It is not clear how the user envision this, but one possibility would be to mark older posts as read according to the dtg the user visits the thread.

Ie, what does it mean that a user has read a post, is it when the user has viewed the post (it has been within the viewport) or potentially could have viewed the post (visited the thread)?

The full list of feedbacks can be found on w:no:Wikipedia:Torget#Flow (permalink)