The Tor Browser: browser/modules/WindowsPreviewPerTab.jsm@b8a032363ba2 (annotated)

browser/modules/WindowsPreviewPerTab.jsm@b8a032363ba2 (annotated)

browser/modules/WindowsPreviewPerTab.jsm

Thu, 22 Jan 2015 13:21:57 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Thu, 22 Jan 2015 13:21:57 +0100
branch: TOR_BUG_9701
changeset 15: b8a032363ba2
permissions: -rw-r--r--

Incorporate requested changes from Mozilla in review:
https://bugzilla.mozilla.org/show_bug.cgi?id=1123480#c6

 /* 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. Starting in
  * Windows Vista, the taskbar began showing live thumbnail previews of windows
  * when the user hovered over the window icon in the taskbar. Starting with
  * Windows 7, the taskbar allows an application to expose its tabbed interface
  * in the taskbar 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/XPCOMUtils.jsm");
 Cu.import("resource://gre/modules/NetUtil.jsm");
 Cu.import("resource://gre/modules/PrivateBrowsingUtils.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, "ioSvc",
                                    "@mozilla.org/network/io-service;1",
                                    "nsIIOService");
 XPCOMUtils.defineLazyServiceGetter(this, "imgTools",
                                    "@mozilla.org/image/tools;1",
                                    "imgITools");
 XPCOMUtils.defineLazyServiceGetter(this, "faviconSvc",
                                    "@mozilla.org/browser/favicon-service;1",
                                    "nsIFaviconService");
 // nsIURI -> imgIContainer
 function _imageFromURI(uri, privateMode, callback) {
   let channel = ioSvc.newChannelFromURI(uri);
   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 = faviconSvc.defaultFavicon;
       if (!defaultURI.equals(uri))
         _imageFromURI(defaultURI, callback);
     }
   });
 }
 // string? -> imgIContainer
 function getFaviconAsImage(iconurl, privateMode, callback) {
   if (iconurl)
     _imageFromURI(NetUtil.newURI(iconurl), privateMode, callback);
   else
     _imageFromURI(faviconSvc.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 the preview.
  *
  * To give greater performance when drawing, the dirty areas of the content
  * window are tracked and drawn on demand into a canvas of the same size.
  * This provides a great increase in responsiveness when drawing a preview
  * for unchanged (or even only slightly changed) tabs.
  *
  * @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.linkedBrowser.addEventListener("MozAfterPaint", this, false);
   this.tab.addEventListener("TabAttrModified", this, false);
   XPCOMUtils.defineLazyGetter(this, "canvasPreview", function () {
     let canvas = this.win.win.document.createElementNS("http://www.w3.org/1999/xhtml", "canvas");
     canvas.mozOpaque = true;
     return canvas;
   });
   XPCOMUtils.defineLazyGetter(this, "dirtyRegion",
     function () {
       let dirtyRegion = Cc["@mozilla.org/gfx/region;1"]
                        .createInstance(Ci.nsIScriptableRegion);
       dirtyRegion.init();
       return dirtyRegion;
     });
   XPCOMUtils.defineLazyGetter(this, "winutils",
     function () {
       let win = tab.linkedBrowser.contentWindow;
       return win.QueryInterface(Ci.nsIInterfaceRequestor)
                 .getInterface(Ci.nsIDOMWindowUtils);
   });
 }
 PreviewController.prototype = {
   QueryInterface: XPCOMUtils.generateQI([Ci.nsITaskbarPreviewController,
                                          Ci.nsIDOMEventListener]),
   destroy: function () {
     this.tab.removeEventListener("TabAttrModified", this, false);
     this.linkedBrowser.removeEventListener("MozAfterPaint", this, false);
     // Break cycles, otherwise we end up leaking the window with everything
     // attached to it.
     delete this.win;
     delete this.preview;
     delete this.dirtyRegion;
   },
   get wrappedJSObject() {
     return this;
   },
   get dirtyRects() {
     let rectstream = this.dirtyRegion.getRects();
     if (!rectstream)
       return [];
     let rects = [];
     for (let i = 0; i < rectstream.length; i+= 4) {
       let r = {x:      rectstream[i],
                y:      rectstream[i+1],
                width:  rectstream[i+2],
                height: rectstream[i+3]};
       rects.push(r);
     }
     return rects;
   },
   // Resizes the canvasPreview to 0x0, essentially freeing its memory.
   // updateCanvasPreview() will detect the size mismatch as a resize event
   // the next time it is called.
   resetCanvasPreview: function () {
     this.canvasPreview.width = 0;
     this.canvasPreview.height = 0;
   },
   get zoom() {
