Thanks for the analysis.  How bizarre!

Your patch is not going to hurt anything and helps hyper-describe-function 
in the short run, so feel free to commit.  But this shouldn't cause a 
problem, so the problem really belongs on `hyper-apropos-get-doc' in 
lisp/hyper.el.  Just delete the comment; the issue is documented in new 
issue 521.

Does this manifest on 21.4?

No big hurry on the commit.

Attached is fix for this problem, marked with a FIXME keyword comment.

I haven't pushed anything into xemacs hg ever, so give me some time if
you want me to commit this fix.

Later,
Adrian

Oops, I have
(fset 'describe-function 'hyper-describe-function)
in my site-start.el

I can confirm that describe-function is actually correct for me too,
while hyper-describe-function shows the reported problem.

I'll see what I can do about this issue.

Adrian

Do you have C-h f rebound? Normally it invokes `describe-function', not hyper-help.

I've reproduced, and also confirmed that `describe-function' provides the right 
info for `interactive', but don't have time to work on it further now.

Here is all that is displayed in the *Hyper Help* buffer for an
xemacs-beta build from hg sources on the default branch (a1dd514df5c6+ tip).

`interactive' (buffer: *Ibuffer*, mode: Ibuffer by alphabetic)


Built-in function:

  arguments: (zmacs-region-stays t)