diff -r 000000000000 -r 6474c204b198 browser/devtools/sourceeditor/test/browser_detectindent.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/devtools/sourceeditor/test/browser_detectindent.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,103 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+
+const TWO_SPACES_CODE = [
+"/*",
+" * tricky comment block",
+" */",
+"div {",
+"  color: red;",
+"  background: blue;",
+"}",
+"     ",
+"span {",
+"  padding-left: 10px;",
+"}"
+].join("\n");
+
+const FOUR_SPACES_CODE = [
+"var obj = {",
+"    addNumbers: function() {",
+"        var x = 5;",
+"        var y = 18;",
+"        return x + y;",
+"    },",
+"   ",
+"    /*",
+"     * Do some stuff to two numbers",
+"     * ",
+"     * @param x",
+"     * @param y",
+"     * ",
+"     * @return the result of doing stuff",
+"     */",
+"    subtractNumbers: function(x, y) {",
+"        var x += 7;",
+"        var y += 18;",
+"        var result = x - y;",
+"        result %= 2;",
+"    }",
+"}"
+].join("\n");
+
+const TABS_CODE = [
+"/*",
+" * tricky comment block",
+" */",
+"div {",
+"\tcolor: red;",
+"\tbackground: blue;",
+"}",
+"",
+"span {",
+"\tpadding-left: 10px;",
+"}"
+].join("\n");
+
+const NONE_CODE = [
+"var x = 0;",
+"           // stray thing",
+"var y = 9;",
+"    ",
+""
+].join("\n");
+
+function test() {
+  waitForExplicitFinish();
+
+  setup((ed, win) => {
+    is(ed.getOption("indentUnit"), 2,
+       "2 spaces before code added");
+    is(ed.getOption("indentWithTabs"), false,
+       "spaces is default");
+
+    ed.setText(NONE_CODE);
+    is(ed.getOption("indentUnit"), 2,
+       "2 spaces after un-detectable code");
+    is(ed.getOption("indentWithTabs"), false,
+       "spaces still set after un-detectable code");
+
+    ed.setText(FOUR_SPACES_CODE);
+    is(ed.getOption("indentUnit"), 4,
+       "4 spaces detected in 4 space code");
+    is(ed.getOption("indentWithTabs"), false,
+       "spaces detected in 4 space code");
+
+    ed.setText(TWO_SPACES_CODE);
+    is(ed.getOption("indentUnit"), 2,
+       "2 spaces detected in 2 space code");
+    is(ed.getOption("indentWithTabs"), false,
+       "spaces detected in 2 space code");
+
+    ed.setText(TABS_CODE);
+    is(ed.getOption("indentUnit"), 2,
+       "2 space indentation unit");
+    is(ed.getOption("indentWithTabs"), true,
+       "tabs detected in majority tabs code");
+
+    teardown(ed, win);
+  });
+}