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diff --git a/base/content/browser-fullZoom.js b/base/content/browser-fullZoom.js
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+/* 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/. */
+
+/**
+ * Controls the "full zoom" setting and its site-specific preferences.
+ */
+var FullZoom = {
+  // Identifies the setting in the content prefs database.
+  name: "browser.content.full-zoom",
+
+  // browser.zoom.siteSpecific preference cache
+  _siteSpecificPref: undefined,
+
+  // browser.zoom.updateBackgroundTabs preference cache
+  updateBackgroundTabs: undefined,
+
+  // This maps the browser to monotonically increasing integer
+  // tokens. _browserTokenMap[browser] is increased each time the zoom is
+  // changed in the browser. See _getBrowserToken and _ignorePendingZoomAccesses.
+  _browserTokenMap: new WeakMap(),
+
+  // Stores initial locations if we receive onLocationChange
+  // events before we're initialized.
+  _initialLocations: new WeakMap(),
+
+  get siteSpecific() {
+    return this._siteSpecificPref;
+  },
+
+  // nsISupports
+
+  QueryInterface: XPCOMUtils.generateQI([Ci.nsIDOMEventListener,
+                                         Ci.nsIObserver,
+                                         Ci.nsIContentPrefObserver,
+                                         Ci.nsISupportsWeakReference,
+                                         Ci.nsISupports]),
+
+  // Initialization & Destruction
+
+  init: function FullZoom_init() {
+    gBrowser.addEventListener("ZoomChangeUsingMouseWheel", this);
+
+    // Register ourselves with the service so we know when our pref changes.
+    this._cps2 = Cc["@mozilla.org/content-pref/service;1"].
+                 getService(Ci.nsIContentPrefService2);
+    this._cps2.addObserverForName(this.name, this);
+
+    this._siteSpecificPref =
+      gPrefService.getBoolPref("browser.zoom.siteSpecific");
+    this.updateBackgroundTabs =
+      gPrefService.getBoolPref("browser.zoom.updateBackgroundTabs");
+    // Listen for changes to the browser.zoom branch so we can enable/disable
+    // updating background tabs and per-site saving and restoring of zoom levels.
+    gPrefService.addObserver("browser.zoom.", this, true);
+
+    // If we received onLocationChange events for any of the current browsers
+    // before we were initialized we want to replay those upon initialization.
+    for (let browser of gBrowser.browsers) {
+      if (this._initialLocations.has(browser)) {
+        this.onLocationChange(...this._initialLocations.get(browser), browser);
+      }
+    }
+
+    // This should be nulled after initialization.
+    this._initialLocations = null;
+  },
+
+  destroy: function FullZoom_destroy() {
+    gPrefService.removeObserver("browser.zoom.", this);
+    this._cps2.removeObserverForName(this.name, this);
+    gBrowser.removeEventListener("ZoomChangeUsingMouseWheel", this);
+  },
+
+
+  // Event Handlers
+
+  // nsIDOMEventListener
+
+  handleEvent: function FullZoom_handleEvent(event) {
+    switch (event.type) {
+      case "ZoomChangeUsingMouseWheel":
+        let browser = this._getTargetedBrowser(event);
+        this._ignorePendingZoomAccesses(browser);
+        this._applyZoomToPref(browser);
+        break;
+    }
+  },
+
+  // nsIObserver
+
+  observe: function (aSubject, aTopic, aData) {
+    switch (aTopic) {
+      case "nsPref:changed":
+        switch (aData) {
+          case "browser.zoom.siteSpecific":
+            this._siteSpecificPref =
+              gPrefService.getBoolPref("browser.zoom.siteSpecific");
+            break;
+          case "browser.zoom.updateBackgroundTabs":
+            this.updateBackgroundTabs =
+              gPrefService.getBoolPref("browser.zoom.updateBackgroundTabs");
+            break;
+        }
+        break;
+    }
+  },
+
+  // nsIContentPrefObserver
+
+  onContentPrefSet: function FullZoom_onContentPrefSet(aGroup, aName, aValue, aIsPrivate) {
+    this._onContentPrefChanged(aGroup, aValue, aIsPrivate);
+  },
+
+  onContentPrefRemoved: function FullZoom_onContentPrefRemoved(aGroup, aName, aIsPrivate) {
+    this._onContentPrefChanged(aGroup, undefined, aIsPrivate);
+  },
+
+  /**
+   * Appropriately updates the zoom level after a content preference has
+   * changed.
