Only messages as comments and email template should keep a white backround since they are not internal.
It is also enough to display fields that changed on subtypes (update message) since emails will be reworked soon and more information will be given there. The support of 'always' track can be removed. (is now considered as a simple tracked field)
The use of self.env.ref('mail.mt_comment').id add some query and the performance tests needs to be update since message_format is called by notify_by_chat. This code will be refactor later
Creating data directly in tests was done when tests were located in mail
module to avoid creating real data or demo. Now that mail tests have their
own module we can create demo data and use them in tests. It is simpler to
have demo data for things like subtype and email templates to ease
understanding and reuse.
Xavier-Do