Blog Engineering How GitLab switched to Headless Chrome for testing
December 19, 2017
18 min read

How GitLab switched to Headless Chrome for testing

A detailed explanation with examples of how GitLab made the switch to headless Chrome.


GitLab recently switched from PhantomJS to headless Chrome for both our frontend tests and our RSpec feature tests. In this post we will detail the reasons we made this transition, the challenges we faced, and the solutions we developed. We hope this will benefit others making the switch.

We now have a truly accurate way to test GitLab within a real, modern browser. The switch has improved our ability to write tests and debug them while running them directly in Chrome. Plus the change forced us to confront and clean up a number of hacks we had been using in our tests.

Switching to headless Chrome from PhantomJS: background

PhantomJS has been a part of GitLab's test framework for almost five years. It has been an immensely useful tool for running browser integration tests in a headless environment at a time when few options were available. However, it had some shortcomings:

The most recent version of PhantomJS (v2.1.1) is compiled with a three-year-old version of QtWebKit (a fork of WebKit v538.1 according to the user-agent string). This puts it on par with something like Safari 7 on macOS 10.9. It resembles a real modern browser, but it's not quite there. It has a different JavaScript engine, an older rendering engine, and a host of missing features and quirks.

At this time, GitLab supports the current and previous major release of Firefox, Chrome, Safari, and Microsoft Edge/IE. This puts PhantomJS and its capabilities somewhere near or below our lowest common denominator. Many modern browser features either do not work, or require vendor prefixes and polyfills that none of our supported browsers require. We could selectively add these polyfills, prefixes, and other workarounds just within our test environment, but doing so would increase technical debt, cause confusion, and make the tests less representative of a true production environment. In most cases we had opted to simply omit them or hack around them (more on this later).

Here's a screenshot of the way PhantomJS renders a page from GitLab, followed by the same page rendered in Google Chrome:

Page Rendered by PhantomJS

Page Rendered by Google Chrome

You can see in PhantomJS the filter tabs are rendered horizontally, the icons in the sidebar render on their own lines, the global search field is overflowing off the navbar, etc.

While it looks ugly, in most cases we could still use this to run functional tests, so long as elements of the page remain visible and clickable, but this disparity with the way GitLab rendered in a real browser did introduce several edge cases.

What is headless Chrome

In April of this year, news spread that Chrome 59 would support a native, cross-platform headless mode. It was previously possible to simulate a headless Chrome browser in CI/CD using virtual frame buffer, but this required a lot of memory and extra complexities. A native headless mode is a game changer. It is now possible to run integration tests in a headless environment on a real, modern web browser that our users actually use!

Soon after this was revealed, Vitaly Slobodin, PhantomJS's chief developer, announced that the project would no longer be maintained:

It became clear that we would need to make the transition away from PhantomJS at some point, so we opened up an issue, downloaded the Chrome 59 beta, and started looking at options.

Frontend tests (Karma)

Our frontend test suite utilizes the Karma test runner, and updating this to work with Google Chrome was surprisingly simple (here's the merge request). The karma-chrome-launcher plugin was very quickly updated to support headless mode starting from version 2.1.0, and it was essentially a drop-in replacement for the PhantomJS launcher. Once we re-built our CI/CD build images to include Google Chrome 59 (and fiddled around with some pesky timeout settings), it worked! We were also able to remove some rather ugly PhantomJS-specific hacks that Jasmine required to spy on some built-in browser functions.

Backend feature tests (RSpec + Capybara)

Our feature tests use RSpec and Capybara to perform full end-to-end integration testing of database, backend, and frontend interactions. Before switching to headless Chrome, we had used Poltergeist which is a PhantomJS driver for Capybara. It would spin up a PhantomJS browser instance and direct it to browse, fill out forms, and click around on pages to verify that everything behaved as it should.

Switching from PhantomJS to Google Chrome required a change in drivers from Poltergeist to Selenium and ChromeDriver. Setting this up was pretty straightforward. You can install ChromeDriver on macOS with brew install chromedriver and the process is similar on any given package manager in Linux. After this we added the selenium-webdriver gem to our test dependencies and configured Capybara like so:

require 'selenium-webdriver'

Capybara.register_driver :chrome do |app|
  options =
    args: %w[headless disable-gpu no-sandbox]
  ), browser: :chrome, options: options)

Capybara.javascript_driver = :chrome

Google says the disable-gpu option is necessary for the time being until some bugs are resolved. The no-sandbox option also appears to be necessary to get Chrome running inside a Docker container for GitLab's CI/CD environment. Google provides a useful guide for working with headless Chrome and Selenium.

In our final implementation we changed this to conditionally add the headless option unless you have CHROME_HEADLESS=false in your environment. This makes it easy to disable headless mode while debugging or writing tests. It's also pretty fun to watch tests execute on the browser window in real time:

export CHROME_HEADLESS=false
bundle exec rspec spec/features/merge_requests/filter_merge_requests_spec.rb

Tests Executing in Chrome

What is the differences between Poltergeist and Selenium?

