When you read a thread to certain point on mobile (not sure about desktop behavior at the moment) and scroll back up a bit, a little arrow appears right on top of the read status bar in the bottom right corner. Clicking on that allows you to jump back to where you are coming from.
While this is most likely done based on the read status of the posts, would it be possible to have something similar when clicking the arrow on a reply post to get up to the post that it relates to? Having an easy way to go back down again would be extremely helpful, especially in longer threads.
The back arrow you mention that lets you jump back to where you are coming from works for going back down too. As long as you use the slider (which opens by clicking the X/X tab) for changing posts, the Back button with have an arrow either going up or down.
Also, I am a bit confused when you say they post that it relates to’.
Sure but my problem isn’t the read/unread posts (where the back arrow appears) but the avatar-link in the top right corner when someone replies to a post in the thread. It lets you jump up (mobile) or opens that post (desktop). On mobile a similar back arrow to the one on unread/read posts would be good.