Gemfile

# frozen_string_literal: true

source "https://rubygems.org"

gem "cuprite", "~> 0.15.1"
gem "sinatra", "~> 4.0"
gem "puma", "~> 6.4"
gem "minitest", "~> 5.24"

Gemfile.lock

GEM
  remote: https://rubygems.org/
  specs:
    addressable (2.8.7)
      public_suffix (>= 2.0.2, < 7.0)
    base64 (0.2.0)
    capybara (3.40.0)
      addressable
      matrix
      mini_mime (>= 0.1.3)
      nokogiri (~> 1.11)
      rack (>= 1.6.0)
      rack-test (>= 0.6.3)
      regexp_parser (>= 1.5, < 3.0)
      xpath (~> 3.2)
    concurrent-ruby (1.3.3)
    cuprite (0.15.1)
      capybara (~> 3.0)
      ferrum (~> 0.15.0)
    ferrum (0.15)
      addressable (~> 2.5)
      concurrent-ruby (~> 1.1)
      webrick (~> 1.7)
      websocket-driver (~> 0.7)
    matrix (0.4.2)
    mini_mime (1.1.5)
    mini_portile2 (2.8.7)
    minitest (5.24.1)
    mustermann (3.0.1)
      ruby2_keywords (~> 0.0.1)
    nio4r (2.7.3)
    nokogiri (1.16.7)
      mini_portile2 (~> 2.8.2)
      racc (~> 1.4)
    nokogiri (1.16.7-arm64-darwin)
      racc (~> 1.4)
    public_suffix (6.0.1)
    puma (6.4.2)
      nio4r (~> 2.0)
    racc (1.8.1)
    rack (3.1.7)
    rack-protection (4.0.0)
      base64 (>= 0.1.0)
      rack (>= 3.0.0, < 4)
    rack-session (2.0.0)
      rack (>= 3.0.0)
    rack-test (2.1.0)
      rack (>= 1.3)
    regexp_parser (2.9.2)
    ruby2_keywords (0.0.5)
    sinatra (4.0.0)
      mustermann (~> 3.0)
      rack (>= 3.0.0, < 4)
      rack-protection (= 4.0.0)
      rack-session (>= 2.0.0, < 3)
      tilt (~> 2.0)
    tilt (2.4.0)
    webrick (1.8.1)
    websocket-driver (0.7.6)
      websocket-extensions (>= 0.1.0)
    websocket-extensions (0.1.5)
    xpath (3.2.0)
      nokogiri (~> 1.8)

PLATFORMS
  arm64-darwin-22
  ruby

DEPENDENCIES
  cuprite (~> 0.15.1)
  minitest (~> 5.24)
  puma (~> 6.4)
  sinatra (~> 4.0)

BUNDLED WITH
   2.5.16

test.rb

require 'irb'
require 'sinatra/base'
require 'capybara/cuprite'
require 'capybara/dsl'
require 'minitest/autorun'

class App < Sinatra::Base
  get '/' do
    erb <<~ERB
      <form id="form">
        <button type="submit">Submit</button>
      </form>

      <script>
        const form = document.getElementById('form');
        form.addEventListener('submit', async event => {
          event.preventDefault();
          fetch('/submit', { method: 'POST' });
          fetch('/submit', { method: 'POST' });
          fetch('/submit', { method: 'POST' });
          fetch('/submit', { method: 'POST' });
          fetch('/submit', { method: 'POST' });
        });
      </script>
    ERB
  end

  post '/submit' do
    'OK'
  end
end

Capybara.app = App
Capybara.current_driver = :cuprite

class Reproduce < Minitest::Test
  include Capybara::DSL

  def test_reproduction
    network = page.driver.browser.network
    network.whitelist = %r{127.0.0.1}

    visit '/'

    100.times do |i|
      click_button 'Submit'

      begin
        network.wait_for_idle
      rescue Ferrum::TimeoutError
        # Find the exchange that isn't finished
        exchange = network.traffic.reject(&:finished?).first

        # Find all exchanges with the same ID
        exchanges = network.traffic.select { _1.id == exchange.id }

        # There will be two exchanges
        assert_equal 2, exchanges.length

        # Poke around
        binding.irb
      end
    end
  end
end