open the view. There is no way to trigger the command while
+      // the widget is in the panel, by design.
+      let button = aNode.firstElementChild;
+      let area = this.getPlacementOfWidget(aNode.id).area;
+      let areaType = CustomizableUI.getAreaType(area);
+      if (
+        areaType == CustomizableUI.TYPE_TOOLBAR &&
+        button.contains(aEvent.target) &&
+        !aNode.hasAttribute("overflowedItem")
+      ) {
+        this.doWidgetCommand(aWidget, aNode, aEvent);
+      } else {
+        this.showWidgetView(aWidget, aNode, aEvent);
+      }
+    }
+  },
+
+  handleWidgetClick(aWidget, aNode, aEvent) {
+    lazy.log.debug("handleWidgetClick");
+    if (aWidget.onClick) {
+      try {
+        aWidget.onClick.call(null, aEvent);
+      } catch (e) {
+        Cu.reportError(e);
+      }
+    } else {
+      // XXXunf Need to think this through more, and formalize.
+      Services.obs.notifyObservers(
+        aNode,
+        "customizedui-widget-click",
+        aWidget.id
+      );
+    }
+  },
+
+  _getPanelForNode(aNode) {
+    return aNode.closest("panel");
+  },
+
+  /*
+   * If people put things in the panel which need more than single-click interaction,
+   * we don't want to close it. Right now we check for text inputs and menu buttons.
+   * We also check for being outside of any toolbaritem/toolbarbutton, ie on a blank
+   * part of the menu, or on another menu (like a context menu inside the panel).
+   */
+  _isOnInteractiveElement(aEvent) {
+    let panel = this._getPanelForNode(aEvent.currentTarget);
+    // This can happen in e.g. customize mode. If there's no panel,
+    // there's clearly nothing for us to close; pretend we're interactive.
+    if (!panel) {
+      return true;
+    }
+
+    function getNextTarget(target) {
+      if (target.nodeType == target.DOCUMENT_NODE) {
+        if (!target.defaultView) {
+          // Err, we're done.
+          return null;
+        }
+        // Find containing browser or iframe element in the parent doc.
+        return target.defaultView.docShell.chromeEventHandler;
+      }
+      // Skip any parent shadow roots
+      return target.parentNode?.host?.parentNode || target.parentNode;
+    }
+
+    // While keeping track of that, we go from the original target back up,
+    // to the panel if we have to. We bail as soon as we find an input,
+    // a toolbarbutton/item, or a menuItem.
+    for (
+      let target = aEvent.originalTarget;
+      target && target != panel;
+      target = getNextTarget(target)
+    ) {
+      if (target.nodeType == target.DOCUMENT_NODE) {
+        // Skip out of iframes etc:
+        continue;
+      }
+
+      // Break out of the loop immediately for disabled items, as we need to
+      // keep the menu open in that case.
+      if (target.getAttribute("disabled") == "true") {
+        return true;
+      }
+
+      let tagName = target.localName;
+      if (tagName == "input" || tagName == "searchbar") {
+        return true;
+      }
+      if (tagName == "toolbaritem" || tagName == "toolbarbutton") {
+        // If we are in a type="menu" toolbarbutton, we'll now interact with
+        // the menu.
+        return target.getAttribute("type") == "menu";
+      }
+      if (tagName == "menuitem") {
+        // If we're in a nested menu we don't need to close this panel.
+        return true;
+      }
+    }
+
+    // We don't know what we interacted with, assume interactive.
+    return true;
+  },
+
+  hidePanelForNode(aNode) {
+    let panel = this._getPanelForNode(aNode);
+    if (panel) {
+      lazy.PanelMultiView.hidePopup(panel);
+    }
+  },
+
+  maybeAutoHidePanel(aEvent) {
+    let eventType = aEvent.type;
+    if (eventType == "keypress" && aEvent.keyCode != aEvent.DOM_VK_RETURN) {
+      return;
+    }
+
+    if (eventType == "click" && aEvent.button != 0) {
+      return;
+    }
+
+    // We don't check preventDefault - it makes sense that this was prevented,
+    // but we probably still want to close the panel. If consumers don't want
+    // this to happen, they should specify the closemenu attribute.
+    if (eventType != "command" && this._isOnInteractiveElement(aEvent)) {
+      return;
+    }
+
+    // We can't use event.target because we might have passed an anonymous
+    // content boundary as well, and so target points to the outer element in
+    // that case. Unfortunately, this means we get anonymous child nodes instead
+    // of the real ones, so looking for the 'stoooop, don't close me' attributes
+    // is more involved.
+    let target = aEvent.originalTarget;
+    while (target.parentNode && target.localName != "panel") {
+      if (
+        target.getAttribute("closemenu") == "none" ||
+        target.getAttribute("widget-type") == "view" ||
+        target.getAttribute("widget-type") == "button-and-view" ||
+        target.hasAttribute("view-button-id")
+      ) {
+        return;
+      }
+      target = target.parentNode;
+    }
+
+    // If we get here, we can actually hide the popup:
+    this.hidePanelForNode(aEvent.target);
+  },
+
+  getUnusedWidgets(aWindowPalette) {
+    let window = aWindowPalette.ownerGlobal;
+    let isWindowPrivate = lazy.PrivateBrowsingUtils.isWindowPrivate(window);
+    // We use a Set because there can be overlap between the widgets in
+    // gPalette and the items in the palette, especially after the first
+    // customization, since programmatically generated widgets will remain
+    // in the toolbox palette.
+    let widgets = new Set();
+
+    // It's possible that some widgets have been defined programmatically and
+    // have not been overlayed into the palette. We can find those inside
+    // gPalette.
+    for (let [id, widget] of gPalette) {
+      if (!widget.currentArea) {
+        if (widget.showInPrivateBrowsing || !isWindowPrivate) {
+          widgets.add(id);
+        }
+      }
+    }
+
+    lazy.log.debug("Iterating the actual nodes of the window palette");
+    for (let node of aWindowPalette.children) {
+      lazy.log.debug("In palette children: " + node.id);
+      if (node.id && !this.getPlacementOfWidget(node.id)) {
+        widgets.add(node.id);
+      }
+    }
+
+    return [...widgets];
+  },
+
+  getPlacementOfWidget(aWidgetId, aOnlyRegistered, aDeadAreas) {
+    if (aOnlyRegistered && !this.widgetExists(aWidgetId)) {
+      return null;
+    }
+
+    for (let [area, placements] of gPlacements) {
+      if (!gAreas.has(area) && !aDeadAreas) {
+        continue;
+      }
+      let index = placements.indexOf(aWidgetId);
+      if (index != -1) {
+        return { area, position: index };
+      }
+    }
+
+    return null;
+  },
+
+  widgetExists(aWidgetId) {
+    if (gPalette.has(aWidgetId) || this.isSpecialWidget(aWidgetId)) {
+      return true;
+    }
+
+    // Destroyed API widgets are in gSeenWidgets, but not in gPalette:
+    // The Pocket button is a default API widget that acts like a custom widget.
+    // If it's not in gPalette, it doesn't exist.
+    if (gSeenWidgets.has(aWidgetId) || aWidgetId === "save-to-pocket-button") {
+      return false;
+    }
+
+    // We're assuming XUL widgets always exist, as it's much harder to check,
+    // and checking would be much more error prone.
+    return true;
+  },
+
+  addWidgetToArea(aWidgetId, aArea, aPosition, aInitialAdd) {
+    if (aArea == CustomizableUI.AREA_NO_AREA) {
+      throw new Error(
+        "AREA_NO_AREA is only used as an argument for " +
+          "canWidgetMoveToArea. Use removeWidgetFromArea instead."
+      );
+    }
+    if (!gAreas.has(aArea)) {
+      throw new Error("Unknown customization area: " + aArea);
+    }
+
+    // Hack: don't want special widgets in the panel (need to check here as well
+    // as in canWidgetMoveToArea because the menu panel is lazy):
+    if (
+      gAreas.get(aArea).get("type") == CustomizableUI.TYPE_PANEL &&
+      this.isSpecialWidget(aWidgetId)
+    ) {
+      return;
+    }
+
+    // If this is a lazy area that hasn't been restored yet, we can't yet modify
+    // it - would would at least like to add to it. So we keep track of it in
+    // gFuturePlacements,  and use that to add it when restoring the area. We
+    // throw away aPosition though, as that can only be bogus if the area hasn't
+    // yet been restorted (caller can't possibly know where its putting the
+    // widget in relation to other widgets).
+    if (this.isAreaLazy(aArea)) {
+      gFuturePlacements.get(aArea).add(aWidgetId);
+      return;
+    }
+
+    if (this.isSpecialWidget(aWidgetId)) {
+      aWidgetId = this.ensureSpecialWidgetId(aWidgetId);
+    }
+
+    let oldPlacement = this.getPlacementOfWidget(aWidgetId, false, true);
+    if (oldPlacement && oldPlacement.area == aArea) {
+      this.moveWidgetWithinArea(aWidgetId, aPosition);
+      return;
+    }
+
+    // Do nothing if the widget is not allowed to move to the target area.
+    if (!this.canWidgetMoveToArea(aWidgetId, aArea)) {
+      return;
+    }
+
+    if (oldPlacement) {
+      this.removeWidgetFromArea(aWidgetId);
+    }
+
+    if (!gPlacements.has(aArea)) {
+      gPlacements.set(aArea, [aWidgetId]);
+      aPosition = 0;
+    } else {
+      let placements = gPlacements.get(aArea);
+      if (typeof aPosition != "number") {
+        aPosition = placements.length;
+      }
+      if (aPosition < 0) {
+        aPosition = 0;
+      }
+      placements.splice(aPosition, 0, aWidgetId);
+    }
+
+    let widget = gPalette.get(aWidgetId);
+    if (widget) {
+      widget.currentArea = aArea;
+      widget.currentPosition = aPosition;
+    }
+
+    // We initially set placements with addWidgetToArea, so in that case
+    // we don't consider the area "dirtied".
+    if (!aInitialAdd) {
+      gDirtyAreaCache.add(aArea);
+    }
+
+    gDirty = true;
+    this.saveState();
+
+    this.notifyListeners("onWidgetAdded", aWidgetId, aArea, aPosition);
+  },
+
+  removeWidgetFromArea(aWidgetId) {
+    let oldPlacement = this.getPlacementOfWidget(aWidgetId, false, true);
+    if (!oldPlacement) {
+      return;
+    }
+
+    if (!this.isWidgetRemovable(aWidgetId)) {
+      return;
+    }
+
+    let placements = gPlacements.get(oldPlacement.area);
+    let position = placements.indexOf(aWidgetId);
+    if (position != -1) {
+      placements.splice(position, 1);
+    }
+
+    let widget = gPalette.get(aWidgetId);
+    if (widget) {
+      widget.currentArea = null;
+      widget.currentPosition = null;
+    }
+
+    gDirty = true;
+    this.saveState();
+    gDirtyAreaCache.add(oldPlacement.area);
+
+    this.notifyListeners("onWidgetRemoved", aWidgetId, oldPlacement.area);
+  },
+
+  moveWidgetWithinArea(aWidgetId, aPosition) {
+    let oldPlacement = this.getPlacementOfWidget(aWidgetId);
+    if (!oldPlacement) {
+      return;
+    }
+
+    let placements = gPlacements.get(oldPlacement.area);
+    if (typeof aPosition != "number") {
+      aPosition = placements.length;
+    } else if (aPosition < 0) {
+      aPosition = 0;
+    } else if (aPosition > placements.length) {
+      aPosition = placements.length;
+    }
+
+    let widget = gPalette.get(aWidgetId);
+    if (widget) {
+      widget.currentPosition = aPosition;
+      widget.currentArea = oldPlacement.area;
+    }
+
+    if (aPosition == oldPlacement.position) {
+      return;
+    }
+
+    placements.splice(oldPlacement.position, 1);
+    // If we just removed the item from *before* where it is now added,
+    // we need to compensate the position offset for that:
+    if (oldPlacement.position < aPosition) {
+      aPosition--;
+    }
+    placements.splice(aPosition, 0, aWidgetId);
+
+    gDirty = true;
+    gDirtyAreaCache.add(oldPlacement.area);
+
+    this.saveState();
+
+    this.notifyListeners(
+      "onWidgetMoved",
+      aWidgetId,
+      oldPlacement.area,
+      oldPlacement.position,
+      aPosition
+    );
+  },
+
+  // Note that this does not populate gPlacements, which is done lazily.
+  // The panel area is an exception here.
+  loadSavedState() {
+    let state = Services.prefs.getCharPref(kPrefCustomizationState, "");
+    if (!state) {
+      lazy.log.debug("No saved state found");
+      // Nothing has been customized, so silently fall back to the defaults.
+      return;
+    }
+    try {
+      gSavedState = JSON.parse(state);
+      if (typeof gSavedState != "object" || gSavedState === null) {
+        throw new Error("Invalid saved state");
+      }
+    } catch (e) {
+      Services.prefs.clearUserPref(kPrefCustomizationState);
+      gSavedState = {};
+      lazy.log.debug(
+        "Error loading saved UI customization state, falling back to defaults."
+      );
+    }
+
+    if (!("placements" in gSavedState)) {
+      gSavedState.placements = {};
+    }
+
+    if (!("currentVersion" in gSavedState)) {
+      gSavedState.currentVersion = 0;
+    }
+
+    gSeenWidgets = new Set(gSavedState.seen || []);
+    gDirtyAreaCache = new Set(gSavedState.dirtyAreaCache || []);
+    gNewElementCount = gSavedState.newElementCount || 0;
+  },
+
+  restoreStateForArea(aArea) {
+    let placementsPreexisted = gPlacements.has(aArea);
+
+    this.beginBatchUpdate();
+    try {
+      gRestoring = true;
+
+      let restored = false;
+      if (placementsPreexisted) {
+        lazy.log.debug("Restoring " + aArea + " from pre-existing placements");
+        for (let [position, id] of gPlacements.get(aArea).entries()) {
+          this.moveWidgetWithinArea(id, position);
+        }
+        gDirty = false;
+        restored = true;
+      } else {
+        gPlacements.set(aArea, []);
+      }
+
+      if (!restored && gSavedState && aArea in gSavedState.placements) {
+        lazy.log.debug("Restoring " + aArea + " from saved state");
+        let placements = gSavedState.placements[aArea];
+        for (let id of placements) {
+          this.addWidgetToArea(id, aArea);
+        }
+        gDirty = false;
+        restored = true;
+      }
+
+      if (!restored) {
+        lazy.log.debug("Restoring " + aArea + " from default state");
+        let defaults = gAreas.get(aArea).get("defaultPlacements");
+        if (defaults) {
+          for (let id of defaults) {
+            this.addWidgetToArea(id, aArea, null, true);
+          }
+        }
+        gDirty = false;
+      }
+
+      // Finally, add widgets to the area that were added before the it was able
+      // to be restored. This can occur when add-ons register widgets for a
+      // lazily-restored area before it's been restored.
+      if (gFuturePlacements.has(aArea)) {
+        let areaPlacements = gPlacements.get(aArea);
+        for (let id of gFuturePlacements.get(aArea)) {
+          if (areaPlacements.includes(id)) {
+            continue;
+          }
+          this.addWidgetToArea(id, aArea);
+        }
+        gFuturePlacements.delete(aArea);
+      }
+
+      lazy.log.debug(
+        "Placements for " +
+          aArea +
+          ":\n\t" +
+          gPlacements.get(aArea).join("\n\t")
+      );
+
+      gRestoring = false;
+    } finally {
+      this.endBatchUpdate();
+    }
+  },
+
+  saveState() {
+    if (gInBatchStack || !gDirty) {
+      return;
+    }
+    // Clone because we want to modify this map:
+    let state = {
+      placements: new Map(gPlacements),
+      seen: gSeenWidgets,
+      dirtyAreaCache: gDirtyAreaCache,
+      currentVersion: kVersion,
+      newElementCount: gNewElementCount,
+    };
+
+    // Merge in previously saved areas if not present in gPlacements.
+    // This way, state is still persisted for e.g. temporarily disabled
+    // add-ons - see bug 989338.
+    if (gSavedState && gSavedState.placements) {
+      for (let area of Object.keys(gSavedState.placements)) {
+        if (!state.placements.has(area)) {
+          let placements = gSavedState.placements[area];
+          state.placements.set(area, placements);
+        }
+      }
+    }
+
+    lazy.log.debug("Saving state.");
+    let serialized = JSON.stringify(state, this.serializerHelper);
+    lazy.log.debug("State saved as: " + serialized);
+    Services.prefs.setCharPref(kPrefCustomizationState, serialized);
+    gDirty = false;
+  },
+
+  serializerHelper(aKey, aValue) {
+    if (typeof aValue == "object" && aValue.constructor.name == "Map") {
+      let result = {};
+      for (let [mapKey, mapValue] of aValue) {
+        result[mapKey] = mapValue;
+      }
+      return result;
+    }
+
+    if (typeof aValue == "object" && aValue.constructor.name == "Set") {
+      return [...aValue];
+    }
+
+    return aValue;
+  },
+
+  beginBatchUpdate() {
+    gInBatchStack++;
+  },
+
+  endBatchUpdate(aForceDirty) {
+    gInBatchStack--;
+    if (aForceDirty === true) {
+      gDirty = true;
+    }
+    if (gInBatchStack == 0) {
+      this.saveState();
+    } else if (gInBatchStack < 0) {
+      throw new Error(
+        "The batch editing stack should never reach a negative number."
