The Tor Browser / file revision

 /* 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();

 }