I’m not reporting this as a problem but I’m just curious. I’ve seen a lot of posts where folks are explaining how to work with them and I haven’t seen anyone else say they don’t have one. I’ve tried every theme and it’s just not there. I finally discovered that if I click on the post count button on the bottom right, a new window will open containing the thread and a slider. I can scroll through the thread and find the post I’m looking for and then back out of the new window. That will work for me since it’s not a feature I use often. But it’s making feel a little crazy that I’m the only one this seems to be happening to. 
(For what it’s worth, I’m using Chrome on a desktop.)
I use mobile view, instead of desktop view, even though I am not on a mobile device. That is one of the effects IME.
I’m also on mobile but that truck works to bring up a slider bar. Nifty!
Some folks have said it didn’t work on mobile. Maybe my trick will help them out. Yay, I helped someone - this wasn’t a useless thread! 
I’m also on Chrome on a laptop. The slider CAN show up as a constant element without hitting the post count button to summon it, but it’s a bit quirky - the browser window needs to be quite wide, and there’s also some dependency on browser font size. So I’d suggest playing around with your browser window. I’m using SDMB Dark, not sure if that’s a factor.
Aha! I changed the browser font size and there it is! That also fixed another problem I found last night. When I hid the reply box, it would disappear and I could only get it back by quoting someone else. I was thinking then that something must be wrong with the page size since both issues involved elements at the edge of the page. I didn’t think of checking the font size. Thank you!
On at least one of my laptops, I see the slider if the browser window is wide “enough”, but not if I pull the browser window in tightly to not waste screen real estate.
Now I can see what people were complaining about when they said there was too much empty real estate around the posts. Before, the post box covered most of the page. I’m using Vincent where the empty space is teal so it’s not so glaring. I can live with that.
I have a problem with my slider bar in that, if I approach the top of the page at anything other than a veritable snail crawl speed, the slider drops back down half the screen.
BtW. I was told that I’m not getting quotes because the board doesn’t quote the immediately previous post, but I don’t get quotes from any post.