+      );
+    }
+  },
+
+  addListener(aListener) {
+    gListeners.add(aListener);
+  },
+
+  removeListener(aListener) {
+    if (aListener == this) {
+      return;
+    }
+
+    gListeners.delete(aListener);
+  },
+
+  notifyListeners(aEvent, ...aArgs) {
+    if (gRestoring) {
+      return;
+    }
+
+    for (let listener of gListeners) {
+      try {
+        if (typeof listener[aEvent] == "function") {
+          listener[aEvent].apply(listener, aArgs);
+        }
+      } catch (e) {
+        lazy.log.error(e + " -- " + e.fileName + ":" + e.lineNumber);
+      }
+    }
+  },
+
+  _dispatchToolboxEventToWindow(aEventType, aDetails, aWindow) {
+    let evt = new aWindow.CustomEvent(aEventType, {
+      bubbles: true,
+      cancelable: true,
+      detail: aDetails,
+    });
+    aWindow.gNavToolbox.dispatchEvent(evt);
+  },
+
+  dispatchToolboxEvent(aEventType, aDetails = {}, aWindow = null) {
+    if (aWindow) {
+      this._dispatchToolboxEventToWindow(aEventType, aDetails, aWindow);
+      return;
+    }
+    for (let [win] of gBuildWindows) {
+      this._dispatchToolboxEventToWindow(aEventType, aDetails, win);
+    }
+  },
+
+  createWidget(aProperties) {
+    let widget = this.normalizeWidget(
+      aProperties,
+      CustomizableUI.SOURCE_EXTERNAL
+    );
+    // XXXunf This should probably throw.
+    if (!widget) {
+      lazy.log.error("unable to normalize widget");
+      return undefined;
+    }
+
+    gPalette.set(widget.id, widget);
+
+    // Clear our caches:
+    gGroupWrapperCache.delete(widget.id);
+    for (let [win] of gBuildWindows) {
+      let cache = gSingleWrapperCache.get(win);
+      if (cache) {
+        cache.delete(widget.id);
+      }
+    }
+
+    this.notifyListeners("onWidgetCreated", widget.id);
+
+    if (widget.defaultArea) {
+      let addToDefaultPlacements = false;
+      let area = gAreas.get(widget.defaultArea);
+      if (
+        !CustomizableUI.isBuiltinToolbar(widget.defaultArea) &&
+        widget.defaultArea != CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+      ) {
+        addToDefaultPlacements = true;
+      }
+
+      if (addToDefaultPlacements) {
+        if (area.has("defaultPlacements")) {
+          area.get("defaultPlacements").push(widget.id);
+        } else {
+          area.set("defaultPlacements", [widget.id]);
+        }
+      }
+    }
+
+    // Look through previously saved state to see if we're restoring a widget.
+    let seenAreas = new Set();
+    let widgetMightNeedAutoAdding = true;
+    for (let [area] of gPlacements) {
+      seenAreas.add(area);
+      let areaIsRegistered = gAreas.has(area);
+      let index = gPlacements.get(area).indexOf(widget.id);
+      if (index != -1) {
+        widgetMightNeedAutoAdding = false;
+        if (areaIsRegistered) {
+          widget.currentArea = area;
+          widget.currentPosition = index;
+        }
+        break;
+      }
+    }
+
+    // Also look at saved state data directly in areas that haven't yet been
+    // restored. Can't rely on this for restored areas, as they may have
+    // changed.
+    if (widgetMightNeedAutoAdding && gSavedState) {
+      for (let area of Object.keys(gSavedState.placements)) {
+        if (seenAreas.has(area)) {
+          continue;
+        }
+
+        let areaIsRegistered = gAreas.has(area);
+        let index = gSavedState.placements[area].indexOf(widget.id);
+        if (index != -1) {
+          widgetMightNeedAutoAdding = false;
+          if (areaIsRegistered) {
+            widget.currentArea = area;
+            widget.currentPosition = index;
+          }
+          break;
+        }
+      }
+    }
+
+    // If we're restoring the widget to it's old placement, fire off the
+    // onWidgetAdded event - our own handler will take care of adding it to
+    // any build areas.
+    this.beginBatchUpdate();
+    try {
+      if (widget.currentArea) {
+        this.notifyListeners(
+          "onWidgetAdded",
+          widget.id,
+          widget.currentArea,
+          widget.currentPosition
+        );
+      } else if (widgetMightNeedAutoAdding) {
+        let autoAdd = Services.prefs.getBoolPref(
+          kPrefCustomizationAutoAdd,
+          true
+        );
+
+        // If the widget doesn't have an existing placement, and it hasn't been
+        // seen before, then add it to its default area so it can be used.
+        // If the widget is not removable, we *have* to add it to its default
+        // area here.
+        let canBeAutoAdded = autoAdd && !gSeenWidgets.has(widget.id);
+        if (!widget.currentArea && (!widget.removable || canBeAutoAdded)) {
+          if (widget.defaultArea) {
+            if (this.isAreaLazy(widget.defaultArea)) {
+              gFuturePlacements.get(widget.defaultArea).add(widget.id);
+            } else {
+              this.addWidgetToArea(widget.id, widget.defaultArea);
+            }
+          }
+        }
+
+        // Extension widgets cannot enter the customization palette, so if
+        // at this point, we haven't found an area for them, move them into
+        // AREA_ADDONS.
+        if (
+          lazy.gUnifiedExtensionsEnabled &&
+          !widget.currentArea &&
+          CustomizableUI.isWebExtensionWidget(widget.id)
+        ) {
+          this.addWidgetToArea(widget.id, CustomizableUI.AREA_ADDONS);
+        }
+      }
+    } finally {
+      // Ensure we always have this widget in gSeenWidgets, and save
+      // state in case this needs to be done here.
+      gSeenWidgets.add(widget.id);
+      this.endBatchUpdate(true);
+    }
+
+    this.notifyListeners(
+      "onWidgetAfterCreation",
+      widget.id,
+      widget.currentArea
+    );
+    return widget.id;
+  },
+
+  createBuiltinWidget(aData) {
+    // This should only ever be called on startup, before any windows are
+    // opened - so we know there's no build areas to handle. Also, builtin
+    // widgets are expected to be (mostly) static, so shouldn't affect the
+    // current placement settings.
+
+    // This allows a widget to be both built-in by default but also able to be
+    // destroyed and removed from the area based on criteria that may not be
+    // available when the widget is created -- for example, because some other
+    // feature in the browser supersedes the widget.
+    let conditionalDestroyPromise = aData.conditionalDestroyPromise || null;
+    delete aData.conditionalDestroyPromise;
+
+    let widget = this.normalizeWidget(aData, CustomizableUI.SOURCE_BUILTIN);
+    if (!widget) {
+      lazy.log.error("Error creating builtin widget: " + aData.id);
+      return;
+    }
+
+    lazy.log.debug("Creating built-in widget with id: " + widget.id);
+    gPalette.set(widget.id, widget);
+
+    if (conditionalDestroyPromise) {
+      conditionalDestroyPromise.then(
+        shouldDestroy => {
+          if (shouldDestroy) {
+            this.destroyWidget(widget.id);
+            this.removeWidgetFromArea(widget.id);
+          }
+        },
+        err => {
+          Cu.reportError(err);
+        }
+      );
+    }
+  },
+
+  // Returns true if the area will eventually lazily restore (but hasn't yet).
+  isAreaLazy(aArea) {
+    if (gPlacements.has(aArea)) {
+      return false;
+    }
+    return gAreas.get(aArea).get("type") == CustomizableUI.TYPE_TOOLBAR;
+  },
+
+  // XXXunf Log some warnings here, when the data provided isn't up to scratch.
+  normalizeWidget(aData, aSource) {
+    let widget = {
+      implementation: aData,
+      source: aSource || CustomizableUI.SOURCE_EXTERNAL,
+      instances: new Map(),
+      currentArea: null,
+      localized: true,
+      removable: true,
+      overflows: true,
+      defaultArea: null,
+      shortcutId: null,
+      tabSpecific: false,
+      locationSpecific: false,
+      tooltiptext: null,
+      l10nId: null,
+      showInPrivateBrowsing: true,
+      _introducedInVersion: -1,
+      keepBroadcastAttributesWhenCustomizing: false,
+      disallowSubView: false,
+      webExtension: false,
+    };
+
+    if (typeof aData.id != "string" || !/^[a-z0-9-_]{1,}$/i.test(aData.id)) {
+      lazy.log.error("Given an illegal id in normalizeWidget: " + aData.id);
+      return null;
+    }
+
+    delete widget.implementation.currentArea;
+    widget.implementation.__defineGetter__(
+      "currentArea",
+      () => widget.currentArea
+    );
+
+    const kReqStringProps = ["id"];
+    for (let prop of kReqStringProps) {
+      if (typeof aData[prop] != "string") {
+        lazy.log.error(
+          "Missing required property '" +
+            prop +
+            "' in normalizeWidget: " +
+            aData.id
+        );
+        return null;
+      }
+      widget[prop] = aData[prop];
+    }
+
+    const kOptStringProps = ["l10nId", "label", "tooltiptext", "shortcutId"];
+    for (let prop of kOptStringProps) {
+      if (typeof aData[prop] == "string") {
+        widget[prop] = aData[prop];
+      }
+    }
+
+    const kOptBoolProps = [
+      "removable",
+      "showInPrivateBrowsing",
+      "overflows",
+      "tabSpecific",
+      "locationSpecific",
+      "localized",
+      "keepBroadcastAttributesWhenCustomizing",
+      "disallowSubView",
+      "webExtension",
+    ];
+    for (let prop of kOptBoolProps) {
+      if (typeof aData[prop] == "boolean") {
+        widget[prop] = aData[prop];
+      }
+    }
+
+    // When we normalize builtin widgets, areas have not yet been registered:
+    if (
+      aData.defaultArea &&
+      (aSource == CustomizableUI.SOURCE_BUILTIN ||
+        gAreas.has(aData.defaultArea))
+    ) {
+      widget.defaultArea = aData.defaultArea;
+    } else if (!widget.removable) {
+      lazy.log.error(
+        "Widget '" +
+          widget.id +
+          "' is not removable but does not specify " +
+          "a valid defaultArea. That's not possible; it must specify a " +
+          "valid defaultArea as well."
+      );
+      return null;
+    }
+
+    if ("type" in aData && gSupportedWidgetTypes.has(aData.type)) {
+      widget.type = aData.type;
+    } else {
+      widget.type = "button";
+    }
+
+    widget.disabled = aData.disabled === true;
+
+    if (aSource == CustomizableUI.SOURCE_BUILTIN) {
+      widget._introducedInVersion = aData.introducedInVersion || 0;
+    }
+
+    this.wrapWidgetEventHandler("onBeforeCreated", widget);
+    this.wrapWidgetEventHandler("onClick", widget);
+    this.wrapWidgetEventHandler("onCreated", widget);
+    this.wrapWidgetEventHandler("onDestroyed", widget);
+
+    if (typeof aData.onBeforeCommand == "function") {
+      widget.onBeforeCommand = aData.onBeforeCommand;
+    }
+
+    if (typeof aData.onCommand == "function") {
+      widget.onCommand = aData.onCommand;
+    }
+    if (
+      widget.type == "view" ||
+      widget.type == "button-and-view" ||
+      aData.viewId
+    ) {
+      if (typeof aData.viewId != "string") {
+        lazy.log.error(
+          "Expected a string for widget " +
+            widget.id +
+            " viewId, but got " +
+            aData.viewId
+        );
+        return null;
+      }
+      widget.viewId = aData.viewId;
+
+      this.wrapWidgetEventHandler("onViewShowing", widget);
+      this.wrapWidgetEventHandler("onViewHiding", widget);
+    }
+    if (widget.type == "custom") {
+      this.wrapWidgetEventHandler("onBuild", widget);
+    }
+
+    if (gPalette.has(widget.id)) {
+      return null;
+    }
+
+    return widget;
+  },
+
+  wrapWidgetEventHandler(aEventName, aWidget) {
+    if (typeof aWidget.implementation[aEventName] != "function") {
+      aWidget[aEventName] = null;
+      return;
+    }
+    aWidget[aEventName] = function(...aArgs) {
+      try {
+        // Don't copy the function to the normalized widget object, instead
+        // keep it on the original object provided to the API so that
+        // additional methods can be implemented and used by the event
+        // handlers.
+        return aWidget.implementation[aEventName].apply(
+          aWidget.implementation,
+          aArgs
+        );
+      } catch (e) {
+        Cu.reportError(e);
+        return undefined;
+      }
+    };
+  },
+
+  destroyWidget(aWidgetId) {
+    let widget = gPalette.get(aWidgetId);
+    if (!widget) {
+      gGroupWrapperCache.delete(aWidgetId);
+      for (let [window] of gBuildWindows) {
+        let windowCache = gSingleWrapperCache.get(window);
+        if (windowCache) {
+          windowCache.delete(aWidgetId);
+        }
+      }
+      return;
+    }
+
+    // Remove it from the default placements of an area if it was added there:
+    if (widget.defaultArea) {
+      let area = gAreas.get(widget.defaultArea);
+      if (area) {
+        let defaultPlacements = area.get("defaultPlacements");
+        // We can assume this is present because if a widget has a defaultArea,
+        // we automatically create a defaultPlacements array for that area.
+        let widgetIndex = defaultPlacements.indexOf(aWidgetId);
+        if (widgetIndex != -1) {
+          defaultPlacements.splice(widgetIndex, 1);
+        }
+      }
+    }
+
+    // This will not remove the widget from gPlacements - we want to keep the
+    // setting so the widget gets put back in it's old position if/when it
+    // returns.
+    for (let [window] of gBuildWindows) {
+      let windowCache = gSingleWrapperCache.get(window);
+      if (windowCache) {
+        windowCache.delete(aWidgetId);
+      }
+      let widgetNode =
+        window.document.getElementById(aWidgetId) ||
+        window.gNavToolbox.palette.getElementsByAttribute("id", aWidgetId)[0];
+      if (widgetNode) {
+        let container = widgetNode.parentNode;
+        this.notifyListeners(
+          "onWidgetBeforeDOMChange",
+          widgetNode,
+          null,
+          container,
+          true
+        );
+        widgetNode.remove();
+        this.notifyListeners(
+          "onWidgetAfterDOMChange",
+          widgetNode,
+          null,
+          container,
+          true
+        );
+      }
+      if (
+        widget.type == "view" ||
+        widget.type == "button-and-view" ||
+        widget.viewId
+      ) {
+        let viewNode = window.document.getElementById(widget.viewId);
+        if (viewNode) {
+          for (let eventName of kSubviewEvents) {
+            let handler = "on" + eventName;
+            if (typeof widget[handler] == "function") {
+              viewNode.removeEventListener(eventName, widget[handler]);
+            }
+          }
+          viewNode._addedEventListeners = false;
+        }
+      }
+      if (widgetNode && widget.onDestroyed) {
+        widget.onDestroyed(window.document);
+      }
+    }
+
+    gPalette.delete(aWidgetId);
+    gGroupWrapperCache.delete(aWidgetId);
+
+    this.notifyListeners("onWidgetDestroyed", aWidgetId);
+  },
+
+  getCustomizeTargetForArea(aArea, aWindow) {
+    let buildAreaNodes = gBuildAreas.get(aArea);
+    if (!buildAreaNodes) {
+      return null;
+    }
+
+    for (let node of buildAreaNodes) {
+      if (node.ownerGlobal == aWindow) {
+        return this.getCustomizationTarget(node) || node;
+      }
+    }
+
+    return null;
+  },
+
+  reset() {
+    gResetting = true;
+    this._resetUIState();
+
+    // Rebuild each registered area (across windows) to reflect the state that
+    // was reset above.
+    this._rebuildRegisteredAreas();
+
+    for (let [widgetId, widget] of gPalette) {
+      if (widget.source == CustomizableUI.SOURCE_EXTERNAL) {
+        gSeenWidgets.add(widgetId);
+      }
+    }
+    if (gSeenWidgets.size || gNewElementCount) {
+      gDirty = true;
+      this.saveState();
+    }
+
+    gResetting = false;
+  },
+
+  _resetUIState() {
+    try {
+      gUIStateBeforeReset.drawInTitlebar = Services.prefs.getIntPref(
+        kPrefDrawInTitlebar
+      );
+      gUIStateBeforeReset.uiCustomizationState = Services.prefs.getCharPref(
+        kPrefCustomizationState
+      );
+      gUIStateBeforeReset.uiDensity = Services.prefs.getIntPref(kPrefUIDensity);
+      gUIStateBeforeReset.autoTouchMode = Services.prefs.getBoolPref(
+        kPrefAutoTouchMode
+      );
+      gUIStateBeforeReset.currentTheme = gSelectedTheme;
+      gUIStateBeforeReset.autoHideDownloadsButton = Services.prefs.getBoolPref(
+        kPrefAutoHideDownloadsButton
+      );
+      gUIStateBeforeReset.newElementCount = gNewElementCount;
+    } catch (e) {}
+
+    Services.prefs.clearUserPref(kPrefCustomizationState);
+    Services.prefs.clearUserPref(kPrefDrawInTitlebar);
+    Services.prefs.clearUserPref(kPrefUIDensity);
+    Services.prefs.clearUserPref(kPrefAutoTouchMode);
+    Services.prefs.clearUserPref(kPrefAutoHideDownloadsButton);
+    gDefaultTheme.enable();
+    gNewElementCount = 0;
+    lazy.log.debug("State reset");
+
+    // Later in the function, we're going to add any area-less extension
+    // buttons to the AREA_ADDONS area. We'll remember the old placements
+    // for that area so that we don't need to re-add widgets that are already
+    // in there in the DOM.
+    let oldAddonPlacements = gPlacements[CustomizableUI.AREA_ADDONS] || [];
+
+    // Reset placements to make restoring default placements possible.
+    gPlacements = new Map();
+    gDirtyAreaCache = new Set();
+    gSeenWidgets = new Set();
+    // Clear the saved state to ensure that defaults will be used.
+    gSavedState = null;
+    // Restore the state for each area to its defaults
+    for (let [areaId] of gAreas) {
+      // If the Unified Extensions UI is enabled, we'll be adding any
+      // extension buttons that aren't already in AREA_ADDONS there,
+      // so we can skip restoring the state for it.
+      if (areaId != CustomizableUI.AREA_ADDONS) {
+        this.restoreStateForArea(areaId);
+      }
+    }
+
+    // restoreStateForArea will have normally set an array for the placements
+    // for each area, but since we skip AREA_ADDONS intentionally, that array
+    // doesn't get set, so we do that manually here.
