Instead of marking items as read once the user starts scrolling, wait until they've scrolled to the next item. This way half-read items will still be "new" (if you accidentally close the window or open on another device).

This would solve https://bazqux.uservoice.com/forums/190752-general/suggestions/6383547-way-to-get-the-previous-read-post, as a side effect, and is probably easier to implement.