Dropdown dropup in backned

In a list_toobar I am trying to add a dropdown and I want it to be display upward and not downard.
So as the doc is mentioning, we need to add the class dropup but doing so does not work in my context as it is cutoff byt the toolbar itself.

<div data-control="toolbar" data-disposable>
    <div class="dropdown dropdown-fixed dropup">
        <a href="#" data-toggle="dropdown" class="btn btn-sm btn-default oc-icon-plus">
            <?= __('engagement.btn_add_field') ?>
        </a>

any idea how to circumvent this ?