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diff --git a/modules/WindowsPreviewPerTab.jsm b/modules/WindowsPreviewPerTab.jsm
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+/* vim: se cin sw=2 ts=2 et filetype=javascript :
+ * 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/. */
+/*
+ * This module implements the front end behavior for AeroPeek. The taskbar
+ * allows an application to expose its tabbed interface by showing thumbnail
+ * previews rather than the default window preview.
+ * Additionally, when a user hovers over a thumbnail (tab or window),
+ * they are shown a live preview of the window (or tab + its containing window).
+ *
+ * In Windows 7, a title, icon, close button and optional toolbar are shown for
+ * each preview. This feature does not make use of the toolbar. For window
+ * previews, the title is the window title and the icon the window icon. For
+ * tab previews, the title is the page title and the page's favicon. In both
+ * cases, the close button "does the right thing."
+ *
+ * The primary objects behind this feature are nsITaskbarTabPreview and
+ * nsITaskbarPreviewController. Each preview has a controller. The controller
+ * responds to the user's interactions on the taskbar and provides the required
+ * data to the preview for determining the size of the tab and thumbnail. The
+ * PreviewController class implements this interface. The preview will request
+ * the controller to provide a thumbnail or preview when the user interacts with
+ * the taskbar. To reduce the overhead of drawing the tab area, the controller
+ * implementation caches the tab's contents in a <canvas> element. If no
+ * previews or thumbnails have been requested for some time, the controller will
+ * discard its cached tab contents.
+ *
+ * Screen real estate is limited so when there are too many thumbnails to fit
+ * on the screen, the taskbar stops displaying thumbnails and instead displays
+ * just the title, icon and close button in a similar fashion to previous
+ * versions of the taskbar. If there are still too many previews to fit on the
+ * screen, the taskbar resorts to a scroll up and scroll down button pair to let
+ * the user scroll through the list of tabs. Since this is undoubtedly
+ * inconvenient for users with many tabs, the AeroPeek objects turns off all of
+ * the tab previews. This tells the taskbar to revert to one preview per window.
+ * If the number of tabs falls below this magic threshold, the preview-per-tab
+ * behavior returns. There is no reliable way to determine when the scroll
+ * buttons appear on the taskbar, so a magic pref-controlled number determines
+ * when this threshold has been crossed.
+ */
+this.EXPORTED_SYMBOLS = ["AeroPeek"];
+
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+const Cu = Components.utils;
+
+Cu.import("resource://gre/modules/NetUtil.jsm");
+Cu.import("resource://gre/modules/PlacesUtils.jsm");
+Cu.import("resource://gre/modules/PrivateBrowsingUtils.jsm");
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+
+// Pref to enable/disable preview-per-tab
+const TOGGLE_PREF_NAME = "browser.taskbar.previews.enable";
+// Pref to determine the magic auto-disable threshold
+const DISABLE_THRESHOLD_PREF_NAME = "browser.taskbar.previews.max";
+// Pref to control the time in seconds that tab contents live in the cache
+const CACHE_EXPIRATION_TIME_PREF_NAME = "browser.taskbar.previews.cachetime";
+
+const WINTASKBAR_CONTRACTID = "@mozilla.org/windows-taskbar;1";
+
+// Various utility properties
+XPCOMUtils.defineLazyServiceGetter(this, "imgTools",
+                                   "@mozilla.org/image/tools;1",
+                                   "imgITools");
+XPCOMUtils.defineLazyModuleGetter(this, "PageThumbs",
+                                  "resource://gre/modules/PageThumbs.jsm");
+
+// nsIURI -> imgIContainer
+function _imageFromURI(uri, privateMode, callback) {
+  let channel = NetUtil.newChannel({
+    uri: uri,
+    loadUsingSystemPrincipal: true,
+    contentPolicyType: Ci.nsIContentPolicy.TYPE_INTERNAL_IMAGE
+  });
+
+  try {
+    channel.QueryInterface(Ci.nsIPrivateBrowsingChannel);
+    channel.setPrivate(privateMode);
+  } catch (e) {
+    // Ignore channels which do not support nsIPrivateBrowsingChannel
+  }
+  NetUtil.asyncFetch(channel, function(inputStream, resultCode) {
+    if (!Components.isSuccessCode(resultCode))
+      return;
+    try {
+      let out_img = { value: null };
+      imgTools.decodeImageData(inputStream, channel.contentType, out_img);
+      callback(out_img.value);
+    } catch (e) {
+      // We failed, so use the default favicon (only if this wasn't the default
+      // favicon).