+    gPlacements.set(CustomizableUI.AREA_ADDONS, []);
+
+    if (lazy.gUnifiedExtensionsEnabled) {
+      for (let [widgetId] of gPalette) {
+        if (
+          CustomizableUI.isWebExtensionWidget(widgetId) &&
+          !oldAddonPlacements.includes(widgetId)
+        ) {
+          this.addWidgetToArea(widgetId, CustomizableUI.AREA_ADDONS);
+        }
+      }
+    }
+  },
+
+  _rebuildRegisteredAreas() {
+    for (let [areaId, areaNodes] of gBuildAreas) {
+      let placements = gPlacements.get(areaId);
+      let isFirstChangedToolbar = true;
+      for (let areaNode of areaNodes) {
+        this.buildArea(areaId, placements, areaNode);
+
+        let area = gAreas.get(areaId);
+        if (area.get("type") == CustomizableUI.TYPE_TOOLBAR) {
+          let defaultCollapsed = area.get("defaultCollapsed");
+          let win = areaNode.ownerGlobal;
+          if (defaultCollapsed !== null) {
+            win.setToolbarVisibility(
+              areaNode,
+              typeof defaultCollapsed == "string"
+                ? defaultCollapsed
+                : !defaultCollapsed,
+              isFirstChangedToolbar
+            );
+          }
+        }
+        isFirstChangedToolbar = false;
+      }
+    }
+  },
+
+  /**
+   * Undoes a previous reset, restoring the state of the UI to the state prior to the reset.
+   */
+  undoReset() {
+    if (
+      gUIStateBeforeReset.uiCustomizationState == null ||
+      gUIStateBeforeReset.drawInTitlebar == null
+    ) {
+      return;
+    }
+    gUndoResetting = true;
+
+    const {
+      uiCustomizationState,
+      drawInTitlebar,
+      currentTheme,
+      uiDensity,
+      autoTouchMode,
+      autoHideDownloadsButton,
+    } = gUIStateBeforeReset;
+    gNewElementCount = gUIStateBeforeReset.newElementCount;
+
+    // Need to clear the previous state before setting the prefs
+    // because pref observers may check if there is a previous UI state.
+    this._clearPreviousUIState();
+
+    Services.prefs.setCharPref(kPrefCustomizationState, uiCustomizationState);
+    Services.prefs.setIntPref(kPrefDrawInTitlebar, drawInTitlebar);
+    Services.prefs.setIntPref(kPrefUIDensity, uiDensity);
+    Services.prefs.setBoolPref(kPrefAutoTouchMode, autoTouchMode);
+    Services.prefs.setBoolPref(
+      kPrefAutoHideDownloadsButton,
+      autoHideDownloadsButton
+    );
+    currentTheme.enable();
+    this.loadSavedState();
+    // If the user just customizes toolbar/titlebar visibility, gSavedState will be null
+    // and we don't need to do anything else here:
+    if (gSavedState) {
+      for (let areaId of Object.keys(gSavedState.placements)) {
+        let placements = gSavedState.placements[areaId];
+        gPlacements.set(areaId, placements);
+      }
+      this._rebuildRegisteredAreas();
+    }
+
+    gUndoResetting = false;
+  },
+
+  _clearPreviousUIState() {
+    Object.getOwnPropertyNames(gUIStateBeforeReset).forEach(prop => {
+      gUIStateBeforeReset[prop] = null;
+    });
+  },
+
+  /**
+   * @param {String|Node} aWidget - widget ID or a widget node (preferred for performance).
+   * @return {Boolean} whether the widget is removable
+   */
+  isWidgetRemovable(aWidget) {
+    let widgetId;
+    let widgetNode;
+    if (typeof aWidget == "string") {
+      widgetId = aWidget;
+    } else {
+      // Skipped items could just not have ids.
+      if (!aWidget.id && aWidget.getAttribute("skipintoolbarset") == "true") {
+        return false;
+      }
+      if (
+        !aWidget.id &&
+        !["toolbarspring", "toolbarspacer", "toolbarseparator"].includes(
+          aWidget.nodeName
+        )
+      ) {
+        throw new Error(
+          "No nodes without ids that aren't special widgets should ever come into contact with CUI"
+        );
+      }
+      // Use "spring" / "spacer" / "separator" for special widgets without ids
+      widgetId =
+        aWidget.id || aWidget.nodeName.substring(7 /* "toolbar".length */);
+      widgetNode = aWidget;
+    }
+    let provider = this.getWidgetProvider(widgetId);
+
+    if (provider == CustomizableUI.PROVIDER_API) {
+      return gPalette.get(widgetId).removable;
+    }
+
+    if (provider == CustomizableUI.PROVIDER_XUL) {
+      if (gBuildWindows.size == 0) {
+        // We don't have any build windows to look at, so just assume for now
+        // that its removable.
+        return true;
+      }
+
+      if (!widgetNode) {
+        // Pick any of the build windows to look at.
+        let [window] = [...gBuildWindows][0];
+        [, widgetNode] = this.getWidgetNode(widgetId, window);
+      }
+      // If we don't have a node, we assume it's removable. This can happen because
+      // getWidgetProvider returns PROVIDER_XUL by default, but this will also happen
+      // for API-provided widgets which have been destroyed.
+      if (!widgetNode) {
+        return true;
+      }
+      return widgetNode.getAttribute("removable") == "true";
+    }
+
+    // Otherwise this is either a special widget, which is always removable, or
+    // an API widget which has already been removed from gPalette. Returning true
+    // here allows us to then remove its ID from any placements where it might
+    // still occur.
+    return true;
+  },
+
+  canWidgetMoveToArea(aWidgetId, aArea) {
+    // Special widgets can't move to the menu panel.
+    if (
+      this.isSpecialWidget(aWidgetId) &&
+      gAreas.has(aArea) &&
+      gAreas.get(aArea).get("type") == CustomizableUI.TYPE_PANEL
+    ) {
+      return false;
+    }
+
+    if (
+      aArea == CustomizableUI.AREA_ADDONS &&
+      !CustomizableUI.isWebExtensionWidget(aWidgetId)
+    ) {
+      return false;
+    }
+
+    if (
+      lazy.gUnifiedExtensionsEnabled &&
+      CustomizableUI.isWebExtensionWidget(aWidgetId)
+    ) {
+      // Extension widgets cannot move to the customization palette.
+      if (aArea == CustomizableUI.AREA_NO_AREA) {
+        return false;
+      }
+
+      // Extension widgets cannot move to panels, with the exception of the
+      // AREA_ADDONS area.
+      if (
+        gAreas.get(aArea).get("type") == CustomizableUI.TYPE_PANEL &&
+        aArea != CustomizableUI.AREA_ADDONS
+      ) {
+        return false;
+      }
+    }
+
+    let placement = this.getPlacementOfWidget(aWidgetId);
+    // Items in the palette can move, and items can move within their area:
+    if (!placement || placement.area == aArea) {
+      return true;
+    }
+    // For everything else, just return whether the widget is removable.
+    return this.isWidgetRemovable(aWidgetId);
+  },
+
+  ensureWidgetPlacedInWindow(aWidgetId, aWindow) {
+    let placement = this.getPlacementOfWidget(aWidgetId);
+    if (!placement) {
+      return false;
+    }
+    let areaNodes = gBuildAreas.get(placement.area);
+    if (!areaNodes) {
+      return false;
+    }
+    let container = [...areaNodes].filter(n => n.ownerGlobal == aWindow);
+    if (!container.length) {
+      return false;
+    }
+    let existingNode = container[0].getElementsByAttribute("id", aWidgetId)[0];
+    if (existingNode) {
+      return true;
+    }
+
+    this.insertNodeInWindow(aWidgetId, container[0], true);
+    return true;
+  },
+
+  _getCurrentWidgetsInContainer(container) {
+    // Get a list of all the widget IDs in this container, including any that
+    // are overflown.
+    let currentWidgets = new Set();
+    function addUnskippedChildren(parent) {
+      for (let node of parent.children) {
+        let realNode =
+          node.localName == "toolbarpaletteitem"
+            ? node.firstElementChild
+            : node;
+        if (realNode.getAttribute("skipintoolbarset") != "true") {
+          currentWidgets.add(realNode.id);
+        }
+      }
+    }
+    addUnskippedChildren(this.getCustomizationTarget(container));
+    if (container.getAttribute("overflowing") == "true") {
+      let overflowTarget = container.getAttribute("default-overflowtarget");
+      addUnskippedChildren(
+        container.ownerDocument.getElementById(overflowTarget)
+      );
+      let webExtOverflowTarget = container.getAttribute(
+        "addon-webext-overflowtarget"
+      );
+      addUnskippedChildren(
+        container.ownerDocument.getElementById(webExtOverflowTarget)
+      );
+    }
+    // Then get the sorted list of placements, and filter based on the nodes
+    // that are present. This avoids including items that don't exist (e.g. ids
+    // of add-on items that the user has uninstalled).
+    let orderedPlacements = CustomizableUI.getWidgetIdsInArea(container.id);
+    return orderedPlacements.filter(w => currentWidgets.has(w));
+  },
+
+  get inDefaultState() {
+    for (let [areaId, props] of gAreas) {
+      let defaultPlacements = props
+        .get("defaultPlacements")
+        .filter(item => this.widgetExists(item));
+      let currentPlacements = gPlacements.get(areaId);
+      // We're excluding all of the placement IDs for items that do not exist,
+      // and items that have removable="false",
+      // because we don't want to consider them when determining if we're
+      // in the default state. This way, if an add-on introduces a widget
+      // and is then uninstalled, the leftover placement doesn't cause us to
+      // automatically assume that the buttons are not in the default state.
+      let buildAreaNodes = gBuildAreas.get(areaId);
+      if (buildAreaNodes && buildAreaNodes.size) {
+        let container = [...buildAreaNodes][0];
+        let removableOrDefault = itemNodeOrItem => {
+          let item = (itemNodeOrItem && itemNodeOrItem.id) || itemNodeOrItem;
+          let isRemovable = this.isWidgetRemovable(itemNodeOrItem);
+          let isInDefault = defaultPlacements.includes(item);
+          return isRemovable || isInDefault;
+        };
+        // Toolbars need to deal with overflown widgets (if any) - so
+        // specialcase them:
+        if (props.get("type") == CustomizableUI.TYPE_TOOLBAR) {
+          currentPlacements = this._getCurrentWidgetsInContainer(
+            container
+          ).filter(removableOrDefault);
+        } else {
+          currentPlacements = currentPlacements.filter(item => {
+            let itemNode = container.getElementsByAttribute("id", item)[0];
+            return itemNode && removableOrDefault(itemNode || item);
+          });
+        }
+
+        if (props.get("type") == CustomizableUI.TYPE_TOOLBAR) {
+          let collapsed = null;
+          let defaultCollapsed = props.get("defaultCollapsed");
+          let nondefaultState = false;
+          if (areaId == CustomizableUI.AREA_BOOKMARKS) {
+            collapsed = Services.prefs.getCharPref(
+              "browser.toolbars.bookmarks.visibility"
+            );
+            nondefaultState = Services.prefs.prefHasUserValue(
+              "browser.toolbars.bookmarks.visibility"
+            );
+          } else {
+            let attribute =
+              container.getAttribute("type") == "menubar"
+                ? "autohide"
+                : "collapsed";
+            collapsed = container.getAttribute(attribute) == "true";
+            nondefaultState = collapsed != defaultCollapsed;
+          }
+          if (defaultCollapsed !== null && nondefaultState) {
+            lazy.log.debug(
+              "Found " +
+                areaId +
+                " had non-default toolbar visibility" +
+                "(expected " +
+                defaultCollapsed +
+                ", was " +
+                collapsed +
+                ")"
+            );
+            return false;
+          }
+        }
+      }
+      lazy.log.debug(
+        "Checking default state for " +
+          areaId +
+          ":\n" +
+          currentPlacements.join(",") +
+          "\nvs.\n" +
+          defaultPlacements.join(",")
+      );
+
+      if (currentPlacements.length != defaultPlacements.length) {
+        return false;
+      }
+
+      for (let i = 0; i < currentPlacements.length; ++i) {
+        if (
+          currentPlacements[i] != defaultPlacements[i] &&
+          !this.matchingSpecials(currentPlacements[i], defaultPlacements[i])
+        ) {
+          lazy.log.debug(
+            "Found " +
+              currentPlacements[i] +
+              " in " +
+              areaId +
+              " where " +
+              defaultPlacements[i] +
+              " was expected!"
+          );
+          return false;
+        }
+      }
+    }
+
+    if (Services.prefs.prefHasUserValue(kPrefUIDensity)) {
+      lazy.log.debug(kPrefUIDensity + " pref is non-default");
+      return false;
+    }
+
+    if (Services.prefs.prefHasUserValue(kPrefAutoTouchMode)) {
+      lazy.log.debug(kPrefAutoTouchMode + " pref is non-default");
+      return false;
+    }
+
+    if (Services.prefs.prefHasUserValue(kPrefDrawInTitlebar)) {
+      lazy.log.debug(kPrefDrawInTitlebar + " pref is non-default");
+      return false;
+    }
+
+    // This should just be `gDefaultTheme.isActive`, but bugs...
+    if (gDefaultTheme && gDefaultTheme.id != gSelectedTheme.id) {
+      lazy.log.debug(gSelectedTheme.id + " theme is non-default");
+      return false;
+    }
+
+    return true;
+  },
+
+  setToolbarVisibility(aToolbarId, aIsVisible) {
+    // We only persist the attribute the first time.
+    let isFirstChangedToolbar = true;
+    for (let window of CustomizableUI.windows) {
+      let toolbar = window.document.getElementById(aToolbarId);
+      if (toolbar) {
+        window.setToolbarVisibility(toolbar, aIsVisible, isFirstChangedToolbar);
+        isFirstChangedToolbar = false;
+      }
+    }
+  },
+
+  observe(aSubject, aTopic, aData) {
+    if (aTopic == "browser-set-toolbar-visibility") {
+      let [toolbar, visibility] = JSON.parse(aData);
+      CustomizableUI.setToolbarVisibility(toolbar, visibility == "true");
+    }
+  },
+};
+Object.freeze(CustomizableUIInternal);
+
+var CustomizableUI = {
+  /**
+   * Constant reference to the ID of the navigation toolbar.
+   */
+  AREA_NAVBAR: "nav-bar",
+  /**
+   * Constant reference to the ID of the menubar's toolbar.
+   */
+  AREA_MENUBAR: "toolbar-menubar",
+  /**
+   * Constant reference to the ID of the tabstrip toolbar.
+   */
+  AREA_TABSTRIP: "TabsToolbar",
+  /**
+   * Constant reference to the ID of the bookmarks toolbar.
+   */
+  AREA_BOOKMARKS: "PersonalToolbar",
+  /**
+   * Constant reference to the ID of the non-dymanic (fixed) list in the overflow panel.
+   */
+  AREA_FIXED_OVERFLOW_PANEL: "widget-overflow-fixed-list",
+  /**
+   * Constant reference to the ID of the addons area.
+   */
+  AREA_ADDONS: "unified-extensions-area",
+  /**
+   * Constant reference to the ID of the customization palette, which is
+   * where widgets go when they're not assigned to an area. Note that this
+   * area is "virtual" in that it's never set as a value for a widgets
+   * currentArea or defaultArea. It's only used for the `canWidgetMoveToArea`
+   * function to check if widgets can be moved to the palette. Callers who
+   * wish to move items to the palette should use `removeWidgetFromArea`.
+   */
+  AREA_NO_AREA: "customization-palette",
+  /**
+   * Constant indicating the area is a panel.
+   */
+  TYPE_PANEL: "panel",
+  /**
+   * Constant indicating the area is a toolbar.
+   */
+  TYPE_TOOLBAR: "toolbar",
+
+  /**
+   * Constant indicating a XUL-type provider.
+   */
+  PROVIDER_XUL: "xul",
+  /**
+   * Constant indicating an API-type provider.
+   */
+  PROVIDER_API: "api",
+  /**
+   * Constant indicating dynamic (special) widgets: spring, spacer, and separator.
+   */
+  PROVIDER_SPECIAL: "special",
+
+  /**
+   * Constant indicating the widget is built-in
+   */
+  SOURCE_BUILTIN: "builtin",
+  /**
+   * Constant indicating the widget is externally provided
+   * (e.g. by add-ons or other items not part of the builtin widget set).
+   */
+  SOURCE_EXTERNAL: "external",
+
+  /**
+   * Constant indicating the reason the event was fired was a window closing
+   */
+  REASON_WINDOW_CLOSED: "window-closed",
+  /**
+   * Constant indicating the reason the event was fired was an area being
+   * unregistered separately from window closing mechanics.
+   */
+  REASON_AREA_UNREGISTERED: "area-unregistered",
+
+  /**
+   * An iteratable property of windows managed by CustomizableUI.
+   * Note that this can *only* be used as an iterator. ie:
+   *     for (let window of CustomizableUI.windows) { ... }
+   */
+  windows: {
+    *[Symbol.iterator]() {
+      for (let [window] of gBuildWindows) {
+        yield window;
+      }
+    },
+  },
+
+  /**
+   * Add a listener object that will get fired for various events regarding
+   * customization.
+   *
+   * @param aListener the listener object to add
+   *
+   * Not all event handler methods need to be defined.
+   * CustomizableUI will catch exceptions. Events are dispatched
+   * synchronously on the UI thread, so if you can delay any/some of your
+   * processing, that is advisable. The following event handlers are supported:
+   *   - onWidgetAdded(aWidgetId, aArea, aPosition)
+   *     Fired when a widget is added to an area. aWidgetId is the widget that
+   *     was added, aArea the area it was added to, and aPosition the position
+   *     in which it was added.
+   *   - onWidgetMoved(aWidgetId, aArea, aOldPosition, aNewPosition)
+   *     Fired when a widget is moved within its area. aWidgetId is the widget
+   *     that was moved, aArea the area it was moved in, aOldPosition its old
+   *     position, and aNewPosition its new position.
+   *   - onWidgetRemoved(aWidgetId, aArea)
+   *     Fired when a widget is removed from its area. aWidgetId is the widget
+   *     that was removed, aArea the area it was removed from.
