web/web_fullscreen/static/src/xml/web_fullscreen_templates.xml

<?xml version="1.0" encoding="utf-8"?>
<templates xml:space="preserve">
    <t t-extend="ControlPanel">
        <t t-jquery=".o_cp_switch_buttons" t-operation="after">
            <div class="btn-group btn-group-sm o_cp_fullscreen">
                <button class="btn btn-icon fa fa-lg fa-tablet"
                   data-toggle="tooltip" data-placement="top"
                   aria-expanded="false" href="#">
                        <i title="Fullscreen Mode"
                           aria-hidden="true"/>
                </button>
            </div>
        </t>
    </t>
</templates>