diff -r 000000000000 -r 6474c204b198 browser/components/customizableui/test/browser_909779_overflow_toolbars_new_window.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/components/customizableui/test/browser_909779_overflow_toolbars_new_window.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,31 @@
+/* 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";
+
+// Resize to a small window, open a new window, check that new window handles overflow properly
+add_task(function() {
+  let originalWindowWidth = window.outerWidth;
+  let navbar = document.getElementById(CustomizableUI.AREA_NAVBAR);
+  ok(!navbar.hasAttribute("overflowing"), "Should start with a non-overflowing toolbar.");
+  let oldChildCount = navbar.customizationTarget.childElementCount;
+  window.resizeTo(400, window.outerHeight);
+  yield waitForCondition(() => navbar.hasAttribute("overflowing"));
+  ok(navbar.hasAttribute("overflowing"), "Should have an overflowing toolbar.");
+
+  ok(navbar.customizationTarget.childElementCount < oldChildCount, "Should have fewer children.");
+  let newWindow = yield openAndLoadWindow();
+  let otherNavBar = newWindow.document.getElementById(CustomizableUI.AREA_NAVBAR);
+  yield waitForCondition(() => otherNavBar.hasAttribute("overflowing"));
+  ok(otherNavBar.hasAttribute("overflowing"), "Other window should have an overflowing toolbar.");
+  yield promiseWindowClosed(newWindow);
+
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+  yield waitForCondition(() => !navbar.hasAttribute("overflowing"));
+  ok(!navbar.hasAttribute("overflowing"), "Should no longer have an overflowing toolbar.");
+});
+
+add_task(function asyncCleanup() {
+  yield resetCustomization();
+});