The process of switching drivers here was not nearly as straightforward as it was with the frontend test suite. Dozens of tests started failing as soon as we changed our Capybara configuration, and this was due to some major differences in the way Selenium/ChromeDriver implemented Capybara's driver API compared to Poltergeist/PhantomJS. Here are some of the challenges we ran into:

  1. JavaScript modals are no longer accepted automatically

    We often use JavaScript confirm("Are you sure you want to do X?"); click events when performing a destructive action such as deleting a branch or removing a user from a group. Under Poltergeist a .click action would automatically accept modals like alert() and confirm(), but under Selenium, you now need to wrap these with one of accept_alert, accept_confirm, or dismiss_confirm. e.g.:

    # Before
    page.within('.some-selector') do
      click_link 'Delete'
    # After
    page.within('.some-selector') do
      accept_confirm { click_link 'Delete' }
  2. Selenium Element.visible? returns false for empty elements

    If you have an empty div or span that you want to access in your test, Selenium does not consider these "visible." This is not much of an issue unless you set Capybara.ignore_hidden_elements = true as we do in our feature tests. Where find('.empty-div') would have worked fine in Poltergeist, we now need to use visible: :any to select such elements.

    # Before
    # After
    find('.empty-div', visible: :any)
    # or
    find('.empty-div', visible: false)

    More on Capybara and hidden elements.

  3. {:#trigger-method} Poltergeist's Element.trigger('click') method does not exist in Selenium

    In Capybara, when you use find('.some-selector').click, the element you are clicking must be both visible and unobscured by any overlapping element. Situations where links could not be clicked would sometimes occur with Poltergeist/PhantomJS due to its poor CSS support sans-prefixes. Here's one example:

    Overlapping elements

    The broken layout of the search form here was actually placing an invisible element over the top of the "Update all" button, making it unclickable. Poltergeist offers a .trigger('click') method to work around this. Rather than actually clicking the element, this method would trigger a DOM event to simulate a click. Utilizing this method was a bad practice, but we ran into similar issues so often that many developers formed a habit of using it everywhere. This began to lead to some lazy and sloppy test writing. For instance, someone might use .trigger as a shortcut to click on an link that was obscured behind an open dropdown menu, when a properly written test should .click somewhere to close the dropdown, and then .click on the item behind it.

    Selenium does not support the .trigger method. Now that we were using a more accurate rendering engine that won't break our layouts, many of these instances could be resolved by simply replacing .trigger('click') with .click, but due to some of the bad practice uses mentioned above, this didn't always work.

    There are of course some ways to hack a .trigger replacement. You could simulate a click by focusing on an element and hitting the "return" key, or use JavaScript to trigger a click event, but in most cases we decided to take the time and actually correct these poorly implemented tests so that a normal .click could again be used. After all, if our tests are meant to simulate a real user interacting with the page, we should limit ourselves to the actions a real user would be expected to use.

    # Before
    # After
    # bad
    # bad
    # good
    # do something to make link accessible, then
  4. Element.send_keys only works on focus-able elements

    We had a few places in our code where we would test out our keyboard shortcuts using something like find('.boards-list').native.send_keys('i'). It turns out Chrome will not allow you to send_keys to any element that cannot be "focused", e.g. links, form elements, the document body, or presumably anything with a tab index.

    In all of the cases where we were doing this, triggering send_keys on the body element would work since that's ultimately where our event handler was listening anyway:

    # Before
    # After
  5. Element.send_keys does not support non-BMP characters (like emoji)

    In a few tests, we needed to fill out forms with emoji characters. With Poltergeist we would do this like so:

    # Before

    In Selenium we would get the following error message:

        unknown error: ChromeDriver only supports characters in the BMP

    To work around this, we added a JavaScript method to our test bundle that would simulate input and fire off the same DOM events that an actual keyboard input would generate on every keystroke, then wrapped this with a ruby helper method that could be called like so:

    # After
    include InputHelper
    simulate_input('#note-body', "@💃username💃")
  6. Setting cookies is much more complicated

    It's quite common to want to set some cookies before visiting a page that you intend to test, whether it's to mock a user session, or toggle a setting. With Poltergeist, this process is really simple. You can use page.driver.set_cookie, provide a simple key/value pair, and it will just work as expected, setting a cookie with the correct domain and scope.

    Selenium is quite a bit more strict. The method is now page.driver.browser.manage.add_cookie, and it comes with two caveats:

    • You cannot set cookies until you visit a page in the domain you intend to scope your cookies to.
    • Annoyingly, you cannot alter the path parameter (or at least we could never get this to work), so it is best to set cookies at the root path.

