[danShumway]

> maybe it's calculating how to minimize threads on the fly completely clientside by iterating over the DOM, and maybe that's why the buttons don't work with JS disabled

Holy crud, I just opened the JS up and this is actually what it's doing. Heck me and my little 'not going to contradict you' statements, you might just be completely entirely right about what's going on I guess.

  function kidvis (tr, hide) {
    var n0 = ind(tr), n = ind(kid1(tr)), coll = false;
    if (n > n0) {
      while (tr = kid1(tr)) {
        if (ind(tr) <= n0) {

I'm still going to reassert that any architecture that leads to someone doing this kind of logic every time a button is pressed is neither simple nor elegant, and at best this is an example of making architectural sacrifices and introducing complexity for the sake of performance; there's no way that this kind of page logic is easier to maintain or to understand than something built around a more semantic structure.

It's also definitely not easier on the client, I've seen some comments on here argue that HN might use all this weird stuff to save client battery life, and I feel more confident now saying that's not the reason, because if someone is somehow, someway legitimately in some incredible situation where they're actually worried about the battery drain of a browser rendering a table vs a some extra CSS, then this is not the code you would want to run every single time you press a button on the page.

But yeah, the "we just base everything off of an indentation integer and comment order" theory does seem a lot more plausible to me now, because I'm having a somewhat difficult time thinking why else collapsing comments would work this way.