+      let defaultURI = PlacesUtils.favicons.defaultFavicon;
+      if (!defaultURI.equals(uri))
+        _imageFromURI(defaultURI, privateMode, callback);
+    }
+  });
+}
+
+// string? -> imgIContainer
+function getFaviconAsImage(iconurl, privateMode, callback) {
+  if (iconurl) {
+    _imageFromURI(NetUtil.newURI(iconurl), privateMode, callback);
+  } else {
+    _imageFromURI(PlacesUtils.favicons.defaultFavicon, privateMode, callback);
+  }
+}
+
+// Snaps the given rectangle to be pixel-aligned at the given scale
+function snapRectAtScale(r, scale) {
+  let x = Math.floor(r.x * scale);
+  let y = Math.floor(r.y * scale);
+  let width = Math.ceil((r.x + r.width) * scale) - x;
+  let height = Math.ceil((r.y + r.height) * scale) - y;
+
+  r.x = x / scale;
+  r.y = y / scale;
+  r.width = width / scale;
+  r.height = height / scale;
+}
+
+// PreviewController
+
+/*
+ * This class manages the behavior of thumbnails and previews. It has the following
+ * responsibilities:
+ * 1) Responding to requests from Windows taskbar for a thumbnail or window
+ *    preview.
+ * 2) Listening for DOM events that result in a thumbnail or window preview needing
+ *    to be refreshed, and communicating this to the taskbar.
+ * 3) Handling queryies and returning new thumbnail or window preview images to the
+ *    taskbar through PageThumbs.
+ *
+ * @param win
+ *        The TabWindow (see below) that owns the preview that this controls
+ * @param tab
+ *        The <tab> that this preview is associated with
+ */
+function PreviewController(win, tab) {
+  this.win = win;
+  this.tab = tab;
+  this.linkedBrowser = tab.linkedBrowser;
+  this.preview = this.win.createTabPreview(this);
+
+  this.tab.addEventListener("TabAttrModified", this, false);
+
+  XPCOMUtils.defineLazyGetter(this, "canvasPreview", function () {
+    let canvas = PageThumbs.createCanvas();
+    canvas.mozOpaque = true;
+    return canvas;
+  });
+}
+
+PreviewController.prototype = {
+  QueryInterface: XPCOMUtils.generateQI([Ci.nsITaskbarPreviewController,
+                                         Ci.nsIDOMEventListener]),
+
+  destroy: function () {
+    this.tab.removeEventListener("TabAttrModified", this, false);
+
+    // Break cycles, otherwise we end up leaking the window with everything
+    // attached to it.
+    delete this.win;
+    delete this.preview;
+  },
+
+  get wrappedJSObject() {
+    return this;
+  },
+
+  // Resizes the canvasPreview to 0x0, essentially freeing its memory.
+  resetCanvasPreview: function () {
+    this.canvasPreview.width = 0;
+    this.canvasPreview.height = 0;
+  },
+
+  /**
+   * Set the canvas dimensions.
+   */
+  resizeCanvasPreview: function (aRequestedWidth, aRequestedHeight) {
+    this.canvasPreview.width = aRequestedWidth;
+    this.canvasPreview.height = aRequestedHeight;
+  },
+
+
+  get zoom() {
+    // Note that winutils.fullZoom accounts for "quantization" of the zoom factor
+    // from nsIContentViewer due to conversion through appUnits.
+    // We do -not- want screenPixelsPerCSSPixel here, because that would -also-
+    // incorporate any scaling that is applied due to hi-dpi resolution options.
+    return this.tab.linkedBrowser.fullZoom;
+  },
+
+  get screenPixelsPerCSSPixel() {
+    let chromeWin = this.tab.ownerGlobal;
+    let windowUtils = chromeWin.getInterface(Ci.nsIDOMWindowUtils);
+    return windowUtils.screenPixelsPerCSSPixel;
+  },
+
+  get browserDims() {
+    return this.tab.linkedBrowser.getBoundingClientRect();
+  },
+
+  cacheBrowserDims: function () {
+    let dims = this.browserDims;
+    this._cachedWidth = dims.width;
+    this._cachedHeight = dims.height;
+  },
+
+  testCacheBrowserDims: function () {
+    let dims = this.browserDims;
+    return this._cachedWidth == dims.width &&
+      this._cachedHeight == dims.height;
+  },
+
+  /**
+   * Capture a new thumbnail image for this preview. Called by the controller
+   * in response to a request for a new thumbnail image.
