Firstly, I don’t really understand why this is desirable. Why make someone scroll back to the top just to press a button that isn’t taking up significant real estate anyways? Also, why hide the sign up button and not the log in button?
I personally think the sign up button should just stay where it is and not disappear.
With that logic, why not remove the log in button to fit in a few more characters? I just think the trade off of having one single button appear and disappear on scroll to accommodate a single word is not worth it. Also, if a title is too long and getting the “more” ellipsis, the few characters that are gained from removing the sign up button will likely not make or break the context of the question. Lastly, it just looks silly/buggy when the topic title is short.
That’s one solution! I really have a hard time understanding why it is not just best to keep it on top though. Again, the log in button doesn’t go anywhere. I think a user would expect that the sign up button stays where they originally saw it. It’s hard enough signing up users without a teleporting sign up button