[later phase] Provide API for finding and UI for surfacing dangling/dead component subtrees — aka garbage clean-up

Created on 14 May 2025, about 5 hours ago

Overview

To avoid broken sites due to dependencies going missing, we did 🐛 Cannot delete JS components due to component depending on them Active .
Prior to that, we did 🌱 [META] Robust component instance error handling Active to protect the site from crashing in case of A) invalid explicit inputs for component instances and B) logic errors in a component's implementation.

But #3519168 introduced a new kind of problem: it's possible for component subtrees in slots to continue to be stored, but never be visible, because the containing component instance itself is relying on the Fallback component, which essentially renders an empty <div> (Fallback::renderComponent()) + HTML comments to allow the XB UI to still visualize the slots (Fallback::setSlots()).

Conceptually related: 📌 `BlockComponent::validateComponentInput()` allows garbage to pile up Active .

Proposed resolution

Provide API (Drupal service?) for finding dangling/dead component subtrees aka garbage
Provide UI in XB for surfacing dangling/dead component subtrees — aka enabling admin users to clean up garbage