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