Primer.css documentation indicates how to show the menu but not how to hide it when clicking inside options.
https://primer.style/css/components/dropdown
<details class="dropdown details-reset details-overlay d-inline-block">
<summary class="btn" aria-haspopup="true">
Dropdown
<div class="dropdown-caret"></div>
</summary>
<ul class="dropdown-menu dropdown-menu-se">
<li><a class="dropdown-item" href="#url">Dropdown item</a></li>
<li><a class="dropdown-item" href="#url">Dropdown item</a></li>
<li><a class="dropdown-item" href="#url">Dropdown item</a></li>
</ul>
</details>
You can get it for all document dropdowns easily removing the "open" attribute of "dropdown" class when the "dropdown-menu" elements are clicked.
This is a quickly JQuery code:
$('.dropdown-menu').click(function() {
$('.dropdown').removeAttr("open");
});