display_nameとidnameの行を分ける。ドメイン先頭のmstdnは自明だから消す。すまほんはすまほん。

double_header.rb

class Gdk::MiraclePainter < Gtk::Object
  alias :original_header_left_markup :header_left_markup

  def header_left_markup
    user = message.user
    if user.respond_to?(:idname)
      idname = user.idname
      parts = idname.split('@')
      if parts[1]
        if parts[1].start_with?('mstdn.')
          idname = "#{parts[0]}@#{parts[1].delete_prefix('mstdn')}"
        elsif parts[1] == "social.mikutter.hachune.net"
          idname = "#{parts[0]}@s.m.h.n"
        end
      end
      Pango.parse_markup("#{Pango.escape(user.name || '')}\n<b>#{Pango.escape(idname)}</b>")
    else
      Pango.parse_markup(Pango.escape(user.name || ''))
    end
  end
end

class Gdk::SubPartsMessageBase < Gdk::SubParts
  alias :original_header_left_content :header_left_content
  def header_left_content(message)
    user = message.user
    if user.respond_to?(:idname)
      idname = user.idname
      parts = idname.split('@')
      if parts[1]
        if parts[1].start_with?('mstdn.')
          idname = "#{parts[0]}@#{parts[1].delete_prefix('mstdn')}"
        elsif parts[1] == "social.mikutter.hachune.net"
          idname = "#{parts[0]}@s.m.h.n"
        end
      end
      attr_list, text = Pango.parse_markup("#{Pango.escape(user.name || '')}\n<b>#{Pango.escape(idname)}</b>")
    else
      attr_list, text = Pango.parse_markup(Pango.escape(user.name || ''))
    end
    return text, header_left_font(message), attr_list
  end
end