xmpp.chapril.org-conversejs/src/templates/spoiler_button.html

<li class="toggle-compose-spoiler fa {[ if (o.composing_spoiler)  { ]} fa-eye-slash {[ } ]} {[ if (!o.composing_spoiler)  { ]} fa-eye {[ } ]}"
    title="{{ o.label_toggle_spoiler }}"></a>
</li>