I spotted this minor issue some days ago and still haven’t find a solution. I attached a video below:
The difference with this community is that the topic scroll/jump to bar (post count) stays at the end of each post, whereas in my community, it stays some posts above.
That’s what i was talking about. Note, it happens both in PMs and topics.
It Must be a bug then. However, it only happens on your forum. On mine it does not seem to happen? I will check
I think the footer in the site in the video causes this issue. If I hide the footer, the topic progress element (i.e. posts count) is placed in the correct location.
If the footer is built using a theme component, let me know, I can test the theme component specifically and see if it can be fixed.
Custom html and css should be fine, but you should use the below-footer plugin outlet. I’m not sure what you are currently using, but the Discourse easy footer component uses below-footer and works fine.
Thanks for the solution, @pmusaraj 
Amazing how you guys can find a solution for everything!
