Linking from PIE entries to their root derivation categories

That's strange, the arrow expands just fine for me in this thread.

It does for me now too, maybe it only apples to text added to the page by JavaScript features (such as when you preview and save a thread, but not after you refresh). Still, the subcategory arrows are grayed out.

Maybe the extension only supports expanding one level? I don't know.

It probably uses the same backend as the category pages themselves, and on those you can expand unlimited levels of subcategories. I'll look in the documentation.

|mode=all fixed it:

Category Terms derived from the PIE root *sed- not found