I was replying to a post and out of curiosity I ticked a small button on the lower right (Looked like angled lines or mountains on the button It is not the window resizing button).

Now when replying to a post I can no longer see any prior posts below it.... All I can view is the current post I am creating nothing prior further below it.

What is this button (It seems to be missing) and how can I bring this feature back?

Thank You