GFork

Failed experiment to use "Mark threadmarks read" as a per-chapter tracking feature.
Can serve as a rough "Mark threadmarks unread" toggle by clicking the main threadlist badges. In Reader mode you can blindly press the "Mark New after" button.

All it does is use an API call to move the Old-New cutoff date up/down to that threadmark (a single per-thread date shared between Threadmarks/Sidestory/etc).

Fundamental issues:

1. When you open a page, all visible threadmarks are marked read already.
   Therefore, if you intended to remember where you left off by looking for the first "New" in the Threadmark list:
   
   • Opening it in Reader mode will mark the first 20 read, loosing the info of where you left off.
     
   • Opening post-by-post may also mark the first 3+ visible posts read.
     
   • Opening the last post will accidentally mark everything read.
     It only works well if you left off farther than that.
     Note: There does seem to be secondary "New" badges above each post that are about to become read. You could scroll down to the first one and keep reading from there even if the list makes it look like they are all read, but it isn't fully robust.
2. Threadmarks may be created or modified out-of-order. When this happens, looking at an old post with a recent date may mark the entire thread read. This is a fundamentally design flaw of the back-end and not solvable. A proper tracking system would need to surface the last like or bookmark sorted by postid.