Source plugins have a getFilterDefinitions() method, the base implementation of which calls $this->getCategories() to produce a list of categories as one of the filters.
The Svelte code also makes an AJAX request to the backend, which...calls getCategories() on the source plugin. Since the categories are available up front when the Svelte app loads, this request is completely pointless, makes the code more confusing, and necessitates an extra route and method of EnabledSourceHandler which is not ultimately needed.
Remove EnabledSourceHandler::getCategories() entirely, and the route which calls it. Then refactor the MultipleChoiceFilter component in the Svelte code so that it uses a set of categories passed to it.
Active
2.0
Code
Postponing on 📌 Export a coherent set of information about a particular Project Browser instance in drupalSettings Active , since it'll be a lot easier to do this once that one is in.