>> Stefan and Eli, do you agree with that conclusion?
>
> I admit that I've lost the line of reasoning here (too much of the 
> previous context is being elided, forcing me to re-read the entire 
> discussion).  Which code is proposed for the release branch, and how 
> will Emacs behave with that code in this particular use case?
>

It's this patch.  It adds an invisible empty line at the beginning of 
*Completions* when completions-header-format is not a string (in 
particular, nil) or an empty string.