Mypal68/browser/base/content/test/general/browser_bug623893.js

add_task(async function test() {
  await BrowserTestUtils.withNewTab(
    "data:text/plain;charset=utf-8,1",
    async function(browser) {
      BrowserTestUtils.loadURI(browser, "data:text/plain;charset=utf-8,2");
      await BrowserTestUtils.browserLoaded(browser);

      BrowserTestUtils.loadURI(browser, "data:text/plain;charset=utf-8,3");
      await BrowserTestUtils.browserLoaded(browser);

      await duplicate(0, "maintained the original index");
      BrowserTestUtils.removeTab(gBrowser.selectedTab);

      await duplicate(-1, "went back");
      await duplicate(1, "went forward");
      BrowserTestUtils.removeTab(gBrowser.selectedTab);
      BrowserTestUtils.removeTab(gBrowser.selectedTab);
    }
  );
});

function promiseGetIndex(browser) {
  return ContentTask.spawn(browser, null, function() {
    let shistory = docShell
      .QueryInterface(Ci.nsIInterfaceRequestor)
      .getInterface(Ci.nsISHistory);
    return shistory.index;
  });
}

let duplicate = async function(delta, msg, cb) {
  var startIndex = await promiseGetIndex(gBrowser.selectedBrowser);

  duplicateTabIn(gBrowser.selectedTab, "tab", delta);

  let tab = gBrowser.selectedTab;
  await BrowserTestUtils.waitForEvent(tab, "SSTabRestored");

  let endIndex = await promiseGetIndex(gBrowser.selectedBrowser);
  is(endIndex, startIndex + delta, msg);
};