    Before you visit your page, Chrome's url is technically sitting at something like about:blank;. When you attempt to set a cookie there, it will refuse because there is no hostname, and you cannot coerce one by providing a domain as an argument. The Selenium documentation suggests that you do the following:

    If you are trying to preset cookies before you start interacting with a site and your homepage is large / takes a while to load, an alternative is to find a smaller page on the site (typically the 404 page is small, e.g.

    # Before
    before do
      page.driver.set_cookie('name', 'value')
    # After
    before do
      visit '/some-root-path'
      page.driver.browser.manage.add_cookie(name: 'name', value: 'value')
  7. Page request/response inspection methods are missing

    Poltergeist very conveniently implemented methods like page.status_code and page.response_headers which are also present in Capybara's default RackTest driver, making it easy to inspect the raw response from the server, in addition to the way that response is rendered by the browser. It also allowed you to inject headers into the requests made to the server, e.g.:

    # Before
    before do
      page.driver.add_header('Accept', '*/*')
    it 'returns a 404 page'
      visit some_path
      expect(page.status_code).to eq(404)
      expect(page).to have_css('.some-selector')

    Selenium does not implement these methods, and the authors do not intend to add support for them, so we needed to develop a workaround. Several people have suggested running a proxy alongside ChromeDriver that would intercept all traffic to and from the server, but this seemed to us like overkill. Instead, we opted to create a lightweight Rack middleware and a corresponding helper class that would intercept the traffic for inspection. This is similar to our RequestBlockerMiddleware that we were already using to intelligently wait_for_requests to complete within our tests. It works like this:

    # After
    it 'returns a 404 page'
      requests = inspect_requests do
        visit some_path
      expect(requests.first.status_code).to eq(404)
      expect(page).to have_css('.some-selector')

    Within the inspect_requests block, the Rack middleware will log all requests and responses, and return them as an array for inspection. This will include the page being visited as well as the subsequent XHR and asset requests, but the initial path request will be the first in the array.

    You can also inject headers using the same helper like so:

    # After
    inspect_requests(inject_headers: { 'Accept' => '*/*' }) do
      visit some_path

    This middleware should be injected early in the stack to ensure any other middleware that might intercept or modify the request/response will be seen by our tests. We include this line in our test environment config:

    config.middleware.insert_before('ActionDispatch::Static', 'Gitlab::Testing::RequestInspectorMiddleware')
  8. Browser console output is no longer output to the terminal

    Poltergeist would automatically output any console messages directly into the terminal in real time as tests were run. If you had a bug in the frontend code that caused a test to fail, this feature would make debugging much easier as you could inspect the terminal output of the test for an error message or a stack trace, or inject your own console.log() into the JavaScript to see what is going on. With Selenium this is sadly no longer the case.

    You can, however, collect browser logs by configuring Capybara like so:

    capabilities =
      loggingPrefs: {
        browser: "ALL",
        client: "ALL",
        driver: "ALL",
        server: "ALL"
    # ...
      browser: :chrome,
      desired_capabilities: capabilities,
      options: options

    This will allow you to access logs with the following, i.e. in the event of a test failure:


    This is far more cumbersome than it was in Poltergeist, but it's the best method we've found so far. Thanks to Larry Reid's blog post for the tip!


Regarding performance, we attempted to quantify the change with a non-scientific analysis of 10 full-suite RSpec test runs before this change, and 10 more runs from after this change, factoring out any tests that were added or removed between these pipelines. The end result was:

Before: 5h 18m 52s After: 5h 12m 34s

A savings of about six minutes, or roughly 2 percent of the total compute time, is statistically insignificant, so I'm not going to claim we improved our test speed with this change.

What we did improve was test accuracy, and we vastly improved the tools at our disposal to write and debug tests. Now, all of the Capybara screenshots generated when a CI/CD job fails look exactly as they do on your own browser rather than resembling the broken PhantomJS screenshot above. Inspecting a failing test locally can now be done interactively by turning off headless mode, dropping a byebug line into the spec file, and watching the browser window as you type commands into the prompt. This technique proved extremely useful while working on this project.

You can find all of the changes we made in the original merge request page on

What are some additional uses for headless Chrome?

We have also been utilizing headless Chrome to analyze frontend performance, and have found it to be useful in detecting issues.

We'd like to make it easier for other companies to embrace as well, so as part of the upcoming 10.3 release of GitLab we are releasing Browser Performance Testing. Leveraging GitLab CI/CD, headless Chrome is launched against a set of pages and an overall performance score is calculated. Then for each merge request the scores are compared between the source and target branches, making it easier detect performance regressions prior to merge.


I sincerely hope this information will prove useful to anybody else looking to make the switch from PhantomJS to headless Chrome for their Rails application.

Thanks to the Google team for their very helpful documentation, thanks to the many bloggers out there who shared their own experiences with hacking headless Chrome in the early days of its availability, and special thanks to Vitaly Slobodin and the rest of the contributors to PhantomJS who provided us with an extremely useful tool that served us for many years. 🙇‍

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