+   *
+   * @param aGroup  The group of the changed preference.
+   * @param aValue  The new value of the changed preference.  Pass undefined to
+   *                indicate the preference's removal.
+   */
+  _onContentPrefChanged: function FullZoom__onContentPrefChanged(aGroup, aValue, aIsPrivate) {
+    if (this._isNextContentPrefChangeInternal) {
+      // Ignore changes that FullZoom itself makes.  This works because the
+      // content pref service calls callbacks before notifying observers, and it
+      // does both in the same turn of the event loop.
+      delete this._isNextContentPrefChangeInternal;
+      return;
+    }
+
+    let browser = gBrowser.selectedBrowser;
+    if (!browser.currentURI)
+      return;
+
+    let ctxt = this._loadContextFromBrowser(browser);
+    let domain = this._cps2.extractDomain(browser.currentURI.spec);
+    if (aGroup) {
+      if (aGroup == domain && ctxt.usePrivateBrowsing == aIsPrivate)
+        this._applyPrefToZoom(aValue, browser);
+      return;
+    }
+
+    this._globalValue = aValue === undefined ? aValue :
+                          this._ensureValid(aValue);
+
+    // If the current page doesn't have a site-specific preference, then its
+    // zoom should be set to the new global preference now that the global
+    // preference has changed.
+    let hasPref = false;
+    let token = this._getBrowserToken(browser);
+    this._cps2.getByDomainAndName(browser.currentURI.spec, this.name, ctxt, {
+      handleResult: function () { hasPref = true; },
+      handleCompletion: function () {
+        if (!hasPref && token.isCurrent)
+          this._applyPrefToZoom(undefined, browser);
+      }.bind(this)
+    });
+  },
+
+  // location change observer
+
+  /**
+   * Called when the location of a tab changes.
+   * When that happens, we need to update the current zoom level if appropriate.
+   *
+   * @param aURI
+   *        A URI object representing the new location.
+   * @param aIsTabSwitch
+   *        Whether this location change has happened because of a tab switch.
+   * @param aBrowser
+   *        (optional) browser object displaying the document
+   */
+  onLocationChange: function FullZoom_onLocationChange(aURI, aIsTabSwitch, aBrowser) {
+    let browser = aBrowser || gBrowser.selectedBrowser;
+
+    // If we haven't been initialized yet but receive an onLocationChange
+    // notification then let's store and replay it upon initialization.
+    if (this._initialLocations) {
+      this._initialLocations.set(browser, [aURI, aIsTabSwitch]);
+      return;
+    }
+
+    // Ignore all pending async zoom accesses in the browser.  Pending accesses
+    // that started before the location change will be prevented from applying
+    // to the new location.
+    this._ignorePendingZoomAccesses(browser);
+
+    if (!aURI || (aIsTabSwitch && !this.siteSpecific)) {
+      this._notifyOnLocationChange(browser);
+      return;
+    }
+
+    // Avoid the cps roundtrip and apply the default/global pref.
+    if (aURI.spec == "about:blank") {
+      this._applyPrefToZoom(undefined, browser,
+                            this._notifyOnLocationChange.bind(this, browser));
+      return;
+    }
+
+    // Media documents should always start at 1, and are not affected by prefs.
+    if (!aIsTabSwitch && browser.isSyntheticDocument) {
+      ZoomManager.setZoomForBrowser(browser, 1);
+      // _ignorePendingZoomAccesses already called above, so no need here.
