Maybe the root is getting filtered out?

I can't imagine how, but...#

brutaldon/threadtree.py

@@ -13,6 +13,7 @@ def maketree(descendants):
             reps.add(descendant.id)
             print("REPLY", descendant.id,
                   descendant.in_reply_to_id,
+                  descendant.in_reply_to_account_id,
                   descendant.in_reply_to_id in lookup)
         else:
             reps = set()

brutaldon/views.py

@@ -692,7 +692,7 @@ def thread(request, id):
     # descendants = [
     #     x for x in context.descendants if not toot_matches_filters(x, filters)
     # ]
-    posts = tuple(threadtree.build(descendants))
+    posts = tuple(threadtree.build(context.descendants))
     from pprint import pprint
     pprint(posts)
     raise SystemExit(0)