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diff --git a/webbrowser/modules/SharedFrame.jsm b/webbrowser/modules/SharedFrame.jsm
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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/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = [ "SharedFrame" ];
+
+const Ci = Components.interfaces;
+const Cu = Components.utils;
+
+/**
+ * The purpose of this module is to create and group various iframe
+ * elements that are meant to all display the same content and only
+ * one at a time. This makes it possible to have the content loaded
+ * only once, while the other iframes can be kept as placeholders to
+ * quickly move the content to them through the swapFrameLoaders function
+ * when another one of the placeholder is meant to be displayed.
+ * */
+
+var Frames = new Map();
+
+/**
+ * The Frames map is the main data structure that holds information
+ * about the groups being tracked. Each entry's key is the group name,
+ * and the object holds information about what is the URL being displayed
+ * on that group, and what is the active element on the group (the frame that
+ * holds the loaded content).
+ * The reference to the activeFrame is a weak reference, which allows the
+ * frame to go away at any time, and when that happens the module considers that
+ * there are no active elements in that group. The group can be reactivated
+ * by changing the URL, calling preload again or adding a new element.
+ *
+ *
+ *  Frames = {
+ *    "messages-panel": {
+ *      url: string,
+ *      activeFrame: weakref
+ *    }
+ *  }
+ *
+ * Each object on the map is called a _SharedFrameGroup, which is an internal
+ * class of this module which does not automatically keep track of its state. This
+ * object should not be used externally, and all control should be handled by the
+ * module's functions.
+ */
+
+function UNLOADED_URL(aStr) "data:text/html;charset=utf-8,<!-- Unloaded frame " + aStr + " -->";
+
+
+this.SharedFrame = {
+  /**
+   * Creates an iframe element and track it as part of the specified group
+   * The module must create the iframe itself because it needs to do some special
+   * handling for the element's src attribute.
+   *
+   * @param aGroupName        the name of the group to which this frame belongs
+   * @param aParent           the parent element to which the frame will be appended to
+   * @param aFrameAttributes  an object with a list of attributes to set in the iframe
+   *                          before appending it to the DOM. The "src" attribute has
+   *                          special meaning here and if it's not blank it specifies
+   *                          the URL that will be initially assigned to this group
+   * @param aPreload          optional, tells if the URL specified in the src attribute
+   *                          should be preloaded in the frame being created, in case
+   *                          it's not yet preloaded in any other frame of the group.
+   *                          This parameter has no meaning if src is blank.
+   */
+  createFrame: function (aGroupName, aParent, aFrameAttributes, aPreload = true) {
+    let frame = aParent.ownerDocument.createElement("iframe");
+
+    for (let [key, val] of Iterator(aFrameAttributes)) {
+      frame.setAttribute(key, val);
+    }
+
+    let src = aFrameAttributes.src;
+    if (!src) {
+      aPreload = false;
+    }
+
+    let group = Frames.get(aGroupName);
+
+    if (group) {
+      // If this group has already been created
+
+      if (aPreload && !group.isAlive) {
+        // If aPreload is set and the group is not already loaded, load it.
+        // This can happen if:
+        // - aPreload was not used while creating the previous frames of this group, or
+        // - the previously active frame went dead in the meantime
+        group.url = src;
+        this.preload(aGroupName, frame);
+      } else {
+        // If aPreload is not set, or the group is already loaded in a different frame,
+        // there's not much that we need to do here: just create this frame as an
+        // inactivate placeholder
+        frame.setAttribute("src", UNLOADED_URL(aGroupName));
+      }
+
+    } else {
+      // This is the first time we hear about this group, so let's start tracking it,
+      // and also preload it if the src attribute was set and aPreload = true
+      group = new _SharedFrameGroup(src);
+      Frames.set(aGroupName, group);
+
+      if (aPreload) {
+        this.preload(aGroupName, frame);
+      } else {
+        frame.setAttribute("src", UNLOADED_URL(aGroupName));
+      }
+    }
+
+    aParent.appendChild(frame);
+    return frame;
+
+  },
+
+  /**
+   * Function that moves the loaded content from one active frame to
+   * another one that is currently a placeholder. If there's no active
+   * frame in the group, the content is loaded/reloaded.
+   *
+   * @param aGroupName   the name of the group
+   * @param aTargetFrame the frame element to which the content should
+   *                     be moved to.
+   */
+  setOwner: function (aGroupName, aTargetFrame) {
+    let group = Frames.get(aGroupName);
+    let frame = group.activeFrame;
+
+    if (frame == aTargetFrame) {
+      // nothing to do here
+      return;
+    }
+
+    if (group.isAlive) {
+      // Move document ownership to the desired frame, and make it the active one
+      frame.QueryInterface(Ci.nsIFrameLoaderOwner).swapFrameLoaders(aTargetFrame);
+      group.activeFrame = aTargetFrame;
+    } else {
+      // Previous owner was dead, reload the document at the new owner and make it the active one
+      aTargetFrame.setAttribute("src", group.url);
+      group.activeFrame = aTargetFrame;
+    }
+  },
+
+  /**
+   * Updates the current URL in use by this group, and loads it into the active frame.
+   *
+   * @param aGroupName  the name of the group
+   * @param aURL        the new url
+   */
+  updateURL: function (aGroupName, aURL) {
+    let group = Frames.get(aGroupName);
+    group.url = aURL;
+
+    if (group.isAlive) {
+      group.activeFrame.setAttribute("src", aURL);
+    }
+  },
+
+  /**
+   * Loads the group's url into a target frame, if the group doesn't have a currently
+   * active frame.
+   *
+   * @param aGroupName    the name of the group
+   * @param aTargetFrame  the frame element which should be made active and
+   *                      have the group's content loaded to
+   */
+  preload: function (aGroupName, aTargetFrame) {
+    let group = Frames.get(aGroupName);
+    if (!group.isAlive) {
+      aTargetFrame.setAttribute("src", group.url);
+      group.activeFrame = aTargetFrame;
+    }
+  },
+
+  /**
+   * Tells if a group currently have an active element.
+   *
+   * @param aGroupName  the name of the group
+   */
+  isGroupAlive: function (aGroupName) {
+    return Frames.get(aGroupName).isAlive;
+  },
+
+  /**
+   * Forgets about this group. This function doesn't need to be used
+   * unless the group's name needs to be reused.
+   *
+   * @param aGroupName  the name of the group
+   */
+  forgetGroup: function (aGroupName) {
+    Frames.delete(aGroupName);
+  }
+}
+
+
+function _SharedFrameGroup(aURL) {
+  this.url = aURL;
+  this._activeFrame = null;
+}
+
+_SharedFrameGroup.prototype = {
+  get isAlive() {
+    let frame = this.activeFrame;
+    return !!(frame &&
+              frame.contentDocument &&
+              frame.contentDocument.location);
+  },
+
+  get activeFrame() {
+    return this._activeFrame &&
+           this._activeFrame.get();
+  },
+
+  set activeFrame(aActiveFrame) {
+    this._activeFrame = aActiveFrame
+                        ? Cu.getWeakReference(aActiveFrame)
+                        : null;
+  }
+}