diff -r 000000000000 -r 6474c204b198 browser/components/customizableui/test/browser_913972_currentset_overflow.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/components/customizableui/test/browser_913972_currentset_overflow.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,55 @@
+/* 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";
+
+let navbar = document.getElementById(CustomizableUI.AREA_NAVBAR);
+
+// Resize to a small window, resize back, shouldn't affect currentSet
+add_task(function() {
+  let originalWindowWidth = window.outerWidth;
+  let oldCurrentSet = navbar.currentSet;
+  ok(!navbar.hasAttribute("overflowing"), "Should start with a non-overflowing toolbar.");
+  ok(CustomizableUI.inDefaultState, "Should start in default state.");
+  let oldChildCount = navbar.customizationTarget.childElementCount;
+  window.resizeTo(400, window.outerHeight);
+  yield waitForCondition(() => navbar.hasAttribute("overflowing"));
+  ok(navbar.hasAttribute("overflowing"), "Should have an overflowing toolbar.");
+  is(navbar.currentSet, oldCurrentSet, "Currentset should be the same when overflowing.");
+  ok(CustomizableUI.inDefaultState, "Should still be in default state when overflowing.");
+  ok(navbar.customizationTarget.childElementCount < oldChildCount, "Should have fewer children.");
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+  yield waitForCondition(() => !navbar.hasAttribute("overflowing"));
+  ok(!navbar.hasAttribute("overflowing"), "Should no longer have an overflowing toolbar.");
+  is(navbar.currentSet, oldCurrentSet, "Currentset should still be the same now we're no longer overflowing.");
+  ok(CustomizableUI.inDefaultState, "Should still be in default state now we're no longer overflowing.");
+
+  // Verify actual physical placements match those of the placement array:
+  let placementCounter = 0;
+  let placements = CustomizableUI.getWidgetIdsInArea(CustomizableUI.AREA_NAVBAR);
+  for (let node of navbar.customizationTarget.childNodes) {
+    if (node.getAttribute("skipintoolbarset") == "true") {
+      continue;
+    }
+    is(placements[placementCounter++], node.id, "Nodes should match after overflow");
+  }
+  is(placements.length, placementCounter, "Should have as many nodes as expected");
+  is(navbar.customizationTarget.childElementCount, oldChildCount, "Number of nodes should match");
+});
+
+// Enter and exit customization mode, check that currentSet works
+add_task(function() {
+  let oldCurrentSet = navbar.currentSet;
+  ok(CustomizableUI.inDefaultState, "Should start in default state.");
+  yield startCustomizing();
+  ok(CustomizableUI.inDefaultState, "Should be in default state in customization mode.");
+  is(navbar.currentSet, oldCurrentSet, "Currentset should be the same in customization mode.");
+  yield endCustomizing();
+  ok(CustomizableUI.inDefaultState, "Should be in default state after customization mode.");
+  is(navbar.currentSet, oldCurrentSet, "Currentset should be the same after customization mode.");
+});
+
+add_task(function asyncCleanup() {
+  yield resetCustomization();
+});