Deprecated but working version of form_buffers removed by https://github.com/activeadmin/activeadmin/pull/3486

gistfile1.rb

module ActiveAdmin
  module Views
    class ActiveAdminForm
      class DeprecatedFormBufferSubstitute
        def initialize(form)
          @form = form
        end
        def << (value)
          @form.text_node value.html_safe
        end
      end
      def form_buffers
        warn "DEPRECATION WARNING: 'form_buffers' is no longer needed."
        @form_buffers ||= [DeprecatedFormBufferSubstitute.new(self)]
      end
    end
  end
end

gistfile2.txt

  context "with deprecated form_buffers workaround" do
    let :body do
      build_form do |f|
        f.inputs
        f.form_buffers.last << '<span>appended to last form buffer</span>'
        f.actions
      end
    end
    it "should allow injection of html" do
      expect(body).to have_tag 'span', 'appended to last form buffer'
      expect(body.index('form buffer')).to be > body.index('label')
      expect(body.index('form buffer')).to be < body.index('Cancel')
    end
  end