fix: JRuby hangs joining leftover server/browser threads at teardown

test/support/setup_capybara.rb

@@ -1,9 +1,46 @@
 # frozen_string_literal: true
 
 require "support/setup_rails_app"
+require "capybara"
+require "puma"
+
+# JRuby joins every live thread at interpreter teardown (`Ruby.tearDown` ->
+# `ThreadService.teardown` -> `Thread#join`) where MRI simply kills them.
+# Puma's reactor thread parks in a native kqueue/epoll poll that no
+# interrupt can wake, so a Capybara-booted Puma keeps the process alive
+# forever after an otherwise green run -- issue #244. Capybara's stock
+# `:puma` block builds its `Puma::Server` in a block-local and joins it,
+# leaving nothing anyone could stop; this equivalent block keeps the handle
+# so the suite can shut the server down for real.
+module StoppablePuma
+  BOOTED = []
+
+  def self.stop_all
+    BOOTED.each { |server| server.stop(true) }
+    BOOTED.clear
+  end
+end
+
+Capybara.register_server :stoppable_puma do |app, port, host, **options|
+  options = {min_threads: 0, max_threads: 4, log_writer: Puma::LogWriter.strings}.merge(options)
+  Puma::Server.new(app, nil, options).tap do |server|
+    server.add_tcp_listener(host, port)
+    StoppablePuma::BOOTED << server
+  end.run.join
+end
 
 Capybara.app = Rails.application
 Capybara.default_max_wait_time = 1
 Capybara.disable_animation = true
-Capybara.server = :puma, {Silent: true}
+Capybara.server = :stoppable_puma
 Capybara.threadsafe = true
+
+# `minitest/autorun` owns this process's exit through its own `at_exit`,
+# registered before this file loads -- a plain `at_exit` here would run
+# BEFORE the suite does. The RSpec fixtures that load this file run
+# standalone without minitest and need the plain hook.
+if defined?(Minitest.after_run)
+  Minitest.after_run { StoppablePuma.stop_all }
+else
+  at_exit { StoppablePuma.stop_all }
+end