To my knowledge this is how reddit handles threading overflow:
On first load, shows max 10 layers of replies — top level post + 9 child comments
On click of “Continue Thread”, shows next 10 replies (replaces content) with lowest reply highlighted at the top (shows siblings of lower level replies too)
On click of “Show parent comments”, shows 10 parent comments with original comment highlighted (max 20 layers shown) (this seems like odd behavior to me)
“View all comments” takes you back up to the top level (max 10 layers shown)
To my knowledge this is how reddit handles threading overflow: