@Lazeez Jiddan (Webmaster)This is probably why I suddenly got switched back to threaded view.
I was going to ask about the possibility of a floating button for 'next unread' in threaded view (flat view takes care of itself), but had second thoughts as it would need too much state to be held the way I could envisage it being done, then I had further thoughts:
The individual posts already have separate IDs used to reference/locate them in replies.
If you build an array with all the IDs in the same order as flat view, and start with an index into the array corresponding to the first unread in the page, the next/prev unread becomes an inc/dec of the index and an in-page jump to that ID location. Although you don't strictly need the post locations from before the first unread, having them would be very useful for the times a page is inadvertently loaded twice as you can the 'prev' back through the unread-but-marked-as-read posts.
Overall I'd prefer threaded view, except for not having a 'next unread' access besides manually scrolling and keeping a careful watch.