I wonder how well it scales to upwards of thousands of comments, though. There are some forums I've seen where it's not unusual to have threads that are several hundred pages of 20+ posts each.
Basically, the thread author (or admins) mark particular posts, and a table of contents to them gets auto-generated, and each of those posts in the thread gets forward/back buttons to skip to the next/previous. It makes long-running threads much more navigable, especially single-purpose ones (game walkthroughs, following the news about specific events, etc).
Massive threads are somewhat of a proving ground for every forum implementation and feature. Like when you first realize that `OFFSET` SQL for pagination just isn't gonna cut it.
For example, I recently read a field report of someone migrating their Xenforo forum to Discourse (https://meta.discourse.org/t/discourse-for-me-6-weeks-in/319...) that had issues with megathreads after the migration since Discourse sends down a list of all post_ids in the topic, a known issue.
That threadmarks plugin is a brilliant idea, like a curated table-of-contents or places-of-interest. I'm going to implement that on my forum.
It also seems like prime territory to have something added in the style of the Xenforo threadmarks plugin: https://github.com/Sidane/xenforo-threadmarks
Basically, the thread author (or admins) mark particular posts, and a table of contents to them gets auto-generated, and each of those posts in the thread gets forward/back buttons to skip to the next/previous. It makes long-running threads much more navigable, especially single-purpose ones (game walkthroughs, following the news about specific events, etc).