diff -r 000000000000 -r 6474c204b198 browser/components/tabview/test/browser_tabview_bug642793.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/components/tabview/test/browser_tabview_bug642793.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,27 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+function test() {
+  waitForExplicitFinish();
+  newWindowWithTabView(testTopOfStack, loadTabs);
+}
+
+function loadTabs (win) {
+  for (let i = 0; i < 4; i++)
+    win.gBrowser.loadOneTab('about:blank', {inBackground: false});
+  win.gBrowser.selectedTab = win.gBrowser.tabs[2];
+}
+
+function testTopOfStack(win) {
+  registerCleanupFunction(function () { win.close(); });
+  let cw = win.TabView.getContentWindow();
+  let groupItem = cw.GroupItems.getActiveGroupItem();
+  ok(!groupItem.isStacked(), 'groupItem is not stacked');
+  groupItem.setSize(150, 150);
+  groupItem.setUserSize();
+  ok(groupItem.isStacked(), 'groupItem is now stacked');
+  ok(groupItem.isTopOfStack(groupItem.getChild(2)),
+    'the third tab is on top of stack');
+  finish();
+}
+