+   */
+  updateCanvasPreview: function (aFullScale, aCallback) {
+    // Update our cached browser dims so that delayed resize
+    // events don't trigger another invalidation if this tab becomes active.
+    this.cacheBrowserDims();
+    PageThumbs.captureToCanvas(this.linkedBrowser, this.canvasPreview,
+                               aCallback, { fullScale: aFullScale });
+    // If we're updating the canvas, then we're in the middle of a peek so
+    // don't discard the cache of previews.
+    AeroPeek.resetCacheTimer();
+  },
+
+  updateTitleAndTooltip: function () {
+    let title = this.win.tabbrowser.getWindowTitleForBrowser(this.linkedBrowser);
+    this.preview.title = title;
+    this.preview.tooltip = title;
+  },
+
+  // nsITaskbarPreviewController
+
+  // window width and height, not browser
+  get width() {
+    return this.win.width;
+  },
+
+  // window width and height, not browser
+  get height() {
+    return this.win.height;
+  },
+
+  get thumbnailAspectRatio() {
+    let browserDims = this.browserDims;
+    // Avoid returning 0
+    let tabWidth = browserDims.width || 1;
+    // Avoid divide by 0
+    let tabHeight = browserDims.height || 1;
+    return tabWidth / tabHeight;
+  },
+
+  /**
+   * Responds to taskbar requests for window previews. Returns the results asynchronously
+   * through updateCanvasPreview.
+   *
+   * @param aTaskbarCallback nsITaskbarPreviewCallback results callback
+   */
+  requestPreview: function (aTaskbarCallback) {
+    // Grab a high res content preview
+    this.resetCanvasPreview();
+    this.updateCanvasPreview(true, (aPreviewCanvas) => {
+      let winWidth = this.win.width;
+      let winHeight = this.win.height;
+
+      let composite = PageThumbs.createCanvas();
+
+      // Use transparency, Aero glass is drawn black without it.
+      composite.mozOpaque = false;
+
+      let ctx = composite.getContext('2d');
+      let scale = this.screenPixelsPerCSSPixel / this.zoom;
+
+      composite.width = winWidth * scale;
+      composite.height = winHeight * scale;
+
+      ctx.save();
+      ctx.scale(scale, scale);
+
+      // Draw chrome. Note we currently do not get scrollbars for remote frames
+      // in the image above.
+      ctx.drawWindow(this.win.win, 0, 0, winWidth, winHeight, "rgba(0,0,0,0)");
+
+      // Draw the content are into the composite canvas at the right location.
+      ctx.drawImage(aPreviewCanvas, this.browserDims.x, this.browserDims.y,
+                    aPreviewCanvas.width, aPreviewCanvas.height);
+      ctx.restore();
+
+      // Deliver the resulting composite canvas to Windows
+      this.win.tabbrowser.previewTab(this.tab, function () {
+        aTaskbarCallback.done(composite, false);
+      });
+    });
+  },
+
+  /**
+   * Responds to taskbar requests for tab thumbnails. Returns the results asynchronously
+   * through updateCanvasPreview.
+   *
+   * Note Windows requests a specific width and height here, if the resulting thumbnail
+   * does not match these dimensions thumbnail display will fail.
