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diff --git a/components/downloads/DownloadsTaskbar.jsm b/components/downloads/DownloadsTaskbar.jsm
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+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80 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/. */
+
+/**
+ * Handles the download progress indicator in the taskbar.
+ */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = [
+  "DownloadsTaskbar",
+];
+
+////////////////////////////////////////////////////////////////////////////////
+//// Globals
+
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+const Cu = Components.utils;
+const Cr = Components.results;
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+
+XPCOMUtils.defineLazyModuleGetter(this, "Downloads",
+                                  "resource://gre/modules/Downloads.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "RecentWindow",
+                                  "resource:///modules/RecentWindow.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "Services",
+                                  "resource://gre/modules/Services.jsm");
+
+XPCOMUtils.defineLazyGetter(this, "gWinTaskbar", function () {
+  if (!("@mozilla.org/windows-taskbar;1" in Cc)) {
+    return null;
+  }
+  let winTaskbar = Cc["@mozilla.org/windows-taskbar;1"]
+                     .getService(Ci.nsIWinTaskbar);
+  return winTaskbar.available && winTaskbar;
+});
+
+XPCOMUtils.defineLazyGetter(this, "gMacTaskbarProgress", function () {
+  return ("@mozilla.org/widget/macdocksupport;1" in Cc) &&
+         Cc["@mozilla.org/widget/macdocksupport;1"]
+           .getService(Ci.nsITaskbarProgress);
+});
+
+////////////////////////////////////////////////////////////////////////////////
+//// DownloadsTaskbar
+
+/**
+ * Handles the download progress indicator in the taskbar.
+ */
+this.DownloadsTaskbar = {
+  /**
+   * Underlying DownloadSummary providing the aggregate download information, or
+   * null if the indicator has never been initialized.
+   */
+  _summary: null,
+
+  /**
+   * nsITaskbarProgress object to which download information is dispatched.
+   * This can be null if the indicator has never been initialized or if the
+   * indicator is currently hidden on Windows.
+   */
+  _taskbarProgress: null,
+
+  /**
+   * This method is called after a new browser window is opened, and ensures
+   * that the download progress indicator is displayed in the taskbar.
+   *
+   * On Windows, the indicator is attached to the first browser window that
+   * calls this method.  When the window is closed, the indicator is moved to
+   * another browser window, if available, in no particular order.  When there
+   * are no browser windows visible, the indicator is hidden.
+   *
+   * On Mac OS X, the indicator is initialized globally when this method is
+   * called for the first time.  Subsequent calls have no effect.
+   *
+   * @param aBrowserWindow
+   *        nsIDOMWindow object of the newly opened browser window to which the
+   *        indicator may be attached.
+   */
+  registerIndicator(aBrowserWindow) {
+    if (!this._taskbarProgress) {
+      if (gMacTaskbarProgress) {
+        // On Mac OS X, we have to register the global indicator only once.
+        this._taskbarProgress = gMacTaskbarProgress;
+        // Free the XPCOM reference on shutdown, to prevent detecting a leak.
+        Services.obs.addObserver(() => {
+          this._taskbarProgress = null;
+          gMacTaskbarProgress = null;
+        }, "quit-application-granted", false);
+      } else if (gWinTaskbar) {
+        // On Windows, the indicator is currently hidden because we have no
+        // previous browser window, thus we should attach the indicator now.
+        this._attachIndicator(aBrowserWindow);
+      } else {
+        // The taskbar indicator is not available on this platform.
+        return;
+      }
+    }
+
+    // Ensure that the DownloadSummary object will be created asynchronously.
+    if (!this._summary) {
+      Downloads.getSummary(Downloads.ALL).then(summary => {
+        // In case the method is re-entered, we simply ignore redundant
+        // invocations of the callback, instead of keeping separate state.
+        if (this._summary) {
+          return;
+        }
+        this._summary = summary;
+        return this._summary.addView(this);
+      }).then(null, Cu.reportError);
+    }
+  },
+
+  /**
+   * On Windows, attaches the taskbar indicator to the specified browser window.
+   */
+  _attachIndicator(aWindow) {
+    // Activate the indicator on the specified window.
+    let docShell = aWindow.QueryInterface(Ci.nsIInterfaceRequestor)
+                          .getInterface(Ci.nsIWebNavigation)
+                          .QueryInterface(Ci.nsIDocShellTreeItem).treeOwner
+                          .QueryInterface(Ci.nsIInterfaceRequestor)
+                          .getInterface(Ci.nsIXULWindow).docShell;
+    this._taskbarProgress = gWinTaskbar.getTaskbarProgress(docShell);
+
+    // If the DownloadSummary object has already been created, we should update
+    // the state of the new indicator, otherwise it will be updated as soon as
+    // the DownloadSummary view is registered.
+    if (this._summary) {
+      this.onSummaryChanged();
+    }
+
+    aWindow.addEventListener("unload", () => {
+      // Locate another browser window, excluding the one being closed.
+      let browserWindow = RecentWindow.getMostRecentBrowserWindow();
+      if (browserWindow) {
+        // Move the progress indicator to the other browser window.
+        this._attachIndicator(browserWindow);
+      } else {
+        // The last browser window has been closed.  We remove the reference to
+        // the taskbar progress object so that the indicator will be registered
+        // again on the next browser window that is opened.
+        this._taskbarProgress = null;
+      }
+    }, false);
+  },
+
+  //////////////////////////////////////////////////////////////////////////////
+  //// DownloadSummary view
+
+  onSummaryChanged() {
+    // If the last browser window has been closed, we have no indicator any more.
+    if (!this._taskbarProgress) {
+      return;
+    }
+
+    if (this._summary.allHaveStopped || this._summary.progressTotalBytes == 0) {
+      this._taskbarProgress.setProgressState(
+                               Ci.nsITaskbarProgress.STATE_NO_PROGRESS, 0, 0);
+    } else {
+      // For a brief moment before completion, some download components may
+      // report more transferred bytes than the total number of bytes.  Thus,
+      // ensure that we never break the expectations of the progress indicator.
+      let progressCurrentBytes = Math.min(this._summary.progressTotalBytes,
+                                          this._summary.progressCurrentBytes);
+      this._taskbarProgress.setProgressState(
+                               Ci.nsITaskbarProgress.STATE_NORMAL,
+                               progressCurrentBytes,
+                               this._summary.progressTotalBytes);
+    }
+  },
+};