social/mail_full_expand/static/src/xml/mail_full_expand.xml

<?xml version="1.0" encoding="UTF-8"?>

<!-- Copyright 2014-2015 Grupo ESOC <http://www.grupoesoc.es>
     License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html). -->

<template>
    <t t-extend="mail.widget.Thread.Message">
        <t t-jquery=".o_mail_info" t-operation="append">
            <span class="o_thread_full_spand_icon">
                <i class="fa o_full_expand fa-arrows-alt" t-att-data-message-id="message.getID()" title="Fully Expand"/>
            </span>
        </t>
    </t>
</template>