attempt1.rb

RSpec.configure do |config|
  ...

  config.after(:each) do
    wait_for_ajax_completion if example.metadata[:type] == :feature
  end
end

def wait_for_ajax_completion
  wait_for('pending AJAX requests complete') do
    num_pending_ajax_calls == 0
  end
end

def num_pending_ajax_calls
  page.evaluate_script("window.$.active").to_i
end

# Waits until the passed block returns true
def wait_for(condition_name, max_wait_time: 30, polling_interval: 0.01)
  wait_until = Time.now + max_wait_time.seconds
  while true
    return if yield
    if Time.now > wait_until
      raise "Condition not met: #{condition_name}"
    else
      sleep(polling_interval)
    end
  end
end

Gemfile

group :test do
  gem 'atomic'
end

rack_request_blocker.rb

require 'atomic'

# Rack middleware that keeps track of the number of active requests and can block new requests.
class RackRequestBlocker

  @@num_active_requests = Atomic.new(0)
  @@block_requests = Atomic.new(false)

  # Returns the number of requests the server is currently processing.
  def self.num_active_requests
    @@num_active_requests.value
  end

  # Prevents the server from accepting new requests. Any new requests will return an HTTP
  # 503 status.
  def self.block_requests!
    @@block_requests.value = true
  end

  # Allows the server to accept requests again.
  def self.allow_requests!
    @@block_requests.value = false
  end

  def initialize(app)
    @app = app
  end

  def call(env)
    increment_active_requests
    if block_requests?
      block_request(env)
    else
      @app.call(env)
    end
  ensure
    decrement_active_requests
  end

  private

  def block_requests?
    @@block_requests.value
  end

  def block_request(env)
    [503, {}, []]
  end

  def increment_active_requests
    @@num_active_requests.update { |v| v + 1 }
  end

  def decrement_active_requests
    @@num_active_requests.update { |v| v - 1 }
  end
end

spec_helper.rb

RSpec.configure do |config|
  ...
  
  config.after(:each) do
    wait_for_requests_complete if example.metadata[:type] == :feature
  end
end

def wait_for_requests_complete
  stop_client
  RackRequestBlocker.block_requests!
  wait_for('pending AJAX requests complete') do
    RackRequestBlocker.num_active_requests == 0
  end
ensure
  RackRequestBlocker.allow_requests!
end

# Navigate away from the current page which will prevent any new requests from being started
def stop_client
  page.execute_script %Q{
    window.location = "about:blank";
  }
end

# Waits until the passed block returns true
def wait_for(condition_name, max_wait_time: 30, polling_interval: 0.01)
  wait_until = Time.now + max_wait_time.seconds
  while true
    return if yield
    if Time.now > wait_until
      raise "Condition not met: #{condition_name}"
    else
      sleep(polling_interval)
    end
  end
end

test.rb

MyProject::Application.configure do
  ...

  require 'rack_request_blocker'
  # Make sure the middleware is inserted first in middleware chain
  config.middleware.insert_before('ActionDispatch::Static', 'RackRequestBlocker')
end