+   *
+   *   - onWidgetBeforeDOMChange(aNode, aNextNode, aContainer, aIsRemoval)
+   *     Fired *before* a widget's DOM node is acted upon by CustomizableUI
+   *     (to add, move or remove it). aNode is the DOM node changed, aNextNode
+   *     the DOM node (if any) before which a widget will be inserted,
+   *     aContainer the *actual* DOM container (could be an overflow panel in
+   *     case of an overflowable toolbar), and aWasRemoval is true iff the
+   *     action about to happen is the removal of the DOM node.
+   *   - onWidgetAfterDOMChange(aNode, aNextNode, aContainer, aWasRemoval)
+   *     Like onWidgetBeforeDOMChange, but fired after the change to the DOM
+   *     node of the widget.
+   *
+   *   - onWidgetReset(aNode, aContainer)
+   *     Fired after a reset to default placements moves a widget's node to a
+   *     different location. aNode is the widget's node, aContainer is the
+   *     area it was moved into (NB: it might already have been there and been
+   *     moved to a different position!)
+   *   - onWidgetUndoMove(aNode, aContainer)
+   *     Fired after undoing a reset to default placements moves a widget's
+   *     node to a different location. aNode is the widget's node, aContainer
+   *     is the area it was moved into (NB: it might already have been there
+   *     and been moved to a different position!)
+   *   - onAreaReset(aArea, aContainer)
+   *     Fired after a reset to default placements is complete on an area's
+   *     DOM node. Note that this is fired for each DOM node. aArea is the area
+   *     that was reset, aContainer the DOM node that was reset.
+   *
+   *   - onWidgetCreated(aWidgetId)
+   *     Fired when a widget with id aWidgetId has been created, but before it
+   *     is added to any placements or any DOM nodes have been constructed.
+   *     Only fired for API-based widgets.
+   *   - onWidgetAfterCreation(aWidgetId, aArea)
+   *     Fired after a widget with id aWidgetId has been created, and has been
+   *     added to either its default area or the area in which it was placed
+   *     previously. If the widget has no default area and/or it has never
+   *     been placed anywhere, aArea may be null. Only fired for API-based
+   *     widgets.
+   *   - onWidgetDestroyed(aWidgetId)
+   *     Fired when widgets are destroyed. aWidgetId is the widget that is
+   *     being destroyed. Only fired for API-based widgets.
+   *   - onWidgetInstanceRemoved(aWidgetId, aDocument)
+   *     Fired when a window is unloaded and a widget's instance is destroyed
+   *     because of this. Only fired for API-based widgets.
+   *
+   *   - onWidgetDrag(aWidgetId, aArea)
+   *     Fired both when and after customize mode drag handling system tries
+   *     to determine the width and height of widget aWidgetId when dragged to a
+   *     different area. aArea will be the area the item is dragged to, or
+   *     undefined after the measurements have been done and the node has been
+   *     moved back to its 'regular' area.
+   *
+   *   - onCustomizeStart(aWindow)
+   *     Fired when opening customize mode in aWindow.
+   *   - onCustomizeEnd(aWindow)
+   *     Fired when exiting customize mode in aWindow.
+   *
+   *   - onWidgetOverflow(aNode, aContainer)
+   *     Fired when a widget's DOM node is overflowing its container, a toolbar,
+   *     and will be displayed in the overflow panel.
+   *   - onWidgetUnderflow(aNode, aContainer)
+   *     Fired when a widget's DOM node is *not* overflowing its container, a
+   *     toolbar, anymore.
+   *   - onWindowOpened(aWindow)
+   *     Fired when a window has been opened that is managed by CustomizableUI,
+   *     once all of the prerequisite setup has been done.
+   *   - onWindowClosed(aWindow)
+   *     Fired when a window that has been managed by CustomizableUI has been
+   *     closed.
+   *   - onAreaNodeRegistered(aArea, aContainer)
+   *     Fired after an area node is first built when it is registered. This
+   *     is often when the window has opened, but in the case of add-ons,
+   *     could fire when the node has just been registered with CustomizableUI
+   *     after an add-on update or disable/enable sequence.
+   *   - onAreaNodeUnregistered(aArea, aContainer, aReason)
+   *     Fired when an area node is explicitly unregistered by an API caller,
+   *     or by a window closing. The aReason parameter indicates which of
+   *     these is the case.
+   */
+  addListener(aListener) {
+    CustomizableUIInternal.addListener(aListener);
+  },
+  /**
+   * Remove a listener added with addListener
+   * @param aListener the listener object to remove
+   */
+  removeListener(aListener) {
+    CustomizableUIInternal.removeListener(aListener);
+  },
+
+  /**
+   * Register a customizable area with CustomizableUI.
+   * @param aName   the name of the area to register. Can only contain
+   *                alphanumeric characters, dashes (-) and underscores (_).
+   * @param aProps  the properties of the area. The following properties are
+   *                recognized:
+   *                - type:   the type of area. Either TYPE_TOOLBAR (default) or
+   *                          TYPE_PANEL;
+   *                - anchor: for a menu panel or overflowable toolbar, the
+   *                          anchoring node for the panel.
+   *                - overflowable: set to true if your toolbar is overflowable.
+   *                                This requires an anchor, and only has an
+   *                                effect for toolbars.
+   *                - defaultPlacements: an array of widget IDs making up the
+   *                                     default contents of the area
+   *                - defaultCollapsed: (INTERNAL ONLY) applies if the type is TYPE_TOOLBAR, specifies
+   *                                    if toolbar is collapsed by default (default to true).
+   *                                    Specify null to ensure that reset/inDefaultArea don't care
+   *                                    about a toolbar's collapsed state
+   */
+  registerArea(aName, aProperties) {
+    CustomizableUIInternal.registerArea(aName, aProperties);
+  },
+  /**
+   * Register a concrete node for a registered area. This method needs to be called
+   * with any toolbar in the main browser window that has its "customizable" attribute
+   * set to true.
+   *
+   * Note that ideally, you should register your toolbar using registerArea
+   * before calling this. If you don't, the node will be saved for processing when
+   * you call registerArea. Note that CustomizableUI won't restore state in the area,
+   * allow the user to customize it in customize mode, or otherwise deal
+   * with it, until the area has been registered.
+   */
+  registerToolbarNode(aToolbar) {
+    CustomizableUIInternal.registerToolbarNode(aToolbar);
+  },
+  /**
+   * Register a panel node. A panel treated slightly differently from a toolbar in
+   * terms of what items can be moved into it. For example, a panel cannot have a
+   * spacer or a spring put into it.
+   *
+   * @param aPanelContents the panel contents DOM node being registered.
+   * @param aArea the area for which to register this node.
+   */
+  registerPanelNode(aNode, aArea) {
+    CustomizableUIInternal.registerPanelNode(aNode, aArea);
+  },
+  /**
+   * Unregister a customizable area. The inverse of registerArea.
+   *
+   * Unregistering an area will remove all the (removable) widgets in the
+   * area, which will return to the panel, and destroy all other traces
+   * of the area within CustomizableUI. Note that this means the *contents*
+   * of the area's DOM nodes will be moved to the panel or removed, but
+   * the area's DOM nodes *themselves* will stay.
+   *
+   * Furthermore, by default the placements of the area will be kept in the
+   * saved state (!) and restored if you re-register the area at a later
+   * point. This is useful for e.g. add-ons that get disabled and then
+   * re-enabled (e.g. when they update).
+   *
+   * You can override this last behaviour (and destroy the placements
+   * information in the saved state) by passing true for aDestroyPlacements.
+   *
+   * @param aName              the name of the area to unregister
+   * @param aDestroyPlacements whether to destroy the placements information
+   *                           for the area, too.
+   */
+  unregisterArea(aName, aDestroyPlacements) {
+    CustomizableUIInternal.unregisterArea(aName, aDestroyPlacements);
+  },
+  /**
+   * Add a widget to an area.
+   * If the area to which you try to add is not known to CustomizableUI,
+   * this will throw.
+   * If the area to which you try to add is the same as the area in which
+   * the widget is currently placed, this will do the same as
+   * moveWidgetWithinArea.
+   * If the widget cannot be removed from its original location, this will
+   * no-op.
+   *
+   * This will fire an onWidgetAdded notification,
+   * and an onWidgetBeforeDOMChange and onWidgetAfterDOMChange notification
+   * for each window CustomizableUI knows about.
+   *
+   * @param aWidgetId the ID of the widget to add
+   * @param aArea     the ID of the area to add the widget to
+   * @param aPosition the position at which to add the widget. If you do not
+   *                  pass a position, the widget will be added to the end
+   *                  of the area.
+   */
+  addWidgetToArea(aWidgetId, aArea, aPosition) {
+    CustomizableUIInternal.addWidgetToArea(aWidgetId, aArea, aPosition);
+  },
+  /**
+   * Remove a widget from its area. If the widget cannot be removed from its
+   * area, or is not in any area, this will no-op. Otherwise, this will fire an
+   * onWidgetRemoved notification, and an onWidgetBeforeDOMChange and
+   * onWidgetAfterDOMChange notification for each window CustomizableUI knows
+   * about.
+   *
+   * @param aWidgetId the ID of the widget to remove
+   */
+  removeWidgetFromArea(aWidgetId) {
+    CustomizableUIInternal.removeWidgetFromArea(aWidgetId);
+  },
+  /**
+   * Move a widget within an area.
+   * If the widget is not in any area, this will no-op.
+   * If the widget is already at the indicated position, this will no-op.
+   *
+   * Otherwise, this will move the widget and fire an onWidgetMoved notification,
+   * and an onWidgetBeforeDOMChange and onWidgetAfterDOMChange notification for
+   * each window CustomizableUI knows about.
+   *
+   * @param aWidgetId the ID of the widget to move
+   * @param aPosition the position to move the widget to.
+   *                  Negative values or values greater than the number of
+   *                  widgets will be interpreted to mean moving the widget to
+   *                  respectively the first or last position.
+   */
+  moveWidgetWithinArea(aWidgetId, aPosition) {
+    CustomizableUIInternal.moveWidgetWithinArea(aWidgetId, aPosition);
+  },
+  /**
+   * Ensure a XUL-based widget created in a window after areas were
+   * initialized moves to its correct position.
+   * Always prefer this over moving items in the DOM yourself.
+   *
+   * @param aWidgetId the ID of the widget that was just created
+   * @param aWindow the window in which you want to ensure it was added.
+   *
+   * NB: why is this API per-window, you wonder? Because if you need this,
+   * presumably you yourself need to create the widget in all the windows
+   * and need to loop through them anyway.
+   */
+  ensureWidgetPlacedInWindow(aWidgetId, aWindow) {
+    return CustomizableUIInternal.ensureWidgetPlacedInWindow(
+      aWidgetId,
+      aWindow
+    );
+  },
+  /**
+   * Start a batch update of items.
+   * During a batch update, the customization state is not saved to the user's
+   * preferences file, in order to reduce (possibly sync) IO.
+   * Calls to begin/endBatchUpdate may be nested.
+   *
+   * Callers should ensure that NO MATTER WHAT they call endBatchUpdate once
+   * for each call to beginBatchUpdate, even if there are exceptions in the
+   * code in the batch update. Otherwise, for the duration of the
+   * Firefox session, customization state is never saved. Typically, you
+   * would do this using a try...finally block.
+   */
+  beginBatchUpdate() {
+    CustomizableUIInternal.beginBatchUpdate();
+  },
+  /**
+   * End a batch update. See the documentation for beginBatchUpdate above.
+   *
+   * State is not saved if we believe it is identical to the last known
+   * saved state. State is only ever saved when all batch updates have
+   * finished (ie there has been 1 endBatchUpdate call for each
+   * beginBatchUpdate call). If any of the endBatchUpdate calls pass
+   * aForceDirty=true, we will flush to the prefs file.
+   *
+   * @param aForceDirty force CustomizableUI to flush to the prefs file when
+   *                    all batch updates have finished.
+   */
+  endBatchUpdate(aForceDirty) {
+    CustomizableUIInternal.endBatchUpdate(aForceDirty);
+  },
+  /**
+   * Create a widget.
+   *
+   * To create a widget, you should pass an object with its desired
+   * properties. The following properties are supported:
+   *
+   * - id:            the ID of the widget (required).
+   * - type:          a string indicating the type of widget. Possible types
+   *                  are:
+   *                  'button' - for simple button widgets (the default)
+   *                  'view'   - for buttons that open a panel or subview,
+   *                             depending on where they are placed.
+   *                  'button-and-view' - A combination of 'button' and 'view',
+   *                             which looks different depending on whether it's
+   *                             located in the toolbar or in the panel: When
+   *                             located in the toolbar, the widget is shown as
+   *                             a combined item of a button and a dropmarker
+   *                             button. The button triggers the command and the
+   *                             dropmarker button opens the view. When located
+   *                             in the panel, shown as one item which opens the
+   *                             view, and the button command cannot be
+   *                             triggered separately.
+   *                  'custom' - for fine-grained control over the creation
+   *                             of the widget.
+   * - viewId:        Only useful for views and button-and-view widgets (and
+   *                  required there): the id of the <panelview> that should be
+   *                  shown when clicking the widget.  If used with a custom
+   *                  widget, the widget must also provide a toolbaritem where
+   *                  the first child is the view button.
+   * - onBuild(aDoc): Only useful for custom widgets (and required there); a
+   *                  function that will be invoked with the document in which
+   *                  to build a widget. Should return the DOM node that has
+   *                  been constructed.
+   * - onBeforeCreated(aDoc): Attached to all non-custom widgets; a function
+   *                  that will be invoked before the widget gets a DOM node
+   *                  constructed, passing the document in which that will happen.
+   *                  This is useful especially for 'view' type widgets that need
+   *                  to construct their views on the fly (e.g. from bootstrapped
+   *                  add-ons). If the function returns `false`, the widget will
+   *                  not be created.
+   * - onCreated(aNode): Attached to all widgets; a function that will be invoked
+   *                  whenever the widget has a DOM node constructed, passing the
+   *                  constructed node as an argument.
+   * - onDestroyed(aDoc): Attached to all non-custom widgets; a function that
+   *                  will be invoked after the widget has a DOM node destroyed,
+   *                  passing the document from which it was removed. This is
+   *                  useful especially for 'view' type widgets that need to
+   *                  cleanup after views that were constructed on the fly.
+   * - onBeforeCommand(aEvt, aNode): A function that will be invoked when the user
+   *                          activates the button but before the command
+   *                          is evaluated. Useful if code needs to run to
+   *                          change the button's icon in preparation to the
+   *                          pending command action. Called for any type that
+   *                          supports the handler.  The command type, either
+   *                          "view" or "command", may be returned to force the
+   *                          action that will occur.  View will open the panel
+   *                          and command will result in calling onCommand.
+   * - onCommand(aEvt): Useful for custom, button and button-and-view widgets; a
+   *                    function that will be invoked when the user activates
+   *                    the button. A custom widget with a view should
+   *                    return "view" or "command" to continue processing
+   *                    the command per the needs of the widget.
+   * - onClick(aEvt): Attached to all widgets; a function that will be invoked
+   *                  when the user clicks the widget.
+   * - onViewShowing(aEvt): Only useful for views and button-and-view widgets; a
+   *                  function that will be invoked when a user shows your view.
+   *                  If any event handler calls aEvt.preventDefault(), the view
+   *                  will not be shown.
+   *
+   *                  The event's `detail` property is an object with an
+   *                  `addBlocker` method. Handlers which need to
+   *                  perform asynchronous operations before the view is
+   *                  shown may pass this method a Promise, which will
+   *                  prevent the view from showing until it resolves.
+   *                  Additionally, if the promise resolves to the exact
+   *                  value `false`, the view will not be shown.
+   * - onViewHiding(aEvt): Only useful for views; a function that will be
+   *                  invoked when a user hides your view.
+   * - l10nId:        fluent string identifier to use for localizing attributes
+   *                  on the widget. If present, preferred over the
+   *                  label/tooltiptext.
+   * - tooltiptext:   string to use for the tooltip of the widget
+   * - label:         string to use for the label of the widget
+   * - localized:     If true, or undefined, attempt to retrieve the
+   *                  widget's string properties from the customizable
+   *                  widgets string bundle.
+   * - removable:     whether the widget is removable (optional, default: true)
+   *                  NB: if you specify false here, you must provide a
+   *                  defaultArea, too.
+   * - overflows:     whether widget can overflow when in an overflowable
+   *                  toolbar (optional, default: true)
+   * - defaultArea:   default area to add the widget to
+   *                  (optional, default: none; required if non-removable)
+   * - shortcutId:    id of an element that has a shortcut for this widget
+   *                  (optional, default: null). This is only used to display
+   *                  the shortcut as part of the tooltip for builtin widgets
+   *                  (which have strings inside
+   *                  customizableWidgets.properties). If you're in an add-on,
+   *                  you should not set this property.
+   *                  If l10nId is provided, the resulting shortcut is passed
+   *                  as the "$shortcut" variable to the fluent message.
+   * - showInPrivateBrowsing: whether to show the widget in private browsing
+   *                          mode (optional, default: true)
+   * - tabSpecific:      If true, closes the panel if the tab changes.
+   * - locationSpecific: If true, closes the panel if the location changes.
+   *                     This is similar to tabSpecific, but also if the location
+   *                     changes in the same tab, we may want to close the panel.
+   * - webExtension:  Set to true if this widget is being created on behalf of an
+   *                  extension.
+   *
+   * @param aProperties the specifications for the widget.
+   * @return a wrapper around the created widget (see getWidget)
+   */
+  createWidget(aProperties) {
+    return CustomizableUIInternal.wrapWidget(
+      CustomizableUIInternal.createWidget(aProperties)
+    );
+  },
+  /**
+   * Destroy a widget
+   *
+   * If the widget is part of the default placements in an area, this will
+   * remove it from there. It will also remove any DOM instances. However,
+   * it will keep the widget in the placements for whatever area it was
+   * in at the time. You can remove it from there yourself by calling
+   * CustomizableUI.removeWidgetFromArea(aWidgetId).
+   *
+   * @param aWidgetId the ID of the widget to destroy
+   */
+  destroyWidget(aWidgetId) {
+    CustomizableUIInternal.destroyWidget(aWidgetId);
+  },
+  /**
+   * Get a wrapper object with information about the widget.
