Minor Description
In Threader.gatherSubjects, it appears to be possible for tail.next = c.child; to be null [line 356]:
349 if (old.threadable == null && c.threadable == null) {
350 // both dummies - merge them
351 ThreadContainer tail;
352 for (tail = old.child;
353 tail != null && tail.next != null;
354 tail = tail.next);
355
356 tail.next = c.child;
It also seems kind of pointless, given that the subsequent line immediately resets tail to c.child. Perhaps the ; is not intended there?