+      this._notifyOnLocationChange(browser);
+      return;
+    }
+
+    // See if the zoom pref is cached.
+    let ctxt = this._loadContextFromBrowser(browser);
+    let pref = this._cps2.getCachedByDomainAndName(aURI.spec, this.name, ctxt);
+    if (pref) {
+      this._applyPrefToZoom(pref.value, browser,
+                            this._notifyOnLocationChange.bind(this, browser));
+      return;
+    }
+
+    // It's not cached, so we have to asynchronously fetch it.
+    let value = undefined;
+    let token = this._getBrowserToken(browser);
+    this._cps2.getByDomainAndName(aURI.spec, this.name, ctxt, {
+      handleResult: function (resultPref) { value = resultPref.value; },
+      handleCompletion: function () {
+        if (!token.isCurrent) {
+          this._notifyOnLocationChange(browser);
+          return;
+        }
+        this._applyPrefToZoom(value, browser,
+                              this._notifyOnLocationChange.bind(this, browser));
+      }.bind(this)
+    });
+  },
+
+  // update state of zoom type menu item
+
+  updateMenu: function FullZoom_updateMenu() {
+    var menuItem = document.getElementById("toggle_zoom");
+
+    menuItem.setAttribute("checked", !ZoomManager.useFullZoom);
+  },
+
+  // Setting & Pref Manipulation
+
+  /**
+   * Reduces the zoom level of the page in the current browser.
+   */
+  reduce: function FullZoom_reduce() {
+    ZoomManager.reduce();
+    let browser = gBrowser.selectedBrowser;
+    this._ignorePendingZoomAccesses(browser);
+    this._applyZoomToPref(browser);
+  },
+
+  /**
+   * Enlarges the zoom level of the page in the current browser.
+   */
+  enlarge: function FullZoom_enlarge() {
+    ZoomManager.enlarge();
+    let browser = gBrowser.selectedBrowser;
+    this._ignorePendingZoomAccesses(browser);
+    this._applyZoomToPref(browser);
+  },
+
+  /**
+   * Sets the zoom level for the given browser to the given floating
+   * point value, where 1 is the default zoom level.
+   */
+  setZoom: function (value, browser = gBrowser.selectedBrowser) {
+    ZoomManager.setZoomForBrowser(browser, value);
+    this._ignorePendingZoomAccesses(browser);
+    this._applyZoomToPref(browser);
+  },
+
+  /**
+   * Sets the zoom level of the page in the given browser to the global zoom
+   * level.
+   *
+   * @return A promise which resolves when the zoom reset has been applied.
+   */
+  reset: function FullZoom_reset(browser = gBrowser.selectedBrowser) {
+    let token = this._getBrowserToken(browser);
+    let result = this._getGlobalValue(browser).then(value => {
+      if (token.isCurrent) {
+        ZoomManager.setZoomForBrowser(browser, value === undefined ? 1 : value);
+        this._ignorePendingZoomAccesses(browser);
+        Services.obs.notifyObservers(browser, "browser-fullZoom:zoomReset", "");
+      }
+    });
+    this._removePref(browser);
+    return result;
+  },
+
+  /**
+   * Set the zoom level for a given browser.
+   *
+   * Per nsPresContext::setFullZoom, we can set the zoom to its current value
+   * without significant impact on performance, as the setting is only applied
+   * if it differs from the current setting.  In fact getting the zoom and then
+   * checking ourselves if it differs costs more.
+   *
+   * And perhaps we should always set the zoom even if it was more expensive,
+   * since nsDocumentViewer::SetTextZoom claims that child documents can have
+   * a different text zoom (although it would be unusual), and it implies that
+   * those child text zooms should get updated when the parent zoom gets set,
+   * and perhaps the same is true for full zoom
+   * (although nsDocumentViewer::SetFullZoom doesn't mention it).
+   *
+   * So when we apply new zoom values to the browser, we simply set the zoom.
+   * We don't check first to see if the new value is the same as the current
+   * one.