+   * The object provides the following properties
+   * (all read-only unless otherwise indicated):
+   *
+   * - id:            the widget's ID;
+   * - type:          the type of widget (button, view, custom). For
+   *                  XUL-provided widgets, this is always 'custom';
+   * - provider:      the provider type of the widget, id est one of
+   *                  PROVIDER_API or PROVIDER_XUL;
+   * - forWindow(w):  a method to obtain a single window wrapper for a widget,
+   *                  in the window w passed as the only argument;
+   * - instances:     an array of all instances (single window wrappers)
+   *                  of the widget. This array is NOT live;
+   * - areaType:      the type of the widget's current area
+   * - isGroup:       true; will be false for wrappers around single widget nodes;
+   * - source:        for API-provided widgets, whether they are built-in to
+   *                  Firefox or add-on-provided;
+   * - disabled:      for API-provided widgets, whether the widget is currently
+   *                  disabled. NB: this property is writable, and will toggle
+   *                  all the widgets' nodes' disabled states;
+   * - label:         for API-provied widgets, the label of the widget;
+   * - tooltiptext:   for API-provided widgets, the tooltip of the widget;
+   * - showInPrivateBrowsing: for API-provided widgets, whether the widget is
+   *                          visible in private browsing;
+   *
+   * Single window wrappers obtained through forWindow(someWindow) or from the
+   * instances array have the following properties
+   * (all read-only unless otherwise indicated):
+   *
+   * - id:            the widget's ID;
+   * - type:          the type of widget (button, view, custom). For
+   *                  XUL-provided widgets, this is always 'custom';
+   * - provider:      the provider type of the widget, id est one of
+   *                  PROVIDER_API or PROVIDER_XUL;
+   * - node:          reference to the corresponding DOM node;
+   * - anchor:        the anchor on which to anchor panels opened from this
+   *                  node. This will point to the overflow chevron on
+   *                  overflowable toolbars if and only if your widget node
+   *                  is overflowed, to the anchor for the panel menu
+   *                  if your widget is inside the panel menu, and to the
+   *                  node itself in all other cases;
+   * - overflowed:    boolean indicating whether the node is currently in the
+   *                  overflow panel of the toolbar;
+   * - isGroup:       false; will be true for the group widget;
+   * - label:         for API-provided widgets, convenience getter for the
+   *                  label attribute of the DOM node;
+   * - tooltiptext:   for API-provided widgets, convenience getter for the
+   *                  tooltiptext attribute of the DOM node;
+   * - disabled:      for API-provided widgets, convenience getter *and setter*
+   *                  for the disabled state of this single widget. Note that
+   *                  you may prefer to use the group wrapper's getter/setter
+   *                  instead.
+   *
+   * @param aWidgetId the ID of the widget whose information you need
+   * @return a wrapper around the widget as described above, or null if the
+   *         widget is known not to exist (anymore). NB: non-null return
+   *         is no guarantee the widget exists because we cannot know in
+   *         advance if a XUL widget exists or not.
+   */
+  getWidget(aWidgetId) {
+    return CustomizableUIInternal.wrapWidget(aWidgetId);
+  },
+  /**
+   * Get an array of widget wrappers (see getWidget) for all the widgets
+   * which are currently not in any area (so which are in the palette).
+   *
+   * @param aWindowPalette the palette (and by extension, the window) in which
+   *                       CustomizableUI should look. This matters because of
+   *                       course XUL-provided widgets could be available in
+   *                       some windows but not others, and likewise
+   *                       API-provided widgets might not exist in a private
+   *                       window (because of the showInPrivateBrowsing
+   *                       property).
+   *
+   * @return an array of widget wrappers (see getWidget)
+   */
+  getUnusedWidgets(aWindowPalette) {
+    return CustomizableUIInternal.getUnusedWidgets(aWindowPalette).map(
+      CustomizableUIInternal.wrapWidget,
+      CustomizableUIInternal
+    );
+  },
+  /**
+   * Get an array of all the widget IDs placed in an area.
+   * Modifying the array will not affect CustomizableUI.
+   *
+   * @param aArea the ID of the area whose placements you want to obtain.
+   * @return an array containing the widget IDs that are in the area.
+   *
+   * NB: will throw if called too early (before placements have been fetched)
+   *     or if the area is not currently known to CustomizableUI.
+   */
+  getWidgetIdsInArea(aArea) {
+    if (!gAreas.has(aArea)) {
+      throw new Error("Unknown customization area: " + aArea);
+    }
+    if (!gPlacements.has(aArea)) {
+      throw new Error("Area not yet restored");
+    }
+
+    // We need to clone this, as we don't want to let consumers muck with placements
+    return [...gPlacements.get(aArea)];
+  },
+  /**
+   * Get an array of widget wrappers for all the widgets in an area. This is
+   * the same as calling getWidgetIdsInArea and .map() ing the result through
+   * CustomizableUI.getWidget. Careful: this means that if there are IDs in there
+   * which don't have corresponding DOM nodes, there might be nulls in this array,
+   * or items for which wrapper.forWindow(win) will return null.
+   *
+   * @param aArea the ID of the area whose widgets you want to obtain.
+   * @return an array of widget wrappers and/or null values for the widget IDs
+   *         placed in an area.
+   *
+   * NB: will throw if called too early (before placements have been fetched)
+   *     or if the area is not currently known to CustomizableUI.
+   */
+  getWidgetsInArea(aArea) {
+    return this.getWidgetIdsInArea(aArea).map(
+      CustomizableUIInternal.wrapWidget,
+      CustomizableUIInternal
+    );
+  },
+
+  /**
+   * Ensure the customizable widget that matches up with this view node
+   * will get the right subview showing/shown/hiding/hidden events when
+   * they fire.
+   * @param aViewNode the view node to add listeners to if they haven't
+   *                  been added already.
+   */
+  ensureSubviewListeners(aViewNode) {
+    return CustomizableUIInternal.ensureSubviewListeners(aViewNode);
+  },
+  /**
+   * Obtain an array of all the area IDs known to CustomizableUI.
+   * This array is created for you, so is modifiable without CustomizableUI
+   * being affected.
+   */
+  get areas() {
+    return [...gAreas.keys()];
+  },
+  /**
+   * Check what kind of area (toolbar or menu panel) an area is. This is
+   * useful if you have a widget that needs to behave differently depending
+   * on its location. Note that widget wrappers have a convenience getter
+   * property (areaType) for this purpose.
+   *
+   * @param aArea the ID of the area whose type you want to know
+   * @return TYPE_TOOLBAR or TYPE_PANEL depending on the area, null if
+   *         the area is unknown.
+   */
+  getAreaType(aArea) {
+    let area = gAreas.get(aArea);
+    return area ? area.get("type") : null;
+  },
+  /**
+   * Check if a toolbar is collapsed by default.
+   *
+   * @param aArea the ID of the area whose default-collapsed state you want to know.
+   * @return `true` or `false` depending on the area, null if the area is unknown,
+   *         or its collapsed state cannot normally be controlled by the user
+   */
+  isToolbarDefaultCollapsed(aArea) {
+    let area = gAreas.get(aArea);
+    return area ? area.get("defaultCollapsed") : null;
+  },
+  /**
+   * Obtain the DOM node that is the customize target for an area in a
+   * specific window.
+   *
+   * Areas can have a customization target that does not correspond to the
+   * node itself. In particular, toolbars that have a customizationtarget
+   * attribute set will have their customization target set to that node.
+   * This means widgets will end up in the customization target, not in the
+   * DOM node with the ID that corresponds to the area ID. This is useful
+   * because it lets you have fixed content in a toolbar (e.g. the panel
+   * menu item in the navbar) and have all the customizable widgets use
+   * the customization target.
+   *
+   * Using this API yourself is discouraged; you should generally not need
+   * to be asking for the DOM container node used for a particular area.
+   * In particular, if you're wanting to check it in relation to a widget's
+   * node, your DOM node might not be a direct child of the customize target
+   * in a window if, for instance, the window is in customization mode, or if
+   * this is an overflowable toolbar and the widget has been overflowed.
+   *
+   * @param aArea   the ID of the area whose customize target you want to have
+   * @param aWindow the window where you want to fetch the DOM node.
+   * @return the customize target DOM node for aArea in aWindow
+   */
+  getCustomizeTargetForArea(aArea, aWindow) {
+    return CustomizableUIInternal.getCustomizeTargetForArea(aArea, aWindow);
+  },
+  /**
+   * Reset the customization state back to its default.
+   *
+   * This is the nuclear option. You should never call this except if the user
+   * explicitly requests it. Firefox does this when the user clicks the
+   * "Restore Defaults" button in customize mode.
+   */
+  reset() {
+    CustomizableUIInternal.reset();
+  },
+
+  /**
+   * Undo the previous reset, can only be called immediately after a reset.
+   * @return a promise that will be resolved when the operation is complete.
+   */
+  undoReset() {
+    CustomizableUIInternal.undoReset();
+  },
+
+  /**
+   * Remove a custom toolbar added in a previous version of Firefox or using
+   * an add-on. NB: only works on the customizable toolbars generated by
+   * the toolbox itself. Intended for use from CustomizeMode, not by
+   * other consumers.
+   * @param aToolbarId the ID of the toolbar to remove
+   */
+  removeExtraToolbar(aToolbarId) {
+    CustomizableUIInternal.removeExtraToolbar(aToolbarId);
+  },
+
+  /**
+   * Can the last Restore Defaults operation be undone.
+   *
+   * @return A boolean stating whether an undo of the
+   *         Restore Defaults can be performed.
+   */
+  get canUndoReset() {
+    return (
+      gUIStateBeforeReset.uiCustomizationState != null ||
+      gUIStateBeforeReset.drawInTitlebar != null ||
+      gUIStateBeforeReset.currentTheme != null ||
+      gUIStateBeforeReset.autoTouchMode != null ||
+      gUIStateBeforeReset.uiDensity != null
+    );
+  },
+
+  /**
+   * Get the placement of a widget. This is by far the best way to obtain
+   * information about what the state of your widget is. The internals of
+   * this call are cheap (no DOM necessary) and you will know where the user
+   * has put your widget.
+   *
+   * @param aWidgetId the ID of the widget whose placement you want to know
+   * @return
+   *   {
+   *     area: "somearea", // The ID of the area where the widget is placed
+   *     position: 42 // the index in the placements array corresponding to
+   *                  // your widget.
+   *   }
+   *
+   *   OR
+   *
+   *   null // if the widget is not placed anywhere (ie in the palette)
+   */
+  getPlacementOfWidget(aWidgetId, aOnlyRegistered = true, aDeadAreas = false) {
+    return CustomizableUIInternal.getPlacementOfWidget(
+      aWidgetId,
+      aOnlyRegistered,
+      aDeadAreas
+    );
+  },
+  /**
+   * Check if a widget can be removed from the area it's in.
+   *
+   * Note that if you're wanting to move the widget somewhere, you should
+   * generally be checking canWidgetMoveToArea, because that will return
+   * true if the widget is already in the area where you want to move it (!).
+   *
+   * NB: oh, also, this method might lie if the widget in question is a
+   *     XUL-provided widget and there are no windows open, because it
+   *     can obviously not check anything in this case. It will return
+   *     true. You will be able to move the widget elsewhere. However,
+   *     once the user reopens a window, the widget will move back to its
+   *     'proper' area automagically.
+   *
+   * @param aWidgetId a widget ID or DOM node to check
+   * @return true if the widget can be removed from its area,
+   *          false otherwise.
+   */
+  isWidgetRemovable(aWidgetId) {
+    return CustomizableUIInternal.isWidgetRemovable(aWidgetId);
+  },
+  /**
+   * Check if a widget can be moved to a particular area. Like
+   * isWidgetRemovable but better, because it'll return true if the widget
+   * is already in the right area.
+   *
+   * @param aWidgetId the widget ID or DOM node you want to move somewhere
+   * @param aArea     the area ID you want to move it to. This can also be
+   *                  AREA_NO_AREA to see if the widget can move to the
+   *                  customization palette, whether it's removable or not.
+   * @return true if this is possible, false if it is not. The same caveats as
+   *              for isWidgetRemovable apply, however, if no windows are open.
+   */
+  canWidgetMoveToArea(aWidgetId, aArea) {
+    return CustomizableUIInternal.canWidgetMoveToArea(aWidgetId, aArea);
+  },
+  /**
+   * Whether we're in a default state. Note that non-removable non-default
+   * widgets and non-existing widgets are not taken into account in determining
+   * whether we're in the default state.
+   *
+   * NB: this is a property with a getter. The getter is NOT cheap, because
+   * it does smart things with non-removable non-default items, non-existent
+   * items, and so forth. Please don't call unless necessary.
+   */
+  get inDefaultState() {
+    return CustomizableUIInternal.inDefaultState;
+  },
+
+  /**
+   * Set a toolbar's visibility state in all windows.
+   * @param aToolbarId    the toolbar whose visibility should be adjusted
+   * @param aIsVisible    whether the toolbar should be visible
+   */
+  setToolbarVisibility(aToolbarId, aIsVisible) {
+    CustomizableUIInternal.setToolbarVisibility(aToolbarId, aIsVisible);
+  },
+
+  /**
+   * DEPRECATED! Use fluent instead.
+   *
+   * Get a localized property off a (widget?) object.
+   *
+   * NB: this is unlikely to be useful unless you're in Firefox code, because
+   *     this code uses the builtin widget stringbundle, and can't be told
+   *     to use add-on-provided strings. It's mainly here as convenience for
+   *     custom builtin widgets that build their own DOM but use the same
+   *     stringbundle as the other builtin widgets.
+   *
+   * @param aWidget     the object whose property we should use to fetch a
+   *                    localizable string;
+   * @param aProp       the property on the object to use for the fetching;
+   * @param aFormatArgs (optional) any extra arguments to use for a formatted
+   *                    string;
+   * @param aDef        (optional) the default to return if we don't find the
+   *                    string in the stringbundle;
+   *
+   * @return the localized string, or aDef if the string isn't in the bundle.
+   *         If no default is provided,
+   *           if aProp exists on aWidget, we'll return that,
+   *           otherwise we'll return the empty string
+   *
+   */
+  getLocalizedProperty(aWidget, aProp, aFormatArgs, aDef) {
+    return CustomizableUIInternal.getLocalizedProperty(
+      aWidget,
+      aProp,
+      aFormatArgs,
+      aDef
+    );
+  },
+  /**
+   * Utility function to detect, find and set a keyboard shortcut for a menuitem
+   * or (toolbar)button.
+   *
+   * @param aShortcutNode the XUL node where the shortcut will be derived from;
+   * @param aTargetNode   (optional) the XUL node on which the `shortcut`
+   *                      attribute will be set. If NULL, the shortcut will be
+   *                      set on aShortcutNode;
+   */
+  addShortcut(aShortcutNode, aTargetNode) {
+    return CustomizableUIInternal.addShortcut(aShortcutNode, aTargetNode);
+  },
+  /**
+   * Given a node, walk up to the first panel in its ancestor chain, and
+   * close it.
+   *
+   * @param aNode a node whose panel should be closed;
+   */
+  hidePanelForNode(aNode) {
+    CustomizableUIInternal.hidePanelForNode(aNode);
+  },
+  /**
+   * Check if a widget is a "special" widget: a spring, spacer or separator.
+   *
+   * @param aWidgetId the widget ID to check.
+   * @return true if the widget is 'special', false otherwise.
+   */
+  isSpecialWidget(aWidgetId) {
+    return CustomizableUIInternal.isSpecialWidget(aWidgetId);
+  },
+  /**
+   * Check if a widget is provided by an extension. This effectively checks
+   * whether `webExtension: true` passed when the widget was being created.
+   *
+   * If the widget being referred to hasn't yet been created, or has been
+   * destroyed, we fallback to checking the ID for the "-browser-action"
+   * suffix.
+   *
+   * @param aWidgetId the widget ID to check.
+   * @return true if the widget was provided by an extension, false otherwise.
+   */
+  isWebExtensionWidget(aWidgetId) {
+    let widget = this.getWidget(aWidgetId);
+    return widget?.webExtension || aWidgetId.endsWith("-browser-action");
+  },
+  /**
+   * Add listeners to a panel that will close it. For use from the menu panel
+   * and overflowable toolbar implementations, unlikely to be useful for
+   * consumers.
+   *
+   * @param aPanel the panel to which listeners should be attached.
+   */
+  addPanelCloseListeners(aPanel) {
+    CustomizableUIInternal.addPanelCloseListeners(aPanel);
+  },
+  /**
+   * Remove close listeners that have been added to a panel with
+   * addPanelCloseListeners. For use from the menu panel and overflowable
+   * toolbar implementations, unlikely to be useful for consumers.
+   *
+   * @param aPanel the panel from which listeners should be removed.
+   */
+  removePanelCloseListeners(aPanel) {
+    CustomizableUIInternal.removePanelCloseListeners(aPanel);
+  },
+  /**
+   * Notify listeners a widget is about to be dragged to an area. For use from
+   * Customize Mode only, do not use otherwise.
+   *
+   * @param aWidgetId the ID of the widget that is being dragged to an area.
+   * @param aArea     the ID of the area to which the widget is being dragged.
+   */
+  onWidgetDrag(aWidgetId, aArea) {
+    CustomizableUIInternal.notifyListeners("onWidgetDrag", aWidgetId, aArea);
+  },
+  /**
+   * Notify listeners that a window is entering customize mode. For use from
+   * Customize Mode only, do not use otherwise.
+   * @param aWindow the window entering customize mode
+   */
+  notifyStartCustomizing(aWindow) {
+    CustomizableUIInternal.notifyListeners("onCustomizeStart", aWindow);
+  },
+  /**
+   * Notify listeners that a window is exiting customize mode. For use from
+   * Customize Mode only, do not use otherwise.
+   * @param aWindow the window exiting customize mode
+   */
+  notifyEndCustomizing(aWindow) {
+    CustomizableUIInternal.notifyListeners("onCustomizeEnd", aWindow);
+  },
+
+  /**
+   * Notify toolbox(es) of a particular event. If you don't pass aWindow,
+   * all toolboxes will be notified. For use from Customize Mode only,
+   * do not use otherwise.
