Clicking on the boundary between workspaces chooses the rightmost, bottommost one, when using the Workspace selector applet

Say I'm using Compiz with the Desktop Wall plugin enabled, and I'm working on the top left workspace.
Then I click, on the workspace selector, exactly on the boundary between said workspace and the one directly below it. Then the workspace below is immediately selected.
Clicking again on the same spot will NOT get me back to the top. Clicking one pixel above the boundary will still not work. The effective boundary where I'm brought back up is somewhere above what is shown on the workspace selector.

The same thing happens with the workspace to the right of my current one.

ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: gnome-panel 1:2.32.1-0ubuntu6.5
ProcVersionSignature: Ubuntu 2.6.38-8.42-generic 2.6.38.2
Uname: Linux 2.6.38-8-generic i686
NonfreeKernelModules: nvidia
Architecture: i386
Date: Sat Jul 2 23:10:05 2011
GConfNonDefault:
 /apps/panel/general/applet_id_list=[fast_user_switch_screen0,trashapplet_screen0,workspace_switcher_screen0,window_list_screen0,show_desktop_button_screen0,indicator_applet_screen0,clock_screen0,applet_1,applet_0]
 /apps/panel/general/object_id_list=[yelp_launcher_screen0,browser_launcher_screen0,menu_bar_screen0,object_0,object_1]
 /apps/panel/general/toplevel_id_list=[bottom_panel_screen0,top_panel_screen0]
ProcEnviron:
 LANGUAGE=it_IT:en
 PATH=(custom, user)
 LANG=it_IT.UTF-8
 SHELL=/bin/bash
SourcePackage: gnome-panel
UpgradeStatus: Upgraded to natty on 2011-04-30 (63 days ago)