How to allow testing beforeunload confirmation dialogs with modern ChromeDrivers

Starting with ChromeDriver 127, if your application displays a beforeunload confirmation dialog, ChromeDriver will immediately close it. In consequence, any automated tests which try to interact with unload prompts will fail.

This is because ChromeDriver now follows the W3C WebDriver spec Show archive.org snapshot which states that any unload prompts should be closed automatically.
However, this applies only to "HTTP" test sessions, i.e. what you're using by default. The spec also defines that bi-directional test sessions (using the "BiDi" WebDriver) should not automatically accept such dialogs. This means that we want to use the BiDi driver when testing beforeunload prompts.

Enabling the BiDi WebDriver

Using Capybara, simply set the :web_socket_url option to true to enable BiDi mode.
Example:

Copyoptions = Selenium::WebDriver::Chrome::Options.new(
  args: ['--headless'],
  unhandled_prompt_behavior: 'ignore',
  # ...
)

if unload_confirmation
  options.add_option(:web_socket_url, true)
end

Capybara::Selenium::Driver.new(app, browser: :chrome, options: options)

In combination with unhandled_prompt_behavior: 'ignore' (which we recommend setting) any unload alerts will now stay open until handled.

Recommendation

We do not recommend BiDi mode as a default because it can have a negative impact on other tests.

For example, if your application shows unload confirmations for forms with unsaved changes, all tests would have to ensure they explicitly save or discard any changes before a test finishes. Otherwise, when the driver closes the page, an unload dialog appears which cannot be handled.
Also, the BiDi driver does not yet implement all features (see https://wpt.fyi/results/webdriver/tests/bidi Show archive.org snapshot ).

Instead, specify a BiDi driver and use that driver when testing unload behavior. Example:

CopyCapybara.register_driver(:with_unload_confirmation) do |app|
  # ...
end

Copydescribe 'unsaved changes confirmation dialog', :js, driver: :with_unload_confirmation do
  # ...
end