+   * @param aEvent the name of the event to send.
+   * @param aDetails optional, the details of the event.
+   * @param aWindow optional, the window in which to send the event.
+   */
+  dispatchToolboxEvent(aEvent, aDetails = {}, aWindow = null) {
+    CustomizableUIInternal.dispatchToolboxEvent(aEvent, aDetails, aWindow);
+  },
+
+  /**
+   * Check whether an area is overflowable.
+   *
+   * @param aAreaId the ID of an area to check for overflowable-ness
+   * @return true if the area is overflowable, false otherwise.
+   */
+  isAreaOverflowable(aAreaId) {
+    let area = gAreas.get(aAreaId);
+    return area
+      ? area.get("type") == this.TYPE_TOOLBAR && area.get("overflowable")
+      : false;
+  },
+  /**
+   * Obtain a string indicating the place of an element. This is intended
+   * for use from customize mode; You should generally use getPlacementOfWidget
+   * instead, which is cheaper because it does not use the DOM.
+   *
+   * @param aElement the DOM node whose place we need to check
+   * @return "toolbar" if the node is in a toolbar, "panel" if it is in the
+   *         menu panel, "palette" if it is in the (visible!) customization
+   *         palette, undefined otherwise.
+   */
+  getPlaceForItem(aElement) {
+    let place;
+    let node = aElement;
+    while (node && !place) {
+      if (node.localName == "toolbar") {
+        place = "toolbar";
+      } else if (node.id == CustomizableUI.AREA_FIXED_OVERFLOW_PANEL) {
+        place = "panel";
+      } else if (node.id == "customization-palette") {
+        place = "palette";
+      }
+
+      node = node.parentNode;
+    }
+    return place;
+  },
+
+  /**
+   * Check if a toolbar is builtin or not.
+   * @param aToolbarId the ID of the toolbar you want to check
+   */
+  isBuiltinToolbar(aToolbarId) {
+    return CustomizableUIInternal._builtinToolbars.has(aToolbarId);
+  },
+
+  /**
+   * Create an instance of a spring, spacer or separator.
+   * @param aId       the type of special widget (spring, spacer or separator)
+   * @param aDocument the document in which to create it.
+   */
+  createSpecialWidget(aId, aDocument) {
+    return CustomizableUIInternal.createSpecialWidget(aId, aDocument);
+  },
+
+  /**
+   * Fills a submenu with menu items.
+   * @param aMenuItems the menu items to display.
+   * @param aSubview   the subview to fill.
+   */
+  fillSubviewFromMenuItems(aMenuItems, aSubview) {
+    let attrs = [
+      "oncommand",
+      "onclick",
+      "label",
+      "key",
+      "disabled",
+      "command",
+      "observes",
+      "hidden",
+      "class",
+      "origin",
+      "image",
+      "checked",
+      "style",
+    ];
+
+    // Use ownerGlobal.document to ensure we get the right doc even for
+    // elements in template tags.
+    let doc = aSubview.ownerGlobal.document;
+    let fragment = doc.createDocumentFragment();
+    for (let menuChild of aMenuItems) {
+      if (menuChild.hidden) {
+        continue;
+      }
+
+      let subviewItem;
+      if (menuChild.localName == "menuseparator") {
+        // Don't insert duplicate or leading separators. This can happen if there are
+        // menus (which we don't copy) above the separator.
+        if (
+          !fragment.lastElementChild ||
+          fragment.lastElementChild.localName == "toolbarseparator"
+        ) {
+          continue;
+        }
+        subviewItem = doc.createXULElement("toolbarseparator");
+      } else if (menuChild.localName == "menuitem") {
+        subviewItem = doc.createXULElement("toolbarbutton");
+        CustomizableUI.addShortcut(menuChild, subviewItem);
+
+        let item = menuChild;
+        if (!item.hasAttribute("onclick")) {
+          subviewItem.addEventListener("click", event => {
+            let newEvent = new doc.defaultView.MouseEvent(event.type, event);
+
+            // Telemetry should only pay attention to the original event.
+            lazy.BrowserUsageTelemetry.ignoreEvent(newEvent);
+            item.dispatchEvent(newEvent);
+          });
+        }
+
+        if (!item.hasAttribute("oncommand")) {
+          subviewItem.addEventListener("command", event => {
+            let newEvent = doc.createEvent("XULCommandEvent");
+            newEvent.initCommandEvent(
+              event.type,
+              event.bubbles,
+              event.cancelable,
+              event.view,
+              event.detail,
+              event.ctrlKey,
+              event.altKey,
+              event.shiftKey,
+              event.metaKey,
+              0,
+              event.sourceEvent,
+              0
+            );
+
+            // Telemetry should only pay attention to the original event.
+            lazy.BrowserUsageTelemetry.ignoreEvent(newEvent);
+            item.dispatchEvent(newEvent);
+          });
+        }
+      } else {
+        continue;
+      }
+      for (let attr of attrs) {
+        let attrVal = menuChild.getAttribute(attr);
+        if (attrVal) {
+          subviewItem.setAttribute(attr, attrVal);
+        }
+      }
+      // We do this after so the .subviewbutton class doesn't get overriden.
+      if (menuChild.localName == "menuitem") {
+        subviewItem.classList.add("subviewbutton");
+      }
+
+      // We make it possible to supply an alternative Fluent key when cloning
+      // this menuitem into the AppMenu or panel contexts. This is because
+      // we often use Title Case in menuitems in native menus, but want to use
+      // Sentence case in the AppMenu / panels.
+      let l10nId = menuChild.getAttribute("appmenu-data-l10n-id");
+      if (l10nId) {
+        subviewItem.setAttribute("data-l10n-id", l10nId);
+      }
+
+      fragment.appendChild(subviewItem);
+    }
+    aSubview.appendChild(fragment);
+  },
+
+  /**
+   * A helper function for clearing subviews.
+   * @param aSubview the subview to clear.
+   */
+  clearSubview(aSubview) {
+    let parent = aSubview.parentNode;
+    // We'll take the container out of the document before cleaning it out
+    // to avoid reflowing each time we remove something.
+    parent.removeChild(aSubview);
+
+    while (aSubview.firstChild) {
+      aSubview.firstChild.remove();
+    }
+
+    parent.appendChild(aSubview);
+  },
+
+  getCustomizationTarget(aElement) {
+    return CustomizableUIInternal.getCustomizationTarget(aElement);
+  },
+
+  getTestOnlyInternalProp(aProp) {
+    if (!Cu.isInAutomation) {
+      return null;
+    }
+    switch (aProp) {
+      case "CustomizableUIInternal":
+        return CustomizableUIInternal;
+      case "gAreas":
+        return gAreas;
+      case "gFuturePlacements":
+        return gFuturePlacements;
+      case "gPalette":
+        return gPalette;
+      case "gPlacements":
+        return gPlacements;
+      case "gSavedState":
+        return gSavedState;
+      case "gSeenWidgets":
+        return gSeenWidgets;
+      case "kVersion":
+        return kVersion;
+    }
+    return null;
+  },
+  setTestOnlyInternalProp(aProp, aValue) {
+    if (!Cu.isInAutomation) {
+      return;
+    }
+    switch (aProp) {
+      case "gSavedState":
+        gSavedState = aValue;
+        break;
+      case "kVersion":
+        kVersion = aValue;
+        break;
+      case "gDirty":
+        gDirty = aValue;
+        break;
+    }
+  },
+};
+Object.freeze(CustomizableUI);
+Object.freeze(CustomizableUI.windows);
+
+/**
+ * All external consumers of widgets are really interacting with these wrappers
+ * which provide a common interface.
+ */
+
+/**
+ * WidgetGroupWrapper is the common interface for interacting with an entire
+ * widget group - AKA, all instances of a widget across a series of windows.
+ * This particular wrapper is only used for widgets created via the provider
+ * API.
+ */
+function WidgetGroupWrapper(aWidget) {
+  this.isGroup = true;
+
+  const kBareProps = [
+    "id",
+    "source",
+    "type",
+    "disabled",
+    "label",
+    "tooltiptext",
+    "showInPrivateBrowsing",
+    "viewId",
+    "disallowSubView",
+    "webExtension",
+  ];
+  for (let prop of kBareProps) {
+    let propertyName = prop;
+    this.__defineGetter__(propertyName, () => aWidget[propertyName]);
+  }
+
+  this.__defineGetter__("provider", () => CustomizableUI.PROVIDER_API);
+
+  this.__defineSetter__("disabled", function(aValue) {
+    aValue = !!aValue;
+    aWidget.disabled = aValue;
+    for (let [, instance] of aWidget.instances) {
+      instance.disabled = aValue;
+    }
+  });
+
+  this.forWindow = function WidgetGroupWrapper_forWindow(aWindow) {
+    let wrapperMap;
+    if (!gSingleWrapperCache.has(aWindow)) {
+      wrapperMap = new Map();
+      gSingleWrapperCache.set(aWindow, wrapperMap);
+    } else {
+      wrapperMap = gSingleWrapperCache.get(aWindow);
+    }
+    if (wrapperMap.has(aWidget.id)) {
+      return wrapperMap.get(aWidget.id);
+    }
+
+    let instance = aWidget.instances.get(aWindow.document);
+    if (!instance) {
+      instance = CustomizableUIInternal.buildWidget(aWindow.document, aWidget);
+    }
+
+    let wrapper = new WidgetSingleWrapper(aWidget, instance);
+    wrapperMap.set(aWidget.id, wrapper);
+    return wrapper;
+  };
+
+  this.__defineGetter__("instances", function() {
+    // Can't use gBuildWindows here because some areas load lazily:
+    let placement = CustomizableUIInternal.getPlacementOfWidget(aWidget.id);
+    if (!placement) {
+      return [];
+    }
+    let area = placement.area;
+    let buildAreas = gBuildAreas.get(area);
+    if (!buildAreas) {
+      return [];
+    }
+    return Array.from(buildAreas, node => this.forWindow(node.ownerGlobal));
+  });
+
+  this.__defineGetter__("areaType", function() {
+    let areaProps = gAreas.get(aWidget.currentArea);
+    return areaProps && areaProps.get("type");
+  });
+
+  Object.freeze(this);
+}
+
+/**
+ * A WidgetSingleWrapper is a wrapper around a single instance of a widget in
+ * a particular window.
+ */
+function WidgetSingleWrapper(aWidget, aNode) {
+  this.isGroup = false;
+
+  this.node = aNode;
+  this.provider = CustomizableUI.PROVIDER_API;
+
+  const kGlobalProps = ["id", "type"];
+  for (let prop of kGlobalProps) {
+    this[prop] = aWidget[prop];
+  }
+
+  const kNodeProps = ["label", "tooltiptext"];
+  for (let prop of kNodeProps) {
+    let propertyName = prop;
+    // Look at the node for these, instead of the widget data, to ensure the
+    // wrapper always reflects this live instance.
+    this.__defineGetter__(propertyName, () => aNode.getAttribute(propertyName));
+  }
+
+  this.__defineGetter__("disabled", () => aNode.disabled);
+  this.__defineSetter__("disabled", function(aValue) {
+    aNode.disabled = !!aValue;
+  });
+
+  this.__defineGetter__("anchor", function() {
+    let anchorId;
+    // First check for an anchor for the area:
+    let placement = CustomizableUIInternal.getPlacementOfWidget(aWidget.id);
+    if (placement) {
+      anchorId = gAreas.get(placement.area).get("anchor");
+    }
+    if (!anchorId) {
+      anchorId = aNode.getAttribute("cui-anchorid");
+    }
+    if (!anchorId) {
+      anchorId = aNode.getAttribute("view-button-id");
+    }
+    if (anchorId) {
+      return aNode.ownerDocument.getElementById(anchorId);
+    }
+    if (aWidget.type == "button-and-view") {
+      return aNode.lastElementChild;
+    }
+    return aNode;
+  });
+
+  this.__defineGetter__("overflowed", function() {
+    return aNode.getAttribute("overflowedItem") == "true";
+  });
+
+  Object.freeze(this);
+}
+
+/**
+ * XULWidgetGroupWrapper is the common interface for interacting with an entire
+ * widget group - AKA, all instances of a widget across a series of windows.
+ * This particular wrapper is only used for widgets created via the old-school
+ * XUL method (overlays, or programmatically injecting toolbaritems, or other
+ * such things).
+ */
+// XXXunf Going to need to hook this up to some events to keep it all live.
+function XULWidgetGroupWrapper(aWidgetId) {
+  this.isGroup = true;
+  this.id = aWidgetId;
+  this.type = "custom";
+  // XUL Widgets can never be provided by extensions.
+  this.webExtension = false;
+  this.provider = CustomizableUI.PROVIDER_XUL;
+
+  this.forWindow = function XULWidgetGroupWrapper_forWindow(aWindow) {
+    let wrapperMap;
+    if (!gSingleWrapperCache.has(aWindow)) {
+      wrapperMap = new Map();
+      gSingleWrapperCache.set(aWindow, wrapperMap);
+    } else {
+      wrapperMap = gSingleWrapperCache.get(aWindow);
+    }
+    if (wrapperMap.has(aWidgetId)) {
+      return wrapperMap.get(aWidgetId);
+    }
+
+    let instance = aWindow.document.getElementById(aWidgetId);
+    if (!instance) {
+      // Toolbar palettes aren't part of the document, so elements in there
+      // won't be found via document.getElementById().
+      instance = aWindow.gNavToolbox.palette.getElementsByAttribute(
+        "id",
+        aWidgetId
+      )[0];
+    }
+
+    let wrapper = new XULWidgetSingleWrapper(
+      aWidgetId,
+      instance,
+      aWindow.document
+    );
+    wrapperMap.set(aWidgetId, wrapper);
+    return wrapper;
+  };
+
+  this.__defineGetter__("areaType", function() {
+    let placement = CustomizableUIInternal.getPlacementOfWidget(aWidgetId);
+    if (!placement) {
+      return null;
+    }
+
+    let areaProps = gAreas.get(placement.area);
+    return areaProps && areaProps.get("type");
+  });
+
+  this.__defineGetter__("instances", function() {
+    return Array.from(gBuildWindows, wins => this.forWindow(wins[0]));
+  });
+
+  Object.freeze(this);
+}
+
+/**
+ * A XULWidgetSingleWrapper is a wrapper around a single instance of a XUL
+ * widget in a particular window.
+ */
+function XULWidgetSingleWrapper(aWidgetId, aNode, aDocument) {
+  this.isGroup = false;
+
+  this.id = aWidgetId;
+  this.type = "custom";
+  this.provider = CustomizableUI.PROVIDER_XUL;
+
+  let weakDoc = Cu.getWeakReference(aDocument);
+  // If we keep a strong ref, the weak ref will never die, so null it out:
+  aDocument = null;
+
+  this.__defineGetter__("node", function() {
+    // If we've set this to null (further down), we're sure there's nothing to
+    // be gotten here, so bail out early:
+    if (!weakDoc) {
+      return null;
+    }
+    if (aNode) {
+      // Return the last known node if it's still in the DOM...
+      if (aNode.isConnected) {
+        return aNode;
+      }
+      // ... or the toolbox
+      let toolbox = aNode.ownerGlobal.gNavToolbox;
+      if (toolbox && toolbox.palette && aNode.parentNode == toolbox.palette) {
+        return aNode;
+      }
+      // If it isn't, clear the cached value and fall through to the "slow" case:
+      aNode = null;
+    }
+
+    let doc = weakDoc.get();
+    if (doc) {
+      // Store locally so we can cache the result:
+      aNode = CustomizableUIInternal.findWidgetInWindow(
+        aWidgetId,
+        doc.defaultView
+      );
+      return aNode;
+    }
+    // The weakref to the document is dead, we're done here forever more:
+    weakDoc = null;
+    return null;
+  });
+
+  this.__defineGetter__("anchor", function() {
+    let anchorId;
+    // First check for an anchor for the area:
+    let placement = CustomizableUIInternal.getPlacementOfWidget(aWidgetId);
+    if (placement) {
+      anchorId = gAreas.get(placement.area).get("anchor");
+    }
+
+    let node = this.node;
+    if (!anchorId && node) {
+      anchorId = node.getAttribute("cui-anchorid");
+    }
+
+    return anchorId && node
+      ? node.ownerDocument.getElementById(anchorId)
+      : node;
+  });
+
+  this.__defineGetter__("overflowed", function() {
+    let node = this.node;
+    if (!node) {
+      return false;
+    }
+    return node.getAttribute("overflowedItem") == "true";
+  });
+
+  Object.freeze(this);
+}
+
+/**
+ * OverflowableToolbar is a class that gives a <xul:toolbar> the ability to send
+ * toolbar items that are "overflowable" to lists in separate panels if and
+ * when the toolbar shrinks enough so that those items overflow out of bounds.
+ * Secondly, this class manages moving things out from those panels and back
+ * into the toolbar once it underflows and has the space to accommodate the
+ * items that had originally overflowed out.
+ *
+ * There are two panels that toolbar items can be overflowed to:
+ *
+ * 1. The default items overflow panel
+ *   This is where built-in default toolbar items will go to.
+ * 2. The Unified Extensions panel
+ *   This is where browser_action toolbar buttons created by extensions will
+ *   go to if the Unified Extensions UI is enabled - otherwise, those items will
+ *   go to the default items overflow panel.
+ *
+ * Finally, OverflowableToolbar manages the showing of the default items
+ * overflow panel when the associated anchor is clicked or dragged over. The
+ * Unified Extensions panel is managed separately by the extension code.
+ *
+ * In theory, we could have multiple overflowable toolbars, but in practice,
+ * only the nav-bar (CustomizableUI.AREA_NAVBAR) makes use of this class.
+ */
+class OverflowableToolbar {
+  /**
+   * The OverflowableToolbar class is constructed during browser window
+   * creation, but to optimize for window painting, we defer most work until
+   * after the window has painted. This property is set to true once
+   * initialization has completed.
+   *
+   * @type {boolean}
+   */
+  #initialized = false;
+
+  /**
+   * A reference to the <xul:toolbar> that is overflowable.
+   *
+   * @type {Element}
+   */
+  #toolbar = null;
+
+  /**
+   * A reference to the part of the <xul:toolbar> that accepts CustomizableUI
+   * widgets.
