Steps to reproduce the issue

Create a menu in position menu
Create some categories with subcategories and some articles.
Create a menu item of type List All Categories in an Article Category Tree

Go to the categories page. You will see a list of categories and a + symbol in the category with a subcategory (Blog)

When you click on the + symbol the category expands and you see the subcategory (General)

Now unpublish the menu

Expected result

The expand / collapse function works

Actual result

The expand / collapse function doesn't work

Explanation

The com_content/tmpl/categories/default.php file loads a script for accordion $wa->useScript('com_categories.shared-categories-accordion'); that only changes the aria-label and the icon depending on the status expanded or collapsed.
The collapse function itself is on a Javascript from Boostrap (vendor/bootstrap/collapse.min.js), that is not being loaded in the view.
In Cassiopeia the collapse Javascript is being loaded when a menu is published in the position menu.
<nav class="navbar navbar-expand-md"> <?php HTMLHelper::_('bootstrap.collapse', '.navbar-toggler'); ?> ...
That is why the collapse in the categories list only works if the menu is present.

Solution

• Load the collapse script from Bootstrap in the view or
• Create an own collapse / accordion function (e.g. extend the functionality of the com_categories.shared-categories-accordion script). A good example can be found here: https://inclusive-components.design/collapsible-sections/

System information (as much as possible)

Additional comments