     // Note that winutils.fullZoom accounts for "quantization" of the zoom factor
     // from nsIMarkupDocumentViewer 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.winutils.fullZoom;
   },
   // Updates the controller's canvas with the parts of the <browser> that need
   // to be redrawn.
   updateCanvasPreview: function () {
     let win = this.linkedBrowser.contentWindow;
     let bx = this.linkedBrowser.boxObject;
     // Check for resize
     if (bx.width != this.canvasPreview.width ||
         bx.height != this.canvasPreview.height) {
       // Invalidate the entire area and repaint
       this.onTabPaint({left:0, top:0, right:win.innerWidth, bottom:win.innerHeight});
       this.canvasPreview.width = bx.width;
       this.canvasPreview.height = bx.height;
     }
     // Draw dirty regions
     let ctx = this.canvasPreview.getContext("2d");
     let scale = this.zoom;
     let flags = this.canvasPreviewFlags;
     // The dirty region may include parts that are offscreen so we clip to the
     // canvas area.
     this.dirtyRegion.intersectRect(0, 0, win.innerWidth, win.innerHeight);
     this.dirtyRects.forEach(function (r) {
       // We need to snap the rectangle to be pixel aligned in the destination
       // coordinate space. Otherwise natively themed widgets might not draw.
       snapRectAtScale(r, scale);
       let x = r.x;
       let y = r.y;
       let width = r.width;
       let height = r.height;
       ctx.save();
       ctx.scale(scale, scale);
       ctx.translate(x, y);
       ctx.drawWindow(win, x, y, width, height, "white", flags);
       ctx.restore();
     });
     this.dirtyRegion.setToRect(0,0,0,0);
     // 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();
   },
   onTabPaint: function (rect) {
     let x = Math.floor(rect.left),
         y = Math.floor(rect.top),
         width = Math.ceil(rect.right) - x,
         height = Math.ceil(rect.bottom) - y;
     this.dirtyRegion.unionRect(x, y, width, height);
   },
   updateTitleAndTooltip: function () {
     let title = this.win.tabbrowser.getWindowTitleForBrowser(this.linkedBrowser);
     this.preview.title = title;
     this.preview.tooltip = title;
   },
   //////////////////////////////////////////////////////////////////////////////
   //// nsITaskbarPreviewController
   get width() {
     return this.win.width;
   },
   get height() {
     return this.win.height;
   },
   get thumbnailAspectRatio() {
     let boxObject = this.tab.linkedBrowser.boxObject;
     // Avoid returning 0
     let tabWidth = boxObject.width || 1;
     // Avoid divide by 0
     let tabHeight = boxObject.height || 1;
     return tabWidth / tabHeight;
   },
   drawPreview: function (ctx) {
     let self = this;
     this.win.tabbrowser.previewTab(this.tab, function () self.previewTabCallback(ctx));
     // We must avoid having the frame drawn around the window. See bug 520807
     return false;
   },
   previewTabCallback: function (ctx) {
     // This will extract the resolution-scale component of the scaling we need,
     // which should be applied to both chrome and content;
     // the page zoom component is applied (to content only) within updateCanvasPreview.
     let scale = this.winutils.screenPixelsPerCSSPixel / this.winutils.fullZoom;
     ctx.save();
     ctx.scale(scale, scale);
     let width = this.win.width;
     let height = this.win.height;
     // Draw our toplevel window
     ctx.drawWindow(this.win.win, 0, 0, width, height, "transparent");
     // XXX (jfkthame): Pending tabs don't seem to draw with the proper scaling
     // unless we use this block of code; but doing this for "normal" (loaded) tabs
     // results in blurry rendering on hidpi systems, so we avoid it if possible.
     // I don't understand why pending and loaded tabs behave differently here...
     // (see bug 857061).
     if (this.tab.hasAttribute("pending")) {
       // Compositor, where art thou?
       // Draw the tab content on top of the toplevel window
       this.updateCanvasPreview();
       let boxObject = this.linkedBrowser.boxObject;
       ctx.translate(boxObject.x, boxObject.y);
       ctx.drawImage(this.canvasPreview, 0, 0);
     }
     ctx.restore();
   },
   drawThumbnail: function (ctx, width, height) {
     this.updateCanvasPreview();
     let scale = width/this.linkedBrowser.boxObject.width;
     ctx.scale(scale, scale);
     ctx.drawImage(this.canvasPreview, 0, 0);
     // Don't draw a frame around the thumbnail
     return false;
   },
   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 "MozAfterPaint":
         if (evt.originalTarget === this.linkedBrowser.contentWindow) {
           let clientRects = evt.clientRects;