+   *
+   * @type {Element}
+   */
+  #target = null;
+
+  /**
+   * A mapping from the ID of a toolbar item that has overflowed to the width
+   * that the toolbar item occupied in the toolbar at the time of overflow. Any
+   * item that is currently overflowed will have an entry in this map.
+   *
+   * @type {Map<string, number>}
+   */
+  #overflowedInfo = new Map();
+
+  /**
+   * The set of overflowed DOM nodes that were hidden at the time of overflowing.
+   */
+  #hiddenOverflowedNodes = new WeakSet();
+
+  /**
+   * True if the overflowable toolbar is actively handling overflows and
+   * underflows. This value is set internally by the private #enable() and
+   * #disable() methods.
+   *
+   * @type {boolean}
+   */
+  #enabled = true;
+
+  /**
+   * A reference to the element that overflowed toolbar items will be
+   * appended to as children upon overflow.
+   *
+   * @type {Element}
+   */
+  #defaultList = null;
+
+  /**
+   * A reference to the button that opens the overflow panel. This is also
+   * the element that the panel will anchor to.
+   *
+   * @type {Element}
+   */
+  #defaultListButton = null;
+
+  /**
+   * A reference to the <xul:panel> overflow panel that contains the #defaultList
+   * element.
+   *
+   * @type {Element}
+   */
+  #defaultListPanel = null;
+
+  /**
+   * A reference to the the element that overflowed extension browser action
+   * toolbar items will be appended to as children upon overflow if the
+   * Unified Extension UI is enabled. This is created lazily and might be null,
+   * so you should use the #webExtList memoizing getter instead to get this.
+   *
+   * @type {Element|null}
+   */
+  #webExtListRef = null;
+
+  /**
+   * An empty object that is created in #checkOverflow to identify individual
+   * calls to #checkOverflow and avoid re-entrancy (since #checkOverflow is
+   * asynchronous, and in theory, could be called multiple times before any of
+   * those times have a chance to fully exit).
+   *
+   * @type {Object}
+   */
+  #checkOverflowHandle = null;
+
+  /**
+   * A timeout ID returned by setTimeout that identifies a timeout function that
+   * runs to hide the #defaultListPanel if the user happened to open the panel by dragging
+   * over the #defaultListButton and then didn't hover any part of the #defaultListPanel.
+   *
+   * @type {number}
+   */
+  #hideTimeoutId = null;
+
+  /**
+   * Public methods start here.
+   */
+
+  /**
+   * OverflowableToolbar constructor. This is run very early on in the lifecycle
+   * of a browser window, so it tries to defer most work to the init() method
+   * instead after first paint.
+   *
+   * Upon construction, a "overflowable" attribute will be set on the
+   * toolbar, set to the value of "true".
+   *
+   * Part of the API for OverflowableToolbar is declarative, in that it expects
+   * certain attributes to be set on the <xul:toolbar> that is overflowable.
+   * Those attributes are:
+   *
+   * default-overflowbutton:
+   *   The ID of the button that is used to open and anchor the overflow panel.
+   * default-overflowtarget:
+   *   The ID of the element that overflowed items will be appended to as
+   *   children. Note that the overflowed toolbar items are moved into and out
+   *   of this overflow target, so it is definitely advisable to let
+   *   OverflowableToolbar own managing the children of default-overflowtarget,
+   *   and to not modify it outside of this class.
+   * default-overflowpanel:
+   *   The ID of the <xul:panel> that contains the default-overflowtarget.
+   * addon-webext-overflowbutton:
+   *   The ID of the button that is used to open and anchor the Unified
+   *   Extensions panel.
+   * addon-webext-overflowtarget:
+   *   The ID of the element that overflowed extension toolbar buttons will
+   *   be appended to as children if the Unified Extensions UI is enabled.
+   *   Note that the overflowed toolbar items are moved into and out of this
+   *   overflow target, so it is definitely advisable to let OverflowableToolbar
+   *   own managing the children of addon-webext-overflowtarget, and to not
+   *   modify it outside of this class.
+   *
+   * @param {Element} aToolbarNode The <xul:toolbar> that will be overflowable.
+   * @throws {Error} Throws if the customization target of the toolbar somehow
+   *   isn't a direct descendent of the toolbar.
+   */
+  constructor(aToolbarNode) {
+    this.#toolbar = aToolbarNode;
+    this.#target = CustomizableUI.getCustomizationTarget(this.#toolbar);
+    if (this.#target.parentNode != this.#toolbar) {
+      throw new Error(
+        "Customization target must be a direct child of an overflowable toolbar."
+      );
+    }
+
+    this.#toolbar.setAttribute("overflowable", "true");
+    let doc = this.#toolbar.ownerDocument;
+    this.#defaultList = doc.getElementById(
+      this.#toolbar.getAttribute("default-overflowtarget")
+    );
+    this.#defaultList._customizationTarget = this.#defaultList;
+
+    let window = this.#toolbar.ownerGlobal;
+
+    if (window.gBrowserInit.delayedStartupFinished) {
+      this.init();
+    } else {
+      Services.obs.addObserver(this, "browser-delayed-startup-finished");
+    }
+  }
+
+  /**
+   * Does final initialization of the OverflowableToolbar after the window has
+   * first painted. This will also kick off the first check to see if overflow
+   * has already occurred at the time of initialization.
+   */
+  init() {
+    let doc = this.#toolbar.ownerDocument;
+    let window = doc.defaultView;
+    window.addEventListener("resize", this);
+    window.gNavToolbox.addEventListener("customizationstarting", this);
+    window.gNavToolbox.addEventListener("aftercustomization", this);
+
+    let defaultListButton = this.#toolbar.getAttribute(
+      "default-overflowbutton"
+    );
+    this.#defaultListButton = doc.getElementById(defaultListButton);
+    this.#defaultListButton.addEventListener("mousedown", this);
+    this.#defaultListButton.addEventListener("keypress", this);
+    this.#defaultListButton.addEventListener("dragover", this);
+    this.#defaultListButton.addEventListener("dragend", this);
+
+    let panelId = this.#toolbar.getAttribute("default-overflowpanel");
+    this.#defaultListPanel = doc.getElementById(panelId);
+    this.#defaultListPanel.addEventListener("popuphiding", this);
+    CustomizableUIInternal.addPanelCloseListeners(this.#defaultListPanel);
+
+    CustomizableUI.addListener(this);
+
+    this.#checkOverflow();
+
+    this.#initialized = true;
+  }
+
+  /**
+   * Almost the exact reverse of init(). This is called when the browser window
+   * is unloading.
+   */
+  uninit() {
+    this.#toolbar.removeAttribute("overflowable");
+
+    if (!this.#initialized) {
+      Services.obs.removeObserver(this, "browser-delayed-startup-finished");
+      return;
+    }
+
+    this.#disable();
+
+    let window = this.#toolbar.ownerGlobal;
+    window.removeEventListener("resize", this);
+    window.gNavToolbox.removeEventListener("customizationstarting", this);
+    window.gNavToolbox.removeEventListener("aftercustomization", this);
+    this.#defaultListButton.removeEventListener("mousedown", this);
+    this.#defaultListButton.removeEventListener("keypress", this);
+    this.#defaultListButton.removeEventListener("dragover", this);
+    this.#defaultListButton.removeEventListener("dragend", this);
+    this.#defaultListPanel.removeEventListener("popuphiding", this);
+
+    CustomizableUI.removeListener(this);
+    CustomizableUIInternal.removePanelCloseListeners(this.#defaultListPanel);
+  }
+
+  /**
+   * Opens the overflow #defaultListPanel if it's not already open. If the panel is in
+   * the midst of hiding when this is called, the panel will be re-opened.
+   *
+   * @returns {Promise}
+   * @resolves {undefined} once the panel is open.
+   */
+  show(aEvent) {
+    if (this.#defaultListPanel.state == "open") {
+      return Promise.resolve();
+    }
+    return new Promise(resolve => {
+      let doc = this.#defaultListPanel.ownerDocument;
+      this.#defaultListPanel.hidden = false;
+      let multiview = this.#defaultListPanel.querySelector("panelmultiview");
+      let mainViewId = multiview.getAttribute("mainViewId");
+      let mainView = doc.getElementById(mainViewId);
+      let contextMenu = doc.getElementById(mainView.getAttribute("context"));
+      Services.els.addSystemEventListener(contextMenu, "command", this, true);
+      let anchor = this.#defaultListButton.icon;
+
+      let popupshown = false;
+      this.#defaultListPanel.addEventListener(
+        "popupshown",
+        () => {
+          popupshown = true;
+          this.#defaultListPanel.addEventListener("dragover", this);
+          this.#defaultListPanel.addEventListener("dragend", this);
+          // Wait until the next tick to resolve so all popupshown
+          // handlers have a chance to run before our promise resolution
+          // handlers do.
+          Services.tm.dispatchToMainThread(resolve);
+        },
+        { once: true }
+      );
+
+      let openPanel = () => {
+        // Ensure we update the gEditUIVisible flag when opening the popup, in
+        // case the edit controls are in it.
+        this.#defaultListPanel.addEventListener(
+          "popupshowing",
+          () => {
+            doc.defaultView.updateEditUIVisibility();
+          },
+          { once: true }
+        );
+
+        this.#defaultListPanel.addEventListener(
+          "popuphidden",
+          () => {
+            if (!popupshown) {
+              // The panel was hidden again before it was shown. This can break
+              // consumers waiting for the panel to show. So we try again.
+              openPanel();
+            }
+          },
+          { once: true }
+        );
+
+        lazy.PanelMultiView.openPopup(
+          this.#defaultListPanel,
+          anchor || this.#defaultListButton,
+          {
+            triggerEvent: aEvent,
+          }
+        );
+        this.#defaultListButton.open = true;
+      };
+
+      openPanel();
+    });
+  }
+
+  /**
+   * Exposes whether #checkOverflow is currently running.
+   *
+   * @returns {boolean} True if #checkOverflow is currently running.
+   */
+  isHandlingOverflow() {
+    return !!this.#checkOverflowHandle;
+  }
+
+  /**
+   * Finds the most appropriate place to insert toolbar item aNode if we've been
+   * asked to put it into the overflowable toolbar without being told exactly
+   * where.
+   *
+   * @param {Element} aNode The toolbar item being inserted.
+   * @returns {Array} [parent, nextNode]
+   *   parent: {Element} The parent element that should contain aNode.
+   *   nextNode: {Element|null} The node that should follow aNode after
+   *     insertion, if any. If this is null, aNode should be placed at the end
+   *     of parent.
+   */
+  findOverflowedInsertionPoints(aNode) {
+    let newNodeCanOverflow = aNode.getAttribute("overflows") != "false";
+    let areaId = this.#toolbar.id;
+    let placements = gPlacements.get(areaId);
+    let nodeIndex = placements.indexOf(aNode.id);
+    let nodeBeforeNewNodeIsOverflown = false;
+
+    let loopIndex = -1;
+    // Loop through placements to find where to insert this item.
+    // As soon as we find an overflown widget, we will only
+    // insert in the overflow panel (this is why we check placements
+    // before the desired location for the new node). Once we pass
+    // the desired location of the widget, we look for placement ids
+    // that actually have DOM equivalents to insert before. If all
+    // else fails, we insert at the end of either the overflow list
+    // or the toolbar target.
+    while (++loopIndex < placements.length) {
+      let nextNodeId = placements[loopIndex];
+      if (loopIndex > nodeIndex) {
+        // Note that if aNode is in a template, its `ownerDocument` is *not*
+        // going to be the browser.xhtml document, so we cannot rely on it.
+        let nextNode = this.#toolbar.ownerDocument.getElementById(nextNodeId);
+        // If the node we're inserting can overflow, and the next node
+        // in the toolbar is overflown, we should insert this node
+        // in the overflow panel before it.
+        if (
+          newNodeCanOverflow &&
+          this.#overflowedInfo.has(nextNodeId) &&
+          nextNode &&
+          nextNode.parentNode == this.#defaultList
+        ) {
+          return [this.#defaultList, nextNode];
+        }
+        // Otherwise (if either we can't overflow, or the previous node
+        // wasn't overflown), and the next node is in the toolbar itself,
+        // insert the node in the toolbar.
+        if (
+          (!nodeBeforeNewNodeIsOverflown || !newNodeCanOverflow) &&
+          nextNode &&
+          (nextNode.parentNode == this.#target ||
+            // Also check if the next node is in a customization wrapper
+            // (toolbarpaletteitem). We don't need to do this for the
+            // overflow case because overflow is disabled in customize mode.
+            (nextNode.parentNode.localName == "toolbarpaletteitem" &&
+              nextNode.parentNode.parentNode == this.#target))
+        ) {
+          return [this.#target, nextNode];
+        }
+      } else if (
+        loopIndex < nodeIndex &&
+        this.#overflowedInfo.has(nextNodeId)
+      ) {
+        nodeBeforeNewNodeIsOverflown = true;
+      }
+    }
+
+    let overflowList = CustomizableUI.isWebExtensionWidget(aNode.id)
+      ? this.#webExtList
+      : this.#defaultList;
+
+    let containerForAppending =
+      this.#overflowedInfo.size && newNodeCanOverflow
+        ? overflowList
+        : this.#target;
+    return [containerForAppending, null];
+  }
+
+  /**
+   * Allows callers to query for the current parent of a toolbar item that may
+   * or may not be overflowed. That parent will either be #defaultList,
+   * #webExtList (if it's an extension button) or #target.
+   *
+   * Note: It is assumed that the caller has verified that aNode is placed
+   * within the toolbar customizable area according to CustomizableUI.
+   *
+   * @param {Element} aNode the node that can be overflowed by this
+   *   OverflowableToolbar.
+   * @returns {Element} The current containing node for aNode.
+   */
+  getContainerFor(aNode) {
+    if (aNode.getAttribute("overflowedItem") == "true") {
+      return CustomizableUI.isWebExtensionWidget(aNode.id)
+        ? this.#webExtList
+        : this.#defaultList;
+    }
+    return this.#target;
+  }
+
+  /**
+   * Private methods start here.
+   */
+
+  /**
+   * Handle overflow in the toolbar by moving items to the overflow menu.
+   */
+  async #onOverflow() {
+    if (!this.#enabled) {
+      return;
+    }
+
+    let win = this.#target.ownerGlobal;
+    let checkOverflowHandle = this.#checkOverflowHandle;
+    let webExtButtonID = this.#toolbar.getAttribute(
+      "addon-webext-overflowbutton"
+    );
+
+    let { isOverflowing, targetContentWidth } = await this.#getOverflowInfo();
+
+    // Stop if the window has closed or if we re-enter while waiting for
+    // layout.
+    if (win.closed || this.#checkOverflowHandle != checkOverflowHandle) {
+      lazy.log.debug("Window closed or another overflow handler started.");
+      return;
+    }
+
+    let webExtList = this.#webExtList;
+
+    let child = this.#target.lastElementChild;
+    while (child && isOverflowing) {
+      let prevChild = child.previousElementSibling;
+
+      if (child.getAttribute("overflows") != "false") {
+        this.#overflowedInfo.set(child.id, targetContentWidth);
+        let { width: childWidth } = win.windowUtils.getBoundsWithoutFlushing(
+          child
+        );
+        if (!childWidth) {
+          this.#hiddenOverflowedNodes.add(child);
+        }
+
+        child.setAttribute("overflowedItem", true);
+        CustomizableUIInternal.ensureButtonContextMenu(
+          child,
+          this.#toolbar,
+          true
+        );
+        CustomizableUIInternal.notifyListeners(
+          "onWidgetOverflow",
+          child,
+          this.#target
+        );
+
+        if (
+          lazy.gUnifiedExtensionsEnabled &&
+          webExtList &&
+          CustomizableUI.isWebExtensionWidget(child.id)
+        ) {
+          child.setAttribute("cui-anchorid", webExtButtonID);
+          webExtList.insertBefore(child, webExtList.firstElementChild);
+        } else {
+          child.setAttribute("cui-anchorid", this.#defaultListButton.id);
+          this.#defaultList.insertBefore(
+            child,
+            this.#defaultList.firstElementChild
+          );
+          if (!CustomizableUI.isSpecialWidget(child.id) && childWidth) {
+            this.#toolbar.setAttribute("overflowing", "true");
+          }
+        }
+      }
+      child = prevChild;
+      ({ isOverflowing, targetContentWidth } = await this.#getOverflowInfo());
+      // Stop if the window has closed or if we re-enter while waiting for
+      // layout.
+      if (win.closed || this.#checkOverflowHandle != checkOverflowHandle) {
+        lazy.log.debug("Window closed or another overflow handler started.");
+        return;
+      }
+    }
+
+    win.UpdateUrlbarSearchSplitterState();
+  }
+
+  /**
+   * Returns a Promise that resolves to a an object that describes the state
+   * that this OverflowableToolbar is currently in.
+   *
+   * @returns {Promise}
+   * @resolves {Object}
+   *   An object with the following properties:
+   *
+   *   isOverflowing: {boolean} True if at least one toolbar item has overflowed
+   *     into an overflow panel.
+   *   targetContentWidth: {number} The total width of the items within the
+   *     customization target area of the toolbar.
+   *   totalAvailWidth: {number} The maximum width items in the toolbar may
+   *     occupy before causing an overflow.
