bug#1582: 23.0.60; GTK+: missing pixel after detaching tool bar

[Stephen Berman <stephen.berman@gmx.net>]

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In GNU Emacs 23.0.60.24 (i686-pc-linux-gnu, GTK+ Version 2.12.9)
 of 2008-12-15 on escher
Windowing system distributor `The X.Org Foundation', version 11.0.10400090

When I detach the tool bar, what looks like a one pixel gap appears in
the bottom border of the menu bar below the `F' in `File'; here is an
image (made from emacs -Q):


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I do not see this when I disable tool-bar-mode; however, in that case
the menu bar has a different appearance than with a detached tool bar:


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I just noticed the missing pixel now, after updating from the trunk, but
I cannot say for sure when it first appeared.  I checked all the GTK+
Emacs binaries I have, going back to 22.1.50, and they all display this
gap, and I'm certain I didn't overlook it all that time.  This suggests
it's a recent bug in GTK+ or a library used by GTK+.  My (openSUSE 11.0)
system updater installed patches to glib2, libglib, libgmodule,
libgthread, libgobject, libgio on 2008-12-10, so this seems a likely
source. I'm reporting it here because I have no other GTK+ app with a
detachable tool bar, so I want to make sure it's not an Emacs bug.  If
Jan or another GTK+ hacker can confirm it's a GTK-related bug, I can
submit this report to the appropriate list or bugtracker.

Steve Berman