In the Kanban, TestManagement and Planning artifact modal, when the tracker has a select2 (open lists) and when it also contains date fields (with flatpickr), the special "+" rule to adjust select2 dropdown z-index does not apply.

The DOM is in the following state:

<form class="tlp-modal tlp-modal></form>
<div class="tlp-modal-backdrop-shown"></div>
<div class="flatpickr-calendar"></div>
<span class="select2-container select2-container--open"></span>

This prevents our special rule to match and the select2 dropdown is under the artifact modal.

This was introduced by a7164362f3f4e65b79404c4d3971877fc0f975b2