openproject/public/templates/work_packages/tabs/_panel_expander.html

<div class="panel-toggler"
     ng-click="collapsed = !collapsed">
  <fieldset>
    <legend align="center">
      <span ng-if="!collapsed">
        <i class="icon-arrow-right5-3"></i>
        <span ng-bind="collapseText"/>
      </span>
      <span ng-if="collapsed">
        <i class="icon-arrow-right5-2"></i>
        <span ng-bind="expandText"/>
      </span>
    </legend>
  </fieldset>
</div>