+   *
+   * @param aValue     The zoom level value.
+   * @param aBrowser   The zoom is set in this browser.  Required.
+   * @param aCallback  If given, it's asynchronously called when complete.
+   */
+  _applyPrefToZoom: function FullZoom__applyPrefToZoom(aValue, aBrowser, aCallback) {
+    if (!this.siteSpecific || gInPrintPreviewMode) {
+      this._executeSoon(aCallback);
+      return;
+    }
+
+    // The browser is sometimes half-destroyed because this method is called
+    // by content pref service callbacks, which themselves can be called at any
+    // time, even after browsers are closed.
+    if (!aBrowser.parentNode || aBrowser.isSyntheticDocument) {
+      this._executeSoon(aCallback);
+      return;
+    }
+
+    if (aValue !== undefined) {
+      ZoomManager.setZoomForBrowser(aBrowser, this._ensureValid(aValue));
+      this._ignorePendingZoomAccesses(aBrowser);
+      this._executeSoon(aCallback);
+      return;
+    }
+
+    let token = this._getBrowserToken(aBrowser);
+    this._getGlobalValue(aBrowser).then(value => {
+      if (token.isCurrent) {
+        ZoomManager.setZoomForBrowser(aBrowser, value === undefined ? 1 : value);
+        this._ignorePendingZoomAccesses(aBrowser);
+      }
+      this._executeSoon(aCallback);
+    });
+  },
+
+  /**
+   * Saves the zoom level of the page in the given browser to the content
+   * prefs store.
+   *
+   * @param browser  The zoom of this browser will be saved.  Required.
+   */
+  _applyZoomToPref: function FullZoom__applyZoomToPref(browser) {
+    Services.obs.notifyObservers(browser, "browser-fullZoom:zoomChange", "");
+    if (!this.siteSpecific ||
+        gInPrintPreviewMode ||
+        browser.isSyntheticDocument)
+      return;
+
+    this._cps2.set(browser.currentURI.spec, this.name,
+                   ZoomManager.getZoomForBrowser(browser),
+                   this._loadContextFromBrowser(browser), {
+      handleCompletion: function () {
+        this._isNextContentPrefChangeInternal = true;
+      }.bind(this),
+    });
+  },
+
+  /**
+   * Removes from the content prefs store the zoom level of the given browser.
+   *
+   * @param browser  The zoom of this browser will be removed.  Required.
+   */
+  _removePref: function FullZoom__removePref(browser) {
+    Services.obs.notifyObservers(browser, "browser-fullZoom:zoomReset", "");
+    if (browser.isSyntheticDocument)
+      return;
+    let ctxt = this._loadContextFromBrowser(browser);
+    this._cps2.removeByDomainAndName(browser.currentURI.spec, this.name, ctxt, {
+      handleCompletion: function () {
+        this._isNextContentPrefChangeInternal = true;
+      }.bind(this),
+    });
+  },
+
+  // Utilities
+
+  /**
+   * Returns the zoom change token of the given browser.  Asynchronous
+   * operations that access the given browser's zoom should use this method to
+   * capture the token before starting and use token.isCurrent to determine if
+   * it's safe to access the zoom when done.  If token.isCurrent is false, then
+   * after the async operation started, either the browser's zoom was changed or
+   * the browser was destroyed, and depending on what the operation is doing, it
+   * may no longer be safe to set and get its zoom.
+   *
+   * @param browser  The token of this browser will be returned.
+   * @return  An object with an "isCurrent" getter.
+   */
+  _getBrowserToken: function FullZoom__getBrowserToken(browser) {
+    let map = this._browserTokenMap;
+    if (!map.has(browser))
+      map.set(browser, 0);
+    return {
+      token: map.get(browser),
+      get isCurrent() {
+        // At this point, the browser may have been destructed and unbound but
+        // its outer ID not removed from the map because outer-window-destroyed
+        // hasn't been received yet.  In that case, the browser is unusable, it
+        // has no properties, so return false.  Check for this case by getting a
+        // property, say, docShell.
