Hi,

after spending hours searching for an answer, I found a solution (for Odoo v13 - but might also work for v11):

By default, the oe_chatter class gets hidden by CSS if the form is viewed in a popup. To resolve this issue, I simply added an inline-style to the oe_chatter-div like this:

<div class="oe_chatter" style="display:block;">
    <field name="message_follower_ids" widget="mail_followers"/>
    <field name="message_ids" widget="mail_thread"/>
</div>

I also placed the chatter inside a notebook-page so that it renders fine.

Note: I also found the suggestion to simply remove the "oe_chatter"-class inside the div. This works to show the e-mail thread but it won't let you add new Notes and E-Mails (see  https://www.odoo.com/fr_FR/forum/aide-1/question/odoo-10-chatter-in-popup-dialogs-129284 )

-Robert