The Tor Browser / file revision

 /* Any copyright is dedicated to the Public Domain.

    http://creativecommons.org/publicdomain/zero/1.0/ */

 function test() {

   waitForExplicitFinish();

   let newTabs = []

   // add enough tabs so the close buttons are hidden and then check the closebuttons attribute

   do {

     let newTab = gBrowser.addTab("about:blank", {skipAnimation: true});

     newTabs.push(newTab);

   } while (gBrowser.visibleTabs[0].getBoundingClientRect().width > gBrowser.tabContainer.mTabClipWidth)

   // a setTimeout() in addTab is used to trigger adjustTabstrip() so we need a delay here as well.

   executeSoon(function() {

     is(gBrowser.tabContainer.getAttribute("closebuttons"), "activetab", "Only show button on selected tab.");

     // move a tab to another group and check the closebuttons attribute

     TabView._initFrame(function() {

       TabView.moveTabTo(newTabs[newTabs.length - 1], null);

       ok(gBrowser.visibleTabs[0].getBoundingClientRect().width > gBrowser.tabContainer.mTabClipWidth, 

          "Tab width is bigger than tab clip width");

       is(gBrowser.tabContainer.getAttribute("closebuttons"), "", "Show button on all tabs.")

       // clean up and finish

       newTabs.forEach(function(tab) {

         gBrowser.removeTab(tab);

       });

       finish();

     });

   });

 }