diff -r 000000000000 -r 6474c204b198 browser/devtools/shared/test/unit/test_undoStack.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/devtools/shared/test/unit/test_undoStack.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,98 @@
+/* -*- Mode: Javascript; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const Cu = Components.utils;
+let {Loader} = Cu.import("resource://gre/modules/commonjs/toolkit/loader.js", {});
+
+let loader = new Loader.Loader({
+  paths: {
+    "": "resource://gre/modules/commonjs/",
+    "devtools": "resource:///modules/devtools",
+  },
+  globals: {},
+});
+let require = Loader.Require(loader, { id: "undo-test" })
+
+let {UndoStack} = require("devtools/shared/undo");
+
+const MAX_SIZE = 5;
+
+function run_test()
+{
+  let str = "";
+  let stack = new UndoStack(MAX_SIZE);
+
+  function add(ch) {
+    stack.do(function() {
+      str += ch;
+    }, function() {
+      str = str.slice(0, -1);
+    });
+  }
+
+  do_check_false(stack.canUndo());
+  do_check_false(stack.canRedo());
+
+  // Check adding up to the limit of the size
+  add("a");
+  do_check_true(stack.canUndo());
+  do_check_false(stack.canRedo());
+
+  add("b");
+  add("c");
+  add("d");
+  add("e");
+
+  do_check_eq(str, "abcde");
+
+  // Check a simple undo+redo
+  stack.undo();
+
+  do_check_eq(str, "abcd");
+  do_check_true(stack.canRedo());
+
+  stack.redo();
+  do_check_eq(str, "abcde")
+  do_check_false(stack.canRedo());
+
+  // Check an undo followed by a new action
+  stack.undo();
+  do_check_eq(str, "abcd");
+
+  add("q");
+  do_check_eq(str, "abcdq");
+  do_check_false(stack.canRedo());
+
+  stack.undo();
+  do_check_eq(str, "abcd");
+  stack.redo();
+  do_check_eq(str, "abcdq");
+
+  // Revert back to the beginning of the queue...
+  while (stack.canUndo()) {
+    stack.undo();
+  }
+  do_check_eq(str, "");
+
+  // Now put it all back....
+  while (stack.canRedo()) {
+    stack.redo();
+  }
+  do_check_eq(str, "abcdq");
+
+  // Now go over the undo limit...
+  add("1");
+  add("2");
+  add("3");
+
+  do_check_eq(str, "abcdq123");
+
+  // And now undoing the whole stack should only undo 5 actions.
+  while (stack.canUndo()) {
+    stack.undo();
+  }
+
+  do_check_eq(str, "abc");
+}