Fix headless chrome modals when page changes

lib/capybara/selenium/driver.rb

@@ -368,6 +368,7 @@ class Capybara::Selenium::Driver < Capybara::Driver::Base
         prompt: window.prompt,
         confirm: window.confirm,
         alert: window.alert,
+        called: false
       }
       window.capybara.add_handler(modal_handler);
 
@@ -420,15 +421,19 @@ class Capybara::Selenium::Driver < Capybara::Driver::Base
       ignore: Selenium::WebDriver::Error::NoAlertPresentError)
     begin
       wait.until do
-        called, alert_text = evaluate_script('window.capybara.current_modal_status()')
+        called, alert_text = evaluate_script('window.capybara && window.capybara.current_modal_status()')
         if called
-          execute_script('window.capybara.modal_handlers.shift()')
+          execute_script('window.capybara && window.capybara.modal_handlers.shift()')
           regexp = options[:text].is_a?(Regexp) ? options[:text] : Regexp.escape(options[:text].to_s)
           if alert_text.match(regexp)
             alert_text
           else
             raise Capybara::ModalNotFound.new("Unable to find modal dialog#{" with #{options[:text]}" if options[:text]}")
           end
+        elsif called.nil?
+          # page changed so modal_handler data has gone away
+          warn "Can't verify modal text when page change occurs - ignoring" if options[:text]
+          ""
         else
           nil
         end

lib/capybara/spec/public/test.js

@@ -87,6 +87,10 @@ $(function() {
       $(link).attr('opened', 'true');
     }, 3000);
   });
+  $('#alert-page-change').click(function() {
+    alert('Page is changing');
+    return true;
+  });
   $('#open-confirm').click(function() {
     if(confirm('Confirm opened')) {
       $(this).attr('confirmed', 'true');

lib/capybara/spec/session/accept_alert_spec.rb

@@ -47,6 +47,14 @@ Capybara::SpecHelper.spec '#accept_alert', requires: [:modals] do
     expect(message).to eq('Alert opened [*Yay?*]')
   end
 
+  it "should handle the alert if the page changes" do
+    @session.accept_alert do
+      @session.click_link('Alert page change')
+      sleep 1 # ensure page change occurs before the accept_alert block exits
+    end
+    expect(@session).to have_current_path('/with_html')
+  end
+
   context "with an asynchronous alert" do
     it "should accept the alert" do
       @session.accept_alert do

lib/capybara/spec/views/with_js.erb

@@ -76,6 +76,7 @@
 
     <p>
       <a href="#" id="open-alert">Open alert</a>
+      <a href="/with_html" id="alert-page-change">Alert page change</a>
     </p>
 
     <p>