After loading new posts, indicate in some way where the old "newest post" now is

## Is your feature request related to a problem? Please describe.
When I load new posts, the current "newest post" often vanishes beyond the bottom of the window. It would be nice to be able to have some visual queue that *right here* is where you can stop reading because you've already seen these posts.

## Describe the solution you'd like
Some kind of visual queue that this is where the old "newest post" is, and you've already seen everything under it.

## Describe alternatives you've considered
An alternative would be to load new posts *above* the view, keeping the existing posts exactly where they are in the view. There could still be a "go all the way to the top" button or whatever. This way, I don't lose my reading position. Even in this case, some indication of where the newer posts start would be nice.

The issue names no files, tests, or entry points. Start by locating the code that loads new posts and manages the current reading position; done means the boundary between already-seen and newly loaded posts is visibly indicated without losing the reader's place.

Written by the indexing model from the issue text.