Ensure modal methods receive a block when using headless chrome

lib/capybara/selenium/driver.rb

@@ -245,8 +245,9 @@ class Capybara::Selenium::Driver < Capybara::Driver::Base
 
   def accept_modal(_type, options={})
     if headless_chrome?
+      raise ArgumentError, "Block that triggers the system modal is missing" unless block_given?
       insert_modal_handlers(true, options[:with], options[:text])
-      yield if block_given?
+      yield
       find_headless_modal(options)
     else
       yield if block_given?
@@ -260,8 +261,9 @@ class Capybara::Selenium::Driver < Capybara::Driver::Base
 
   def dismiss_modal(_type, options={})
     if headless_chrome?
+      raise ArgumentError, "Block that triggers the system modal is missing" unless block_given?
       insert_modal_handlers(false, options[:with], options[:text])
-      yield if block_given?
+      yield
       find_headless_modal(options)
     else
       yield if block_given?

spec/shared_selenium_session.rb

@@ -57,12 +57,18 @@ RSpec.shared_examples "Capybara::Session" do |session, mode|
 
     describe "#accept_alert" do
       it "supports a blockless mode" do
-        skip "Headless Chrome doesn't support blockless modal methods" if @session.driver.send(:headless_chrome?)
         @session.visit('/with_js')
+        skip "Headless Chrome doesn't support blockless modal methods" if @session.driver.send(:headless_chrome?)
         @session.click_link('Open alert')
         @session.accept_alert
         expect{@session.driver.browser.switch_to.alert}.to raise_error(Selenium::WebDriver::Error::NoAlertPresentError)
       end
+
+      it "raises if block is missing" do
+        @session.visit('/with_js')
+        skip "Only Headless Chrome requires the block due to system modal JS injection" unless @session.driver.send(:headless_chrome?)
+        expect { @session.accept_alert }.to raise_error(ArgumentError)
+      end
     end
 
     context "#fill_in with { :clear => :backspace } fill_option", requires: [:js] do