+   */
+  async #getOverflowInfo() {
+    function getInlineSize(aElement) {
+      return aElement.getBoundingClientRect().width;
+    }
+
+    function sumChildrenInlineSize(aParent, aExceptChild = null) {
+      let sum = 0;
+      for (let child of aParent.children) {
+        let style = win.getComputedStyle(child);
+        if (
+          style.display == "none" ||
+          win.XULPopupElement.isInstance(child) ||
+          (style.position != "static" && style.position != "relative")
+        ) {
+          continue;
+        }
+        sum += parseFloat(style.marginLeft) + parseFloat(style.marginRight);
+        if (child != aExceptChild) {
+          sum += getInlineSize(child);
+        }
+      }
+      return sum;
+    }
+
+    let win = this.#target.ownerGlobal;
+    let totalAvailWidth;
+    let targetWidth;
+    let targetChildrenWidth;
+
+    await win.promiseDocumentFlushed(() => {
+      let style = win.getComputedStyle(this.#toolbar);
+      let toolbarChildrenWidth = sumChildrenInlineSize(
+        this.#toolbar,
+        this.#target
+      );
+      totalAvailWidth =
+        getInlineSize(this.#toolbar) -
+        parseFloat(style.paddingLeft) -
+        parseFloat(style.paddingRight) -
+        toolbarChildrenWidth;
+      targetWidth = getInlineSize(this.#target);
+      targetChildrenWidth =
+        this.#target == this.#toolbar
+          ? toolbarChildrenWidth
+          : sumChildrenInlineSize(this.#target);
+    });
+
+    lazy.log.debug(
+      `Getting overflow info: target width: ${targetWidth} (${targetChildrenWidth}); avail: ${totalAvailWidth}`
+    );
+
+    // If the target has min-width: 0, their children might actually overflow
+    // it, so check for both cases explicitly.
+    let targetContentWidth = Math.max(targetWidth, targetChildrenWidth);
+    let isOverflowing = Math.floor(targetContentWidth) > totalAvailWidth;
+    return { isOverflowing, targetContentWidth, totalAvailWidth };
+  }
+
+  /**
+   * Tries to move toolbar items back to the toolbar from the overflow panel.
+   *
+   * @param {boolean} shouldMoveAllItems
+   *        Whether we should move everything (e.g. because we're being
+   *        disabled)
+   * @param {number} [totalAvailWidth=undefined]
+   *        Optional; the width of the toolbar area in which we can put things.
+   *        Some consumers pass this to avoid reflows.
+   *
+   *        While there are items in the list, this width won't change, and so
+   *        we can avoid flushing layout by providing it and/or caching it.
+   *        Note that if `shouldMoveAllItems` is true, we never need the width
+   *        anyway, and this value is ignored.
+   * @returns {Promise}
+   * @resolves {undefined} Once moving of items has completed.
+   */
+  async #moveItemsBackToTheirOrigin(shouldMoveAllItems, totalAvailWidth) {
+    lazy.log.debug(
+      `Attempting to move ${shouldMoveAllItems ? "all" : "some"} items back`
+    );
+    let placements = gPlacements.get(this.#toolbar.id);
+    let win = this.#target.ownerGlobal;
+    let doc = this.#target.ownerDocument;
+    let checkOverflowHandle = this.#checkOverflowHandle;
+
+    let overflowedItemStack = Array.from(this.#overflowedInfo.entries());
+
+    for (let i = overflowedItemStack.length - 1; i >= 0; --i) {
+      let [childID, minSize] = overflowedItemStack[i];
+
+      // The item may have been placed inside of a <xul:panel> that is lazily
+      // loaded and still in the view cache. PanelMultiView.getViewNode will
+      // do the work of checking the DOM for the child, and then falling back to
+      // the cache if that is the case.
+      let child = lazy.PanelMultiView.getViewNode(doc, childID);
+
+      if (!child) {
+        this.#overflowedInfo.delete(childID);
+        continue;
+      }
+
+      lazy.log.debug(
+        `Considering moving ${child.id} back, minSize: ${minSize}`
+      );
+
+      if (!shouldMoveAllItems && minSize) {
+        if (!totalAvailWidth) {
+          ({ totalAvailWidth } = await this.#getOverflowInfo());
+
+          // If the window has closed or if we re-enter because we were waiting
+          // for layout, stop.
+          if (win.closed || this.#checkOverflowHandle != checkOverflowHandle) {
+            lazy.log.debug("Window closed or #checkOverflow called again.");
+            return;
+          }
+        }
+        if (totalAvailWidth <= minSize) {
+          lazy.log.debug(
+            `Need ${minSize} but width is ${totalAvailWidth} so bailing`
+          );
+          break;
+        }
+      }
+
+      lazy.log.debug(`Moving ${child.id} back`);
+      this.#overflowedInfo.delete(child.id);
+      let beforeNodeIndex = placements.indexOf(child.id) + 1;
+      // If this is a skipintoolbarset item, meaning it doesn't occur in the placements list,
+      // we're inserting it at the end. This will mean first-in, first-out (more or less)
+      // leading to as little change in order as possible.
+      if (beforeNodeIndex == 0) {
+        beforeNodeIndex = placements.length;
+      }
+      let inserted = false;
+      for (; beforeNodeIndex < placements.length; beforeNodeIndex++) {
+        let beforeNode = this.#target.getElementsByAttribute(
+          "id",
+          placements[beforeNodeIndex]
+        )[0];
+        // Unfortunately, XUL add-ons can mess with nodes after they are inserted,
+        // and this breaks the following code if the button isn't where we expect
+        // it to be (ie not a child of the target). In this case, ignore the node.
+        if (beforeNode && this.#target == beforeNode.parentElement) {
+          this.#target.insertBefore(child, beforeNode);
+          inserted = true;
+          break;
+        }
+      }
+      if (!inserted) {
+        this.#target.appendChild(child);
+      }
+      child.removeAttribute("cui-anchorid");
+      child.removeAttribute("overflowedItem");
+      CustomizableUIInternal.ensureButtonContextMenu(child, this.#target);
+      CustomizableUIInternal.notifyListeners(
+        "onWidgetUnderflow",
+        child,
+        this.#target
+      );
+    }
+
+    win.UpdateUrlbarSearchSplitterState();
+
+    let defaultListItems = Array.from(this.#defaultList.children);
+    if (
+      defaultListItems.every(
+        item =>
+          CustomizableUI.isSpecialWidget(item.id) ||
+          this.#hiddenOverflowedNodes.has(item)
+      )
+    ) {
+      this.#toolbar.removeAttribute("overflowing");
+    }
+  }
+
+  /**
+   * Checks to see if there are overflowable items within the customization
+   * target of the toolbar that should be moved into the overflow panel, and
+   * if there are, moves them.
+   *
+   * Note that since this is an async function that can be called in bursts
+   * by resize events on the window, this function is often re-called even
+   * when a prior call hasn't yet resolved. In that situation, the older calls
+   * resolve early without doing any work and leave any DOM manipulation to the
+   * most recent call.
+   *
+   * This function is a no-op if the OverflowableToolbar is disabled or the
+   * DOM fullscreen UI is currently being used.
+   *
+   * @returns {Promise}
+   * @resolves {undefined} Once any movement of toolbar items has completed.
+   */
+  async #checkOverflow() {
+    if (!this.#enabled) {
+      return;
+    }
+
+    let win = this.#target.ownerGlobal;
+    if (win.document.documentElement.hasAttribute("inDOMFullscreen")) {
+      // Toolbars are hidden and cannot be made visible in DOM fullscreen mode
+      // so there's nothing to do here.
+      return;
+    }
+
+    let checkOverflowHandle = (this.#checkOverflowHandle = {});
+
+    lazy.log.debug("Checking overflow");
+    let { isOverflowing, totalAvailWidth } = await this.#getOverflowInfo();
+    if (win.closed || this.#checkOverflowHandle != checkOverflowHandle) {
+      return;
+    }
+
+    if (isOverflowing) {
+      await this.#onOverflow();
+    } else {
+      await this.#moveItemsBackToTheirOrigin(false, totalAvailWidth);
+    }
+
+    if (checkOverflowHandle == this.#checkOverflowHandle) {
+      this.#checkOverflowHandle = null;
+    }
+  }
+
+  /**
+   * Makes the OverflowableToolbar inert and moves all overflowable items back
+   * into the customization target of the toolbar.
+   */
+  #disable() {
+    // Abort any ongoing overflow check. #enable() will #checkOverflow()
+    // anyways, so this is enough.
+    this.#checkOverflowHandle = {};
+    this.#moveItemsBackToTheirOrigin(true);
+    this.#enabled = false;
+  }
+
+  /**
+   * Puts the OverflowableToolbar into the enabled state and then checks to see
+   * if any of the items in the customization target should be overflowed into
+   * the overflow panel list.
+   */
+  #enable() {
+    this.#enabled = true;
+    this.#checkOverflow();
+  }
+
+  /**
+   * Shows the overflow panel and sets a timeout to automatically re-hide the
+   * panel if it is not being hovered.
+   */
+  #showWithTimeout() {
+    const OVERFLOW_PANEL_HIDE_DELAY_MS = 500;
+
+    this.show().then(() => {
+      let window = this.#toolbar.ownerGlobal;
+      if (this.#hideTimeoutId) {
+        window.clearTimeout(this.#hideTimeoutId);
+      }
+      this.#hideTimeoutId = window.setTimeout(() => {
+        if (!this.#defaultListPanel.firstElementChild.matches(":hover")) {
+          lazy.PanelMultiView.hidePopup(this.#defaultListPanel);
+        }
+      }, OVERFLOW_PANEL_HIDE_DELAY_MS);
+    });
+  }
+
+  /**
+   * Gets and caches a reference to the DOM node with the ID set as the value
+   * of addon-webext-overflowtarget. If a cache already exists, that's returned
+   * instead. If addon-webext-overflowtarget has no value, null is returned.
+   *
+   * @returns {Element|null} the list that overflowed extension toolbar
+   *   buttons should go to if the Unified Extensions UI is enabled, or null
+   *   if no such list exists.
+   */
+  get #webExtList() {
+    if (!this.#webExtListRef) {
+      let targetID = this.#toolbar.getAttribute("addon-webext-overflowtarget");
+      if (!targetID) {
+        throw new Error(
+          "addon-webext-overflowtarget was not defined on the " +
+            `overflowable toolbar with id: ${this.#toolbar.id}`
+        );
+      }
+      let win = this.#toolbar.ownerGlobal;
+      let { panel } = win.gUnifiedExtensions;
+      this.#webExtListRef = panel.querySelector(`#${targetID}`);
+    }
+    return this.#webExtListRef;
+  }
+
+  /**
+   * Returns true if aNode is not null and is one of either this.#webExtList or
+   * this.#defaultList.
+   *
+   * @param {DOMElement} aNode The node to test.
+   * @returns {boolean}
+   */
+  #isOverflowList(aNode) {
+    return aNode == this.#defaultList || aNode == this.#webExtList;
+  }
+
+  /**
+   * Private event handlers start here.
+   */
+
+  /**
+   * Handles clicks on the #defaultListButton element.
+   *
+   * @param {MouseEvent} aEvent the click event.
+   */
+  #onClickDefaultListButton(aEvent) {
+    if (this.#defaultListButton.open) {
+      this.#defaultListButton.open = false;
+      lazy.PanelMultiView.hidePopup(this.#defaultListPanel);
+    } else if (
+      this.#defaultListPanel.state != "hiding" &&
+      !this.#defaultListButton.disabled
+    ) {
+      this.show(aEvent);
+    }
+  }
+
+  /**
+   * Handles the popuphiding event firing on the #defaultListPanel.
+   *
+   * @param {WidgetMouseEvent} aEvent the popuphiding event that fired on the
+   *   #defaultListPanel.
+   */
+  #onPanelHiding(aEvent) {
+    if (aEvent.target != this.#defaultListPanel) {
+      // Ignore context menus, <select> popups, etc.
+      return;
+    }
+    this.#defaultListButton.open = false;
+    this.#defaultListPanel.removeEventListener("dragover", this);
+    this.#defaultListPanel.removeEventListener("dragend", this);
+    let doc = aEvent.target.ownerDocument;
+    doc.defaultView.updateEditUIVisibility();
+    let contextMenuId = this.#defaultListPanel.getAttribute("context");
+    if (contextMenuId) {
+      let contextMenu = doc.getElementById(contextMenuId);
+      Services.els.removeSystemEventListener(
+        contextMenu,
+        "command",
+        this,
+        true
+      );
+    }
+  }
+
+  /**
+   * Handles a resize event fired on the window hosting this
+   * OverflowableToolbar.
+   *
+   * @param {UIEvent} aEvent the resize event.
+   */
+  #onResize(aEvent) {
+    // Ignore bubbled-up resize events.
+    if (aEvent.target != aEvent.currentTarget) {
+      return;
+    }
+    this.#checkOverflow();
+  }
+
+  /**
+   * CustomizableUI listener methods start here.
+   */
+
+  onWidgetBeforeDOMChange(aNode, aNextNode, aContainer) {
+    // This listener method is used to handle the case where a widget is
+    // moved or removed from an area via the CustomizableUI API while
+    // overflowed. It reorganizes the internal state of this OverflowableToolbar
+    // to handle that change.
+    if (!this.#enabled || !this.#isOverflowList(aContainer)) {
+      return;
+    }
+    // When we (re)move an item, update all the items that come after it in the list
+    // with the minsize *of the item before the to-be-removed node*. This way, we
+    // ensure that we try to move items back as soon as that's possible.
+    let updatedMinSize;
+    if (aNode.previousElementSibling) {
+      updatedMinSize = this.#overflowedInfo.get(
+        aNode.previousElementSibling.id
+      );
+    } else {
+      // Force (these) items to try to flow back into the bar:
+      updatedMinSize = 1;
+    }
+    let nextItem = aNode.nextElementSibling;
+    while (nextItem) {
+      this.#overflowedInfo.set(nextItem.id, updatedMinSize);
+      nextItem = nextItem.nextElementSibling;
+    }
+  }
+
+  onWidgetAfterDOMChange(aNode, aNextNode, aContainer) {
+    // This listener method is used to handle the case where a widget is
+    // moved or removed from an area via the CustomizableUI API while
+    // overflowed. It updates the DOM in the event that the movement or removal
+    // causes overflow or underflow of the toolbar.
+    if (
+      !this.#enabled ||
+      (aContainer != this.#target && !this.#isOverflowList(aContainer))
+    ) {
+      return;
+    }
+
+    let nowOverflowed = this.#isOverflowList(aNode.parentNode);
+    let wasOverflowed = this.#overflowedInfo.has(aNode.id);
+
+    // If this wasn't overflowed before...
+    if (!wasOverflowed) {
+      // ... but it is now, then we added to one of the overflow panels.
+      if (nowOverflowed) {
+        // We could be the first item in the overflow panel if we're being inserted
+        // before the previous first item in it. We can't assume the minimum
+        // size is the same (because the other item might be much wider), so if
+        // there is no previous item, just allow this item to be put back in the
+        // toolbar immediately by specifying a very low minimum size.
+        let sourceOfMinSize = aNode.previousElementSibling;
+        let minSize = sourceOfMinSize
+          ? this.#overflowedInfo.get(sourceOfMinSize.id)
+          : 1;
+        this.#overflowedInfo.set(aNode.id, minSize);
+        aNode.setAttribute("cui-anchorid", this.#defaultListButton.id);
+        aNode.setAttribute("overflowedItem", true);
+        CustomizableUIInternal.ensureButtonContextMenu(aNode, aContainer, true);
+        CustomizableUIInternal.notifyListeners(
+          "onWidgetOverflow",
+          aNode,
+          this.#target
+        );
+      }
+    } else if (!nowOverflowed) {
+      // If it used to be overflowed...
+      // ... and isn't anymore, let's remove our bookkeeping:
+      this.#overflowedInfo.delete(aNode.id);
+      aNode.removeAttribute("cui-anchorid");
+      aNode.removeAttribute("overflowedItem");
+      CustomizableUIInternal.ensureButtonContextMenu(aNode, aContainer);
+      CustomizableUIInternal.notifyListeners(
+        "onWidgetUnderflow",
+        aNode,
+        this.#target
+      );
+
+      let collapsedWidgetIds = Array.from(this.#overflowedInfo.keys());
+      if (collapsedWidgetIds.every(w => CustomizableUI.isSpecialWidget(w))) {
+        this.#toolbar.removeAttribute("overflowing");
+      }
+    } else if (aNode.previousElementSibling) {
+      // but if it still is, it must have changed places. Bookkeep:
+      let prevId = aNode.previousElementSibling.id;
+      let minSize = this.#overflowedInfo.get(prevId);
+      this.#overflowedInfo.set(aNode.id, minSize);
+    }
+
+    // We might overflow now if an item was added, or we may be able to move
+    // stuff back into the toolbar if an item was removed.
+    this.#checkOverflow();
+  }
+
+  /**
+   * @returns {Boolean} whether the given node is in the overflow list.
+   */
+  isInOverflowList(node) {
+    return node.parentNode == this.#defaultList;
+  }
+
+  /**
+   * nsIObserver implementation starts here.
+   */
+
+  observe(aSubject, aTopic, aData) {
+    // This nsIObserver method allows us to defer initialization until after
+    // this window has finished painting and starting up.
+    if (
+      aTopic == "browser-delayed-startup-finished" &&
+      aSubject == this.#toolbar.ownerGlobal
+    ) {
+      Services.obs.removeObserver(this, "browser-delayed-startup-finished");
+      this.init();
+    }
+  }
+
+  /**
+   * nsIDOMEventListener implementation starts here.
+   */
+
+  handleEvent(aEvent) {
+    switch (aEvent.type) {
+      case "aftercustomization": {
+        this.#enable();
+        break;
+      }
+      case "mousedown": {
+        if (aEvent.button != 0) {
+          break;
+        }
+        if (aEvent.target == this.#defaultListButton) {
+          this.#onClickDefaultListButton(aEvent);
+        } else {
+          lazy.PanelMultiView.hidePopup(this.#defaultListPanel);
+        }
+        break;
+      }
+      case "keypress": {
+        if (
+          aEvent.target == this.#defaultListButton &&
+          (aEvent.key == " " || aEvent.key == "Enter")
+        ) {
+          this.#onClickDefaultListButton(aEvent);
+        }
+        break;
+      }
+      case "customizationstarting": {
+        this.#disable();
+        break;
+      }
+      case "dragover": {
+        if (this.#enabled) {
+          this.#showWithTimeout();
+        }
+        break;
+      }
+      case "dragend": {
+        lazy.PanelMultiView.hidePopup(this.#defaultListPanel);
+        break;
+      }
+      case "popuphiding": {
+        this.#onPanelHiding(aEvent);
+        break;
+      }
+      case "resize": {
+        this.#onResize(aEvent);
+        break;
+      }
+    }
+  }
+}
+
+CustomizableUIInternal.initialize();