Please describe the problem:
When using a two screens, stacked vertically, my panel is at the bottom of the top screen. When I maximize a window on the top screen, the window maximizes behind the bottom panel.

Xrandr commands for my dual screen setup:
xrandr --output TMDS-1 --mode 1400x1050 --pos 0x0
xrandr --output LVDS --pos 0x1050

This bug is a follow-up of https://bugs.launchpad.net/ubuntu/+source/metacity/+bug/120818 (not originally reported by me)

Steps to reproduce:
1. attach external monitor
2. execute xrandr commands
3. maximize a window


Actual results:
The window maximizes behind the bottom panel.

Expected results:
The window should not be partially behind the bottom panel, its lower border should be just above the bottom panel.

Does this happen every time?
Yes.

Other information:
Hardware: Macbook with Intel graphics (GMA 945) + Samsung SyncMaster 203b external monitor