Modifying an engine using the overrides directory / class_eval approach

application.rb

# config/application.rb

require_relative "boot"

require "rails/all"

# Require the gems listed in Gemfile, including any gems
# you've limited to :test, :development, or :production.
Bundler.require(*Rails.groups)

module MyApp
  class Application < Rails::Application
    
    # ...
    
    overrides = Rails.root.join('app', 'overrides')
    Rails.autoloaders.main.ignore(overrides)

    config.to_prepare do
      Dir.glob("#{overrides}/**/*_override.rb").sort.each do |override|
        load override
      end
    end
  end
end

override.rb

# app/overrides/controllers/engine/application_controller_override.rb

Engine::ApplicationController.class_eval do
  before_action :turn_off_csp_nonce_generation

  content_security_policy do |policy|
    policy.style_src  :self,
                      :https,
                      :unsafe_inline
  end

  private

  def turn_off_csp_nonce_generation
    request.content_security_policy_nonce_directives = []
  end
end