+   *
+   * @param aTaskbarCallback nsITaskbarPreviewCallback results callback
+   * @param aRequestedWidth width of the requested thumbnail
+   * @param aRequestedHeight height of the requested thumbnail
+   */
+  requestThumbnail: function (aTaskbarCallback, aRequestedWidth, aRequestedHeight) {
+    this.resizeCanvasPreview(aRequestedWidth, aRequestedHeight);
+    this.updateCanvasPreview(false, (aThumbnailCanvas) => {
+      aTaskbarCallback.done(aThumbnailCanvas, false);
+    });
+  },
+
+  // Event handling
+
+  onClose: function () {
+    this.win.tabbrowser.removeTab(this.tab);
+  },
+
+  onActivate: function () {
+    this.win.tabbrowser.selectedTab = this.tab;
+
+    // Accept activation - this will restore the browser window
+    // if it's minimized
+    return true;
+  },
+
+  // nsIDOMEventListener
+  handleEvent: function (evt) {
+    switch (evt.type) {
+      case "TabAttrModified":
+        this.updateTitleAndTooltip();
+        break;
+    }
+  }
+};
+
+XPCOMUtils.defineLazyGetter(PreviewController.prototype, "canvasPreviewFlags",
+  function () { let canvasInterface = Ci.nsIDOMCanvasRenderingContext2D;
+                return canvasInterface.DRAWWINDOW_DRAW_VIEW
+                     | canvasInterface.DRAWWINDOW_DRAW_CARET
+                     | canvasInterface.DRAWWINDOW_ASYNC_DECODE_IMAGES
+                     | canvasInterface.DRAWWINDOW_DO_NOT_FLUSH;
+});
+
+// TabWindow
+
+/*
+ * This class monitors a browser window for changes to its tabs
+ *
+ * @param win
+ *        The nsIDOMWindow browser window
+ */
+function TabWindow(win) {
+  this.win = win;
+  this.tabbrowser = win.gBrowser;
+
+  this.previews = new Map();
+
+  for (let i = 0; i < this.tabEvents.length; i++)
+    this.tabbrowser.tabContainer.addEventListener(this.tabEvents[i], this, false);
+
+  for (let i = 0; i < this.winEvents.length; i++)
+    this.win.addEventListener(this.winEvents[i], this, false);
+
+  this.tabbrowser.addTabsProgressListener(this);
+
+  AeroPeek.windows.push(this);
+  let tabs = this.tabbrowser.tabs;
+  for (let i = 0; i < tabs.length; i++)
+    this.newTab(tabs[i]);
+
+  this.updateTabOrdering();
+  AeroPeek.checkPreviewCount();
+}
+
+TabWindow.prototype = {
+  _enabled: false,
+  _cachedWidth: 0,
+  _cachedHeight: 0,
+  tabEvents: ["TabOpen", "TabClose", "TabSelect", "TabMove"],
+  winEvents: ["resize"],
+
+  destroy: function () {
+    this._destroying = true;
+
+    let tabs = this.tabbrowser.tabs;
+
+    this.tabbrowser.removeTabsProgressListener(this);
+
+    for (let i = 0; i < this.winEvents.length; i++)
+      this.win.removeEventListener(this.winEvents[i], this, false);
+
+    for (let i = 0; i < this.tabEvents.length; i++)
+      this.tabbrowser.tabContainer.removeEventListener(this.tabEvents[i], this, false);
+
+    for (let i = 0; i < tabs.length; i++)
+      this.removeTab(tabs[i]);
+
+    let idx = AeroPeek.windows.indexOf(this.win.gTaskbarTabGroup);
+    AeroPeek.windows.splice(idx, 1);
+    AeroPeek.checkPreviewCount();
+  },
+
+  get width () {
+    return this.win.innerWidth;
+  },
+  get height () {
+    return this.win.innerHeight;
+  },
+
+  cacheDims: function () {
+    this._cachedWidth = this.width;
+    this._cachedHeight = this.height;
+  },
+
+  testCacheDims: function () {
+    return this._cachedWidth == this.width && this._cachedHeight == this.height;
+  },
+
+  // Invoked when the given tab is added to this window
+  newTab: function (tab) {
+    let controller = new PreviewController(this, tab);
+    // It's OK to add the preview now while the favicon still loads.
+    this.previews.set(tab, controller.preview);
+    AeroPeek.addPreview(controller.preview);
+    // updateTitleAndTooltip relies on having controller.preview which is lazily resolved.
+    // Now that we've updated this.previews, it will resolve successfully.
+    controller.updateTitleAndTooltip();
+  },
+
+  createTabPreview: function (controller) {
+    let docShell = this.win
+                  .QueryInterface(Ci.nsIInterfaceRequestor)
+                  .getInterface(Ci.nsIWebNavigation)
+                  .QueryInterface(Ci.nsIDocShell);
+    let preview = AeroPeek.taskbar.createTaskbarTabPreview(docShell, controller);
+    preview.visible = AeroPeek.enabled;
+    preview.active = this.tabbrowser.selectedTab == controller.tab;
+    this.onLinkIconAvailable(controller.tab.linkedBrowser,
+                             controller.tab.getAttribute("image"));
+    return preview;
+  },
+
+  // Invoked when the given tab is closed
+  removeTab: function (tab) {
+    let preview = this.previewFromTab(tab);
+    preview.active = false;
+    preview.visible = false;
+    preview.move(null);
+    preview.controller.wrappedJSObject.destroy();
+
+    this.previews.delete(tab);
+    AeroPeek.removePreview(preview);
+  },
+
+  get enabled () {
+    return this._enabled;
+  },
+
+  set enabled (enable) {
+    this._enabled = enable;
+    // Because making a tab visible requires that the tab it is next to be
+    // visible, it is far simpler to unset the 'next' tab and recreate them all
+    // at once.
