We have recently changed the way the "Add topic" button appears/disappears in T316175. While it's mostly working fine in all four cases as directed in the mockups on the task, there are some inconsistencies in the way it re-appears when scrolled to the top of a talk page. It seems sometimes it does not re-appear at all. When it does it's not persistent and disappears immediately, which isn't helpful.
Check the screen capture below.