+        return map.get(browser) === this.token && browser.parentNode;
+      },
+    };
+  },
+
+  /**
+   * Returns the browser that the supplied zoom event is associated with.
+   * @param event  The ZoomChangeUsingMouseWheel event.
+   * @return  The associated browser element, if one exists, otherwise null.
+   */
+  _getTargetedBrowser: function FullZoom__getTargetedBrowser(event) {
+    let target = event.originalTarget;
+
+    // With remote content browsers, the event's target is the browser
+    // we're looking for.
+    const XUL_NS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
+    if (target instanceof window.XULElement &&
+        target.localName == "browser" &&
+        target.namespaceURI == XUL_NS)
+      return target;
+
+    // With in-process content browsers, the event's target is the content
+    // document.
+    if (target.nodeType == Node.DOCUMENT_NODE)
+      return gBrowser.getBrowserForDocument(target);
+
+    throw new Error("Unexpected ZoomChangeUsingMouseWheel event source");
+  },
+
+  /**
+   * Increments the zoom change token for the given browser so that pending
+   * async operations know that it may be unsafe to access they zoom when they
+   * finish.
+   *
+   * @param browser  Pending accesses in this browser will be ignored.
+   */
+  _ignorePendingZoomAccesses: function FullZoom__ignorePendingZoomAccesses(browser) {
+    let map = this._browserTokenMap;
+    map.set(browser, (map.get(browser) || 0) + 1);
+  },
+
+  _ensureValid: function FullZoom__ensureValid(aValue) {
+    // Note that undefined is a valid value for aValue that indicates a known-
+    // not-to-exist value.
+    if (isNaN(aValue))
+      return 1;
+
+    if (aValue < ZoomManager.MIN)
+      return ZoomManager.MIN;
+
+    if (aValue > ZoomManager.MAX)
+      return ZoomManager.MAX;
+
+    return aValue;
+  },
+
+  /**
+   * Gets the global browser.content.full-zoom content preference.
+   *
+   * @param browser   The browser pertaining to the zoom.
+   * @returns Promise<prefValue>
+   *                  Resolves to the preference value when done.
+   */
+  _getGlobalValue: function FullZoom__getGlobalValue(browser) {
+    // * !("_globalValue" in this) => global value not yet cached.
+    // * this._globalValue === undefined => global value known not to exist.
+    // * Otherwise, this._globalValue is a number, the global value.
+    return new Promise(resolve => {
+      if ("_globalValue" in this) {
+        resolve(this._globalValue);
+        return;
+      }
+      let value = undefined;
+      this._cps2.getGlobal(this.name, this._loadContextFromBrowser(browser), {
+        handleResult: function (pref) { value = pref.value; },
+        handleCompletion: (reason) => {
+          this._globalValue = this._ensureValid(value);
+          resolve(this._globalValue);
+        }
+      });
+    });
+  },
+
+  /**
+   * Gets the load context from the given Browser.
+   *
+   * @param Browser  The Browser whose load context will be returned.
+   * @return        The nsILoadContext of the given Browser.
+   */
+  _loadContextFromBrowser: function FullZoom__loadContextFromBrowser(browser) {
+    return browser.loadContext;
+  },
+
+  /**
+   * Asynchronously broadcasts "browser-fullZoom:location-change" so that
+   * listeners can be notified when the zoom levels on those pages change.
+   * The notification is always asynchronous so that observers are guaranteed a
+   * consistent behavior.
+   */
+  _notifyOnLocationChange: function FullZoom__notifyOnLocationChange(browser) {
+    this._executeSoon(function () {
+      Services.obs.notifyObservers(browser, "browser-fullZoom:location-change", "");
+    });
+  },
+
+  _executeSoon: function FullZoom__executeSoon(callback) {
+    if (!callback)
+      return;
+    Services.tm.mainThread.dispatch(callback, Ci.nsIThread.DISPATCH_NORMAL);
+  },
+};