+    for (let [, preview] of this.previews) {
+      preview.move(null);
+      preview.visible = enable;
+    }
+    this.updateTabOrdering();
+  },
+
+  previewFromTab: function (tab) {
+    return this.previews.get(tab);
+  },
+
+  updateTabOrdering: function () {
+    let previews = this.previews;
+    let tabs = this.tabbrowser.tabs;
+
+    // Previews are internally stored using a map, so we need to iterate the
+    // tabbrowser's array of tabs to retrieve previews in the same order.
+    let inorder = [];
+    for (let t of tabs) {
+      if (previews.has(t)) {
+        inorder.push(previews.get(t));
+      }
+    }
+
+    // Since the internal taskbar array has not yet been updated we must force
+    // on it the sorting order of our local array.  To do so we must walk
+    // the local array backwards, otherwise we would send move requests in the
+    // wrong order.  See bug 522610 for details.
+    for (let i = inorder.length - 1; i >= 0; i--) {
+      inorder[i].move(inorder[i + 1] || null);
+    }
+  },
+
+  // nsIDOMEventListener
+  handleEvent: function (evt) {
+    let tab = evt.originalTarget;
+    switch (evt.type) {
+      case "TabOpen":
+        this.newTab(tab);
+        this.updateTabOrdering();
+        break;
+      case "TabClose":
+        this.removeTab(tab);
+        this.updateTabOrdering();
+        break;
+      case "TabSelect":
+        this.previewFromTab(tab).active = true;
+        break;
+      case "TabMove":
+        this.updateTabOrdering();
+        break;
+      case "resize":
+        if (!AeroPeek._prefenabled)
+          return;
+        this.onResize();
+        break;
+    }
+  },
+
+  // Set or reset a timer that will invalidate visible thumbnails soon.
+  setInvalidationTimer: function () {
+    if (!this.invalidateTimer) {
+      this.invalidateTimer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+    }
+    this.invalidateTimer.cancel();
+
+    // delay 1 second before invalidating
+    this.invalidateTimer.initWithCallback(() => {
+      // invalidate every preview. note the internal implementation of
+      // invalidate ignores thumbnails that aren't visible.
+      this.previews.forEach(function (aPreview) {
+        let controller = aPreview.controller.wrappedJSObject;
+        if (!controller.testCacheBrowserDims()) {
+          controller.cacheBrowserDims();
+          aPreview.invalidate();
+        }
+      });
+    }, 1000, Ci.nsITimer.TYPE_ONE_SHOT);
+  },
+
+  onResize: function () {
+    // Specific to a window.
+
+    // Call invalidate on each tab thumbnail so that Windows will request an
+    // updated image. However don't do this repeatedly across multiple resize
+    // events triggered during window border drags.
+
+    if (this.testCacheDims()) {
+      return;
+    }
+
+    // update the window dims on our TabWindow object.
+    this.cacheDims();
+
+    // invalidate soon
+    this.setInvalidationTimer();
+  },
+
+  invalidateTabPreview: function(aBrowser) {
+    for (let [tab, preview] of this.previews) {
+      if (aBrowser == tab.linkedBrowser) {
+        preview.invalidate();
+        break;
+      }
+    }
+  },
+
+  // Browser progress listener
+
+  onLocationChange: function (aBrowser) {
+    // I'm not sure we need this, onStateChange does a really good job
+    // of picking up page changes.
