Minitest's assert_difference not working with Hotwire's Turbo in a Rails 7 app

I'm upgrading a Rails App in order to use Hotwire but tests like the following, since I enabled Turbo, are now failing:

class UserApprovalTest < ApplicationSystemTestCase
  driven_by :selenium_chrome_headless
 
  test 'welcome email gets sent on new user approval' do
    #...

    assert_difference('ActionMailer::Base.deliveries.count', 1) do # <= this used to PASS before enabling Turbo
    click_link("super_admin_approves_user_#{user.id}") # link with data-turbo="true"

    # ...
  end
end

The expected behavior is that when a super user approves a new user, the app sends a welcome email. This still works in the development environment when I test by hand, but now the system test above fails.

Why?
How can I fix it, please?

If it's not an error in your JS then it's likely because the actions are now being executed asynchronously, and your assertion is being executed before the action has occurred. You need to assert for something that indicates the action has completed to keep your test in sync (assert_difference isn't provided by Capybara and therefore doesn't have retrying behavior)

assert_difference('ActionMailer::Base.deliveries.count', 1) do    
  click_link("super_admin_approves_user_#{user.id}")
  assert_content(...) # any Capybara assertion for something that changes on the page to indicate the action has completed
end