Let me start with saying that I quite like the infinite scrolling-mode. It’s easy to read through an entire discussion (yes, I sometimes do that) without having to click “Next page” even once. However, I do think there is some room for improvement to how the ‘progress’ is displayed to the end user.
To the bottom of the page you can find this widget, for which I think there is some room for improvement:
I ran across a ddifferent forum software package, where they have also implemented the infinite scrolling feature. They have also created a topic progress tracker, but theirs somewhat resembles a more familiar concept: the scrollbar. Their implementation looks as follows:
Go here to give it a go. By grabbing the progress indicator and dragging it up or down, the forum scrolls up or down and loads the associated posts. Because of its vertical orientation, this feels liek a better combination of scrollbars and infinite scrolling than how Discourse currently handles it.