+    // this.invalidateTabPreview(aBrowser);
+  },
+
+  onStateChange: function (aBrowser, aWebProgress, aRequest, aStateFlags, aStatus) {
+    if (aStateFlags & Ci.nsIWebProgressListener.STATE_STOP &&
+        aStateFlags & Ci.nsIWebProgressListener.STATE_IS_NETWORK) {
+      this.invalidateTabPreview(aBrowser);
+    }
+  },
+
+  directRequestProtocols: new Set([
+    "file", "chrome", "resource", "about"
+  ]),
+  onLinkIconAvailable: function (aBrowser, aIconURL) {
+    let self = this;
+    let requestURL = null;
+    if (aIconURL) {
+      let shouldRequestFaviconURL = true;
+      try {
+        let urlObject = NetUtil.newURI(aIconURL);
+        shouldRequestFaviconURL =
+          !this.directRequestProtocols.has(urlObject.scheme);
+      } catch (ex) {}
+
+      requestURL = shouldRequestFaviconURL ?
+        "moz-anno:favicon:" + aIconURL :
+        aIconURL;
+    }
+    let isDefaultFavicon = !requestURL;
+    getFaviconAsImage(
+      requestURL,
+      PrivateBrowsingUtils.isWindowPrivate(self.win),
+      img => {
+        let index = self.tabbrowser.browsers.indexOf(aBrowser);
+        // Only add it if we've found the index and the URI is still the same.
+        // The tab could have closed, and there's no guarantee the icons
+        // will have finished fetching 'in order'.
+        if (index != -1) {
+          let tab = self.tabbrowser.tabs[index];
+          let preview = self.previews.get(tab);
+          if (tab.getAttribute("image") == aIconURL ||
+              (!preview.icon && isDefaultFavicon)) {
+            preview.icon = img;
+          }
+        }
+      }
+    );
+  }
+}
+
+// AeroPeek
+
+/*
+ * This object acts as global storage and external interface for this feature.
+ * It maintains the values of the prefs.
+ */
+this.AeroPeek = {
+  available: false,
+  // Does the pref say we're enabled?
+  __prefenabled: false,
+
+  _enabled: true,
+
+  initialized: false,
+
+  // nsITaskbarTabPreview array
+  previews: [],
+
+  // TabWindow array
+  windows: [],
+
+  // nsIWinTaskbar service
+  taskbar: null,
+
+  // Maximum number of previews
+  maxpreviews: 20,
+
+  // Length of time in seconds that previews are cached
+  cacheLifespan: 20,
+
+  initialize: function () {
+    if (!(WINTASKBAR_CONTRACTID in Cc))
+      return;
+    this.taskbar = Cc[WINTASKBAR_CONTRACTID].getService(Ci.nsIWinTaskbar);
+    this.available = this.taskbar.available;
+    if (!this.available)
+      return;
+
+    this.prefs.addObserver(TOGGLE_PREF_NAME, this, true);
+    this.enabled = this._prefenabled = this.prefs.getBoolPref(TOGGLE_PREF_NAME);
+    this.initialized = true;
+  },
+
+  destroy: function destroy() {
+    this._enabled = false;
+
+    if (this.cacheTimer)
+      this.cacheTimer.cancel();
+  },
+
+  get enabled() {
+    return this._enabled;
+  },
+
+  set enabled(enable) {
+    if (this._enabled == enable)
+      return;
+
+    this._enabled = enable;
+
+    this.windows.forEach(function (win) {
+      win.enabled = enable;
+    });
+  },
+
+  get _prefenabled() {
+    return this.__prefenabled;
+  },
+
+  set _prefenabled(enable) {
+    if (enable == this.__prefenabled) {
+      return;
+    }
+    this.__prefenabled = enable;
+
+    if (enable) {
+      this.enable();
+    } else {
+      this.disable();
+    }
+  },
+
+  _observersAdded: false,
+
+  enable() {
+    if (!this._observersAdded) {
+      this.prefs.addObserver(DISABLE_THRESHOLD_PREF_NAME, this, true);
+      this.prefs.addObserver(CACHE_EXPIRATION_TIME_PREF_NAME, this, true);
+      PlacesUtils.history.addObserver(this, true);
+      this._observersAdded = true;
+    }
+
+    this.cacheLifespan = this.prefs.getIntPref(CACHE_EXPIRATION_TIME_PREF_NAME);
+
+    this.maxpreviews = this.prefs.getIntPref(DISABLE_THRESHOLD_PREF_NAME);
+
+    // If the user toggled us on/off while the browser was already up
+    // (rather than this code running on startup because the pref was
+    // already set to true), we must initialize previews for open windows:
+    if (this.initialized) {
+      let browserWindows = Services.wm.getEnumerator("navigator:browser");
+      while (browserWindows.hasMoreElements()) {
+        let win = browserWindows.getNext();
+        if (!win.closed) {
+          this.onOpenWindow(win);
+        }
+      }
+    }
+  },
+
+  disable() {
+    while (this.windows.length) {
+      // We can't call onCloseWindow here because it'll bail if we're not
+      // enabled.
