There must be a pony... speculations on design.
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How's about a "hot mode" toggle? When switched on, the post's sort order changes from chronological to most recent posted comments. It could be maintained by a lookup table linked to post record headers that gets a bubble-sort type swap with the next higher record in the que every time a comment is posted.
There are interpreters that do this to the reserved word symbol table to optimise the lookup speed for keywords.
This would prevent active threads from scrolling off, but the default chronological mode would keep things moving. Maybe even limit the que length to 10 or 20 to prevent monster threads from eating Cleveland.