It's hard to say, don't know how this scrubber will perform on such a big numbers. My biggest concern is one small move will move by hundreds of posts. So easy moving will be problematic.
Maybe add some switcher, keydown trick or something to dynamically change range of scrubber?
Like, hold shift to change scope of scrubber from default 100% of discussion (from first to last post) to only some part of discussion, like nearest 600 posts only. So if you are on 6000 post of 12000 posts. Scrubber will change it's position to be on vertical middle, allow to move up/down. But move only around current post in between 5700 and 6300 of posts (or more range).
When release LMB (mouse) then scrubber will re-position itself to be on middle again. So 300 posts up and 300 down to move.
Anyway, with key shortcut or some other way like some mode button under scrubber user should be able to switch between 100% mode and fixed XXX posts mode. Or any other way like fluid slider where we can choose how many %% scrubber will handle at once.
Like Logitech mouse have wheel scroll with flow/speed mode and classic (slow) mode.
It's just simple ideas. Toby know better how it works now...
