Either this morning or yesterday, while scrolling down in a thread, the [Back] button that normally holds your place if you leave, shows up in the middle of the scroll bar, about halfway down the responses. Again, this happens while actively reading through a topic. It doesn’t go away until reaching the end of the thread.
Is this something you already know about, and are working to fix, or did something break recently, and I’m the only one reporting it? 
Hmm, it’s possible I’m not fully understanding what you’re describing, but I’m not personally seeing this behavior.
The “Back” button appears in the scroll bar if I start scrolling back up through a long thread, then immediately goes away when I scroll down and reach the place where the “Back” button originally appeared. This is in Chrome and FF on desktop. Chrome on Android shows the back button at the bottom of screen, so I think that’s different from what you’re describing.
What browser are you using?
Using Chrome 92.0.4515.107 on a work computer. Haven’t had the time to check at home.
Mine is showing up during my down scrolling, and disappears when I get to the bottom. Before I shut down today, maybe I’ll clear cache, and check again after the weekend. I’ll also check on my home PC, to see if I am seeing the same thing.