odoo18/addons/mail/static/tests/mail_test_helpers.scss

.o-mail-Discuss-asTabContainer {
    height: 100%;
    width: 100%;
    display: flex;
    flex-direction: column;
    position: absolute;
    top: 0;
    left: 0;

    & .o_action_manager {
        height: 100%;
    }

}