Comment 39 for bug 43066

This problem can be solved with by adding 1 line of code to window-list.c within the gnome-panel source.

First ensure you have the latest versions of gnome-panel and libwnck22.

dpkg -l gnome-panel libwnck22
ii gnome-panel 1:2.30.2-1ubuntu3 launcher and docking facility for GNOME
ii libwnck22 1:2.30.5-0ubuntu1 Window Navigator Construction Kit - runtime files

Then get the source and build dependencies

apt-get source gnome-panel
apt-get build-dep gnome-panel

Edit applets/wncklet/window-list.c in your favourite editor and add this line of code to the beginning of function tasklist_update, just before the if statement.

wnck_tasklist_set_orientation(tasklist->tasklist, tasklist->orientation);

Build the package with dpkg-buildpackage, and install the .deb file for gnome-panel.
Alternatively, use the attached patch file instead of manually editing the source code.
Restart the task list applet using;
killall wnck-applet

I hope this helps.
--
Craig Shelley