diff -r 000000000000 -r 6474c204b198 browser/components/tabview/test/browser_tabview_pending_tabs.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/components/tabview/test/browser_tabview_pending_tabs.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,119 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const STATE = {
+  windows: [{
+    tabs: [{
+      entries: [{ url: "about:mozilla" }],
+      hidden: true,
+      extData: {"tabview-tab": '{"url":"about:mozilla","groupID":1}'}
+    },{
+      entries: [{ url: "about:robots" }],
+      hidden: false,
+      extData: {"tabview-tab": '{"url":"about:robots","groupID":1}'},
+    }],
+    selected: 1,
+    extData: {
+      "tabview-groups": '{"nextID":2,"activeGroupId":1, "totalNumber":1}',
+      "tabview-group":
+        '{"1":{"bounds":{"left":15,"top":5,"width":280,"height":232},"id":1}}'
+    }
+  }]
+};
+
+/**
+ * Make sure that tabs are restored on demand as otherwise the tab will start
+ * loading immediately and we can't check whether it shows cached data.
+ */
+add_task(function setup() {
+  Services.prefs.setBoolPref("browser.sessionstore.restore_on_demand", true);
+
+  registerCleanupFunction(() => {
+    Services.prefs.clearUserPref("browser.sessionstore.restore_on_demand");
+  });
+});
+
+/**
+ * Ensure that a pending tab shows cached data.
+ */
+add_task(function () {
+  // Open a new window.
+  let win = OpenBrowserWindow();
+  yield promiseDelayedStartupFinished(win);
+
+  // Set the window to a specific state.
+  let ss = Cc["@mozilla.org/browser/sessionstore;1"]
+             .getService(Ci.nsISessionStore)
+             .setWindowState(win, JSON.stringify(STATE), true);
+
+  // Open Panorama.
+  yield promiseTabViewShown(win);
+
+  let [tab1, tab2] = win.gBrowser.tabs;
+  let cw = win.TabView.getContentWindow();
+
+  // Update the two tabs in reverse order. Panorama will first try to update
+  // the second tab but will put it back onto the queue once it detects that
+  // it hasn't loaded yet. It will then try to update the first tab.
+  cw.TabItems.update(tab2);
+  cw.TabItems.update(tab1);
+
+  let tabItem1 = tab1._tabViewTabItem;
+  let tabItem2 = tab2._tabViewTabItem;
+
+  // Wait for the first tabItem to be updated. Calling update() on the second
+  // tabItem won't send a notification as that is pushed back onto the queue.
+  yield promiseTabItemUpdated(tabItem1);
+
+  // Check that the first tab doesn't show cached data, the second one does.
+  ok(!tabItem1.isShowingCachedData(), "doesn't show cached data");
+  ok(tabItem2.isShowingCachedData(), "shows cached data");
+
+  // Cleanup.
+  yield promiseWindowClosed(win);
+});
+
+function promiseTabItemUpdated(tabItem) {
+  let deferred = Promise.defer();
+
+  tabItem.addSubscriber("updated", function onUpdated() {
+    tabItem.removeSubscriber("updated", onUpdated);
+    deferred.resolve();
+  });
+
+  return deferred.promise;
+}
+
+function promiseAllTabItemsUpdated(win) {
+  let deferred = Promise.defer();
+  afterAllTabItemsUpdated(deferred.resolve, win);
+  return deferred.promise;
+}
+
+function promiseDelayedStartupFinished(win) {
+  let deferred = Promise.defer();
+  whenDelayedStartupFinished(win, deferred.resolve);
+  return deferred.promise;
+}
+
+function promiseTabViewShown(win) {
+  let deferred = Promise.defer();
+  showTabView(deferred.resolve, win);
+  return deferred.promise;
+}
+
+function promiseWindowClosed(win) {
+  let deferred = Promise.defer();
+
+  Services.obs.addObserver(function obs(subject, topic) {
+    if (subject == win) {
+      Services.obs.removeObserver(obs, topic);
+      deferred.resolve();
+    }
+  }, "domwindowclosed", false);
+
+  win.close();
+  return deferred.promise;
+}