+      let tabWinObject = this.windows[0];
+      tabWinObject.destroy(); // This will remove us from the array.
+      delete tabWinObject.win.gTaskbarTabGroup; // Tidy up the window.
+    }
+  },
+
+  addPreview: function (preview) {
+    this.previews.push(preview);
+    this.checkPreviewCount();
+  },
+
+  removePreview: function (preview) {
+    let idx = this.previews.indexOf(preview);
+    this.previews.splice(idx, 1);
+    this.checkPreviewCount();
+  },
+
+  checkPreviewCount: function () {
+    if (!this._prefenabled) {
+      return;
+    }
+    this.enabled = this.previews.length <= this.maxpreviews;
+  },
+
+  onOpenWindow: function (win) {
+    // This occurs when the taskbar service is not available (xp, vista)
+    if (!this.available || !this._prefenabled)
+      return;
+
+    win.gTaskbarTabGroup = new TabWindow(win);
+  },
+
+  onCloseWindow: function (win) {
+    // This occurs when the taskbar service is not available (xp, vista)
+    if (!this.available || !this._prefenabled)
+      return;
+
+    win.gTaskbarTabGroup.destroy();
+    delete win.gTaskbarTabGroup;
+
+    if (this.windows.length == 0)
+      this.destroy();
+  },
+
+  resetCacheTimer: function () {
+    this.cacheTimer.cancel();
+    this.cacheTimer.init(this, 1000*this.cacheLifespan, Ci.nsITimer.TYPE_ONE_SHOT);
+  },
+
+  // nsIObserver
+  observe: function (aSubject, aTopic, aData) {
+    if (aTopic == "nsPref:changed" && aData == TOGGLE_PREF_NAME) {
+      this._prefenabled = this.prefs.getBoolPref(TOGGLE_PREF_NAME);
+    }
+    if (!this._prefenabled) {
+      return;
+    }
+    switch (aTopic) {
+      case "nsPref:changed":
+        if (aData == CACHE_EXPIRATION_TIME_PREF_NAME)
+          break;
+
+        if (aData == DISABLE_THRESHOLD_PREF_NAME)
+          this.maxpreviews = this.prefs.getIntPref(DISABLE_THRESHOLD_PREF_NAME);
+        // Might need to enable/disable ourselves
+        this.checkPreviewCount();
+        break;
+      case "timer-callback":
+        this.previews.forEach(function (preview) {
+          let controller = preview.controller.wrappedJSObject;
+          controller.resetCanvasPreview();
+        });
+        break;
+    }
+  },
+
+  /* nsINavHistoryObserver implementation */
+  onBeginUpdateBatch() {},
+  onEndUpdateBatch() {},
+  onVisit() {},
+  onTitleChanged() {},
+  onFrecencyChanged() {},
+  onManyFrecenciesChanged() {},
+  onDeleteURI() {},
+  onClearHistory() {},
+  onDeleteVisits() {},
+  onPageChanged(uri, changedConst, newValue) {
+    if (this.enabled && changedConst == Ci.nsINavHistoryObserver.ATTRIBUTE_FAVICON) {
+      for (let win of this.windows) {
+        for (let [tab, ] of win.previews) {
+          if (tab.getAttribute("image") == newValue) {
+            win.onLinkIconAvailable(tab.linkedBrowser, newValue);
+          }
+        }
+      }
+    }
+  },
+
+  QueryInterface: XPCOMUtils.generateQI([
+    Ci.nsISupportsWeakReference,
+    Ci.nsINavHistoryObserver,
+    Ci.nsIObserver
+  ]),
+};
+
+XPCOMUtils.defineLazyGetter(AeroPeek, "cacheTimer", () =>
+  Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer)
+);
+
+XPCOMUtils.defineLazyServiceGetter(AeroPeek, "prefs",
+                                   "@mozilla.org/preferences-service;1",
+                                   "nsIPrefBranch");
+
+AeroPeek.initialize();