Mouse-face overlay bleeds into header line -- except in Info

[Stephen Berman]

I sent a report about this to emacs-pretest-bug a couple of days ago,
but there hasn't been a response yet, and I'm not sure it is a bug,
because it doesn't happen in Info (see below).  So I'm asking here to
see if there's something about the interaction between overlays and
header lines that I'm not understanding.  Do the following in CVS
Emacs: 

1. Start Emacs with -q -no-site-file.  

2. In *scratch* eval (setq ov (make-overlay 66 92)), (overlay-put ov
'mouse-face 'highlight), and (setq header-line-format "test").  

3. Drag the mouse over the string "evaluation.\n;; If you want" and
notice the highlighting of only this string.  

4. Now click on the down arrow in the scroll bar until the line
beginning ";; If you want" is directly below the header line.  

5. Drag the mouse over ";; If you want" and notice that not only it
but also the header line are highlighted.

I have only gotten this with mouse-face overlays; it doesn't happen
with e.g. face overlays, nor with mouse-face text properties.  It also
doesn't happen with mode lines.  It happens with header lines whether
or not the front-advance and rear-advance arguments of make-overlay
are set to t and nil, respectively.  It also happens with header lines
from packages such as ruler.el and tabbar.el.  But it does not happen
with header lines in Info: type C-h i and go to e.g. (elisp) Lisp Data
Types and scroll down until line 22 "Types::.)" is directly below the
header line, so that the beginning of the mouse-face highlighting
"Editing" is hidden by the header line; now dragging the mouse over
"Types" highlights only it and not the header line.  And if you invoke
ruler-mode or tabbar-local-mode in Info, so that the header line
changes, it still it doesn't become highlighted.  So even if what I
described above is a bug, something in Info prevents it.  I'd like to
know what that is and would be grateful for any help.

--Steve Berman