Manual "Mark threadmarks read" control. Click the "New"/"Old" indicators to shift the cutoff date. Add button "Mark New after" below each post.
Experiment to make threadmarks work manually alongside Reader Mode.
Cannot toggle per-threadmark, all it does is let you manually move the Old-New cutoff date up and down (a single date shared between Threadmarks/Sidestory/etc).
Fundamental issues:
Unless you press the new "Mark New after" button below a post and exit the page without reloading, all
Cannot fully prevent page-loads from auto-marking all visible threadmarks. Instead it detects "New" badges and sends a query to put the date back before those. This sends an extra web request on most "/threads/*" navigation.
Without this feature, opening in Reader Mode would instantly check all 20+ threadmarks on the page, making it useless. With this fix, marks should be somewhat usable with Reader mode auto link.
By default RELOAD_AFTER_BUTTON_PRESS = false; to minimize extra requests, but you have to trust that it worked.
If you jump to a latter page e.g. to the end of the story, the most it can do is move the date to the oldest threadmark on screen.
Note: Not a robust solution, some threadmarks may have been created later and cause some weird out of order issues that I cannot fix. Also, there are sometimes issues where the first few chapters cannot become New, or where the posts and threadmarks date are mismatched.
