In an endlessly scrolling implementation, you’d just scroll up.

Without endless scrolling, it could behave as follows:

1. Moving back to page 1 takes the user to where they were on page 1 when they navigated away, with the same items visible
2. When automatically fetching new posts, either expand the page (pushing nothing off) or make it visually clear that pages will be pushed to the next page. Or just don’t fetch new posts automatically, and only reset the first / last post of a page if the user clicks Refresh.