This bug is related to the mode-line erasure problem I mentioned in
http://lists.gnu.org/archive/html/emacs-devel/2015-01/msg01040.html .

To reproduce the problem by the instruction below, you would need to
make sure that your X11 compositing manager is turned off.  I tested
on Cent OS 5.11, default setting.  See also
http://lists.gnu.org/archive/html/emacs-devel/2013-04/msg00600.html .

Steps to reproduce:

1. Create a file (say, ~/test.el) containing the following contents:

  (custom-set-faces
   '(mode-line ((((class color) (min-colors 88)) (:background
     "grey75" :foreground "black" :box (:line-width 2 :color
     "grey75" :style released-button)))))
   )

2. $ emacs -Q -D -l ~/test.el
3. C-x 2 C-x 2 C-x 2
4. C-x o C-x o C-x o
5. Move another window (e.g., the terminal window from which Emacs is
   invoked) so that it hovers over the Emacs frame.

Result:

The upper two mode-lines among four are not redrawn after their hidden
part is revealed (see the attachment).  They are not redrawn in
response to expose events because the flag `enabled_p' for these
mode-line glyph rows have been set to false.

				     YAMAMOTO Mitsuharu
				mituharu@math.s.chiba-u.ac.jp

In GNU Emacs 25.0.50.1 (i686-pc-linux-gnu, GTK+ Version 2.10.4)
 of 2015-01-29 on localhost.localdomain
Windowing system distributor `The X.Org Foundation', version 11.0.70101000
System Description:	CentOS release 5.11 (Final)

Configured features:
XPM JPEG TIFF GIF PNG SOUND LIBSELINUX FREETYPE XFT ZLIB