Manual "Mark threadmarks read" control. Click the "New"/"Old" indicators to shift the cutoff date. Add button "Mark New after" below each post.
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: