In GNU Emacs 23.0.60.21 (i686-pc-linux-gnu, GTK+ Version 2.12.9)
 of 2008-12-03 on escher

1. emacs -Q

2. Eval this:
   (define-minor-mode my-mm ()
     "My minor mode."
     :global t
     (if my-mm
         (setq default-header-line-format
   	       '(:eval (propertize "test" 'face '(:overline t))))
       (setq default-header-line-format nil)))

3. Enable my-mm.

4. C-x d foo (where `foo' is some directory with at least three files,
   e.g. emacs/lisp).

5. Mark three files in foo.

6. Type `C'
   ==> The popped up buffer *Marked Files* shows only two of the three
   marked files.

Likewise if the value of the 'face property in my-mm has a non-nil :box
attribute.  In contrast, if the value of the 'face property is
'(:underline t) (but not '(:underline t :overline t)), then in step 6
all three marked files are shown, though with no room to spare.

I guess the basic problem here is that Emacs calculates window-height in
integral equally sized line numbers.  But less fundamentally and more
directly, this problem arises, AFAICT, because dired-pop-to-buffer's
shrink-to-fit code fails to account for header lines.  To fix this it
would suffice to add this sexp above the last sexp in
dired-pop-to-buffer: 
    (with-current-buffer buf 
      (and header-line-format
           (progn (select-window w2) (enlarge-window 1))))

However, unless I'm missing something, fit-window-to-buffer does what
dired-pop-to-buffer's shrink-to-fit code does, and moreover also handles
header lines.  From perusing the change logs, I found several instances
where fit-window-to-buffer replaces library-specific code, and this
appears to be another candidate (dired-pop-to-buffer was introduced many
years before fit-window-to-buffer), in addition to fixing the above bug.
I'm not able to fix the display code to let window-height return
fractional line numbers, so I propose the following patch: