<templates xml:space="preserve">
     <t t-name="ChatterTopbarResponsive" t-inherit="mail.ChatterTopbar" t-inherit-mode="extension" owl="1">
        <xpath expr="//div[contains(@class, 'o_ChatterTopbar') and contains(@class, 'justify-content-between')]//button[contains(@class, 'o_ChatterTopbar_buttonSendMessage')]" position="attributes">
            <attribute name="style" add="display:none;"/>
        </xpath>
    </t>
</templates>