I can reproduce this consistently in both Edge and Chrome. I've been assuming that it was a change to how Chromium redraws pages that was introduced a few months ago, but I haven't really looked into it. I just started hitting Next instead of collapsing anything.

I just timed it real quick in Edge 134.0.3124.93 and collapsing svara's top level thread hangs the browser and lags my system for 1 minute 36 seconds.