application_controller.rb

class ApplicationController < ActionController::Base
  include EtagWithLayout
end

etag_with_layout.rb

module EtagWithLayout
  extend ActiveSupport::Concern

  include ActionController::ConditionalGet

  included { etag { layout_etag if request.format.html? } }

  private

  def layout_etag
    layout_name = determine_layout_name

    if layout_name
      ActionView::Digestor.digest name: layout_name,
                                  format: nil,
                                  finder: lookup_context
    end
  end

  def determine_layout_name
    current_layout = _layout lookup_context, %i[html]

    case current_layout
    when ActionView::Template
      current_layout.virtual_path
    when String
      "layouts/#{current_layout}"
    end
  end
end