           let length = clientRects.length;
           for (let i = 0; i < length; i++) {
             let r = clientRects.item(i);
             this.onTabPaint(r);
           }
         }
         let preview = this.preview;
         if (preview.visible)
           preview.invalidate();
         break;
       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);
   this.tabbrowser.addTabsProgressListener(this);
   for (let i = 0; i < this.winEvents.length; i++)
     this.win.addEventListener(this.winEvents[i], this, false);
   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,
   tabEvents: ["TabOpen", "TabClose", "TabSelect", "TabMove"],
   winEvents: ["tabviewshown", "tabviewhidden"],
   destroy: function () {
     this._destroying = true;
     let tabs = this.tabbrowser.tabs;
     this.tabbrowser.removeTabsProgressListener(this);
     for (let i = 0; i < this.tabEvents.length; i++)
       this.tabbrowser.tabContainer.removeEventListener(this.tabEvents[i], this, false);
     for (let i = 0; i < this.winEvents.length; i++)
       this.win.removeEventListener(this.winEvents[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;
   },
   // 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;
     // Grab the default favicon
     getFaviconAsImage(null, PrivateBrowsingUtils.isWindowPrivate(this.win), function (img) {
       // It is possible that we've already gotten the real favicon, so make sure
       // we have not set one before setting this default one.
       if (!preview.icon)
         preview.icon = img;
     });
     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 [tab, 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 = [previews.get(t) for (t of tabs) if (previews.has(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 "tabviewshown":
         this.enabled = false;
         break;
       case "tabviewhidden":
         if (!AeroPeek._prefenabled)
           return;
         this.enabled = true;
         break;
     }
   },
   //// Browser progress listener
   onLinkIconAvailable: function (aBrowser, aIconURL) {
     let self = this;
     getFaviconAsImage(aIconURL, PrivateBrowsingUtils.isWindowPrivate(this.win), function (img) {
       let index = self.tabbrowser.browsers.indexOf(aBrowser);
       // Only add it if we've found the index.  The tab could have closed!
       if (index != -1) {
         let tab = self.tabbrowser.tabs[index];
         self.previews.get(tab).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: true,
   _enabled: true,
   // 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, false);
     this.prefs.addObserver(DISABLE_THRESHOLD_PREF_NAME, this, false);
     this.prefs.addObserver(CACHE_EXPIRATION_TIME_PREF_NAME, this, false);
     this.cacheLifespan = this.prefs.getIntPref(CACHE_EXPIRATION_TIME_PREF_NAME);
     this.maxpreviews = this.prefs.getIntPref(DISABLE_THRESHOLD_PREF_NAME);
     this.enabled = this._prefenabled = this.prefs.getBoolPref(TOGGLE_PREF_NAME);
   },
   destroy: function destroy() {
     this._enabled = false;
     this.prefs.removeObserver(TOGGLE_PREF_NAME, this);
     this.prefs.removeObserver(DISABLE_THRESHOLD_PREF_NAME, this);
     this.prefs.removeObserver(CACHE_EXPIRATION_TIME_PREF_NAME, this);
     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;
     });
   },
   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.previews.length > this.maxpreviews)
       this.enabled = false;
     else
       this.enabled = this._prefenabled;
   },
   onOpenWindow: function (win) {
     // This occurs when the taskbar service is not available (xp, vista)
     if (!this.available)
       return;
     win.gTaskbarTabGroup = new TabWindow(win);
   },
   onCloseWindow: function (win) {
     // This occurs when the taskbar service is not available (xp, vista)
     if (!this.available)
       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) {
     switch (aTopic) {
       case "nsPref:changed":
         if (aData == CACHE_EXPIRATION_TIME_PREF_NAME)
           break;
         if (aData == TOGGLE_PREF_NAME)
           this._prefenabled = this.prefs.getBoolPref(TOGGLE_PREF_NAME);
         else 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;
     }
   }
 };
 XPCOMUtils.defineLazyGetter(AeroPeek, "cacheTimer", function ()
   Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer)
 );
 XPCOMUtils.defineLazyServiceGetter(AeroPeek, "prefs",
                                    "@mozilla.org/preferences-service;1",
                                    "nsIPrefBranch");
 AeroPeek.initialize();

The Tor Browser / annotate

browser/modules/WindowsPreviewPerTab.jsm@b8a032363ba2 (annotated)

browser/modules/WindowsPreviewPerTab.jsm