diff -r 000000000000 -r 6474c204b198 browser/devtools/debugger/test/browser_dbg_host-layout.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/devtools/debugger/test/browser_dbg_host-layout.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,104 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * This if the debugger's layout is correctly modified when the toolbox's
+ * host changes.
+ */
+
+let gDefaultHostType = Services.prefs.getCharPref("devtools.toolbox.host");
+
+function test() {
+  Task.spawn(function() {
+    yield testHosts(["bottom", "side", "window"], ["horizontal", "vertical", "horizontal"]);
+    yield testHosts(["side", "bottom", "side"], ["vertical", "horizontal", "vertical"]);
+    yield testHosts(["bottom", "side", "bottom"], ["horizontal", "vertical", "horizontal"]);
+    yield testHosts(["side", "window", "side"], ["vertical", "horizontal", "vertical"]);
+    yield testHosts(["window", "side", "window"], ["horizontal", "vertical", "horizontal"]);
+    finish();
+  });
+}
+
+function testHosts(aHostTypes, aLayoutTypes) {
+  let [firstHost, secondHost, thirdHost] = aHostTypes;
+  let [firstLayout, secondLayout, thirdLayout] = aLayoutTypes;
+
+  Services.prefs.setCharPref("devtools.toolbox.host", firstHost);
+
+  return Task.spawn(function() {
+    let [tab, debuggee, panel] = yield initDebugger("about:blank");
+    yield testHost(tab, debuggee, panel, firstHost, firstLayout);
+    yield switchAndTestHost(tab, debuggee, panel, secondHost, secondLayout);
+    yield switchAndTestHost(tab, debuggee, panel, thirdHost, thirdLayout);
+    yield teardown(panel);
+  });
+}
+
+function switchAndTestHost(aTab, aDebuggee, aPanel, aHostType, aLayoutType) {
+  let gToolbox = aPanel._toolbox;
+  let gDebugger = aPanel.panelWin;
+
+  return Task.spawn(function() {
+    let layoutChanged = once(gDebugger, gDebugger.EVENTS.LAYOUT_CHANGED);
+    let hostChanged = gToolbox.switchHost(aHostType);
+
+    yield hostChanged;
+    ok(true, "The toolbox's host has changed.");
+
+    yield layoutChanged;
+    ok(true, "The debugger's layout has changed.");
+
+    yield testHost(aTab, aDebuggee, aPanel, aHostType, aLayoutType);
+  });
+
+  function once(aTarget, aEvent) {
+    let deferred = promise.defer();
+    aTarget.once(aEvent, deferred.resolve);
+    return deferred.promise;
+  }
+}
+
+function testHost(aTab, aDebuggee, aPanel, aHostType, aLayoutType) {
+  let gDebugger = aPanel.panelWin;
+  let gView = gDebugger.DebuggerView;
+
+  is(gView._hostType, aHostType,
+    "The default host type should've been set on the panel window (1).");
+  is(gDebugger.gHostType, aHostType,
+    "The default host type should've been set on the panel window (2).");
+
+  is(gView._body.getAttribute("layout"), aLayoutType,
+    "The default host type is present as an attribute on the panel's body.");
+
+  if (aLayoutType == "horizontal") {
+    is(gView._sourcesPane.parentNode.id, "debugger-widgets",
+      "The sources pane's parent is correct for the horizontal layout.");
+    is(gView._instrumentsPane.parentNode.id, "debugger-widgets",
+      "The instruments pane's parent is correct for the horizontal layout.");
+  } else {
+    is(gView._sourcesPane.parentNode.id, "vertical-layout-panes-container",
+      "The sources pane's parent is correct for the vertical layout.");
+    is(gView._instrumentsPane.parentNode.id, "vertical-layout-panes-container",
+      "The instruments pane's parent is correct for the vertical layout.");
+  }
+
+  let widgets = gDebugger.document.getElementById("debugger-widgets").childNodes;
+  let panes = gDebugger.document.getElementById("vertical-layout-panes-container").childNodes;
+
+  if (aLayoutType == "horizontal") {
+    is(widgets.length, 7, // 2 panes, 1 editor, 3 splitters and a phantom box.
+      "Found the correct number of debugger widgets.");
+    is(panes.length, 1, // 1 lonely splitter in the phantom box.
+      "Found the correct number of debugger panes.");
+  } else {
+    is(widgets.length, 5, // 1 editor, 3 splitters and a phantom box.
+      "Found the correct number of debugger widgets.");
+    is(panes.length, 3, // 2 panes and 1 splitter in the phantom box.
+      "Found the correct number of debugger panes.");
+  }
+}
+
+registerCleanupFunction(function() {
+  Services.prefs.setCharPref("devtools.toolbox.host", gDefaultHostType);
+  gDefaultHostType = null;
+});