gnome-panel interacts badly when dual-desktop set a vertical overlap.

[davidzhang.amd]

I have a 2 monitors and create a external desktop environment, and the external panel stacked vertically above the primary panel. following is some information about xrandr -q
DFP1 disconnected (normal left inverted right x axis y axis)
DFP2 connected 1920x1200+0+0 (normal left inverted right x axis y axis) 518mm x 324mm
   1920x1200 60.0*+
   1920x1080 60.0
   1600x1200 60.0
DFP5 disconnected (normal left inverted right x axis y axis)
DFP6 connected 1920x1200+0+1200 (normal left inverted right x axis y axis) 582mm x 364mm
   1920x1200 60.0*+
   1920x1080 60.0
   1600x1200 60.0

So the primary panel locate at (0,0) while the external locate at (0,1200), and the gnome-panel bar locate at the bottom of the primary display.
The I want to set the two panel vertical overlapped a small area( reset the position of the external display to(0, 1160) ). use the command: xrandr --output DFP6 --pos 0x1160
The issue observed, gnome-panel don't think the panel-bar on the bottom of the primary desktop, it reconfigured it a wrong position and a wrong big area. all the window and icons can't move to this area.
and if we configure this bar to left,right,top, it won't observed this issue. so the issue only happened when the bar was be set overlapped
May be there some logic mistake in the file ./gnome-panel/panel-mutiscreen.c , in the function panel_multiscreen_compress_overlapping_monitors, it thought we do a full desktop overlap, and subtract a monitor while we just set a part of overlap.

ProblemType: Bug
DistroRelease: Ubuntu 10.10
Package: evince 2.32.0-0ubuntu1
ProcVersionSignature: Ubuntu 2.6.35-22.33-generic 2.6.35.4
Uname: Linux 2.6.35-22-generic i686
NonfreeKernelModules: fglrx
Architecture: i386
Date: Mon Oct 31 14:19:30 2011
InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Release i386 (20101007)
ProcEnviron:
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: evince