Window List resizes buttons arbitrarily when vertical

Binary package hint: gnome-panel

When the Gnome panel is in the vertical position and a Window List applet is visible, opening several windows results in weird behaviour.

Desired behaviour:
Buttons are scaled so that they fill the entire panel. Buttons are only re-scaled if a window is opened or closed, or if several windows are grouped/ungrouped under one button.

Actual behaviour:
Buttons are arbitrarily re-scaled when a window is maximised or minimised. Sometimes the entire panel is used, sometimes not.

Steps to reproduce:
1. Move Panel to right hand edge of screen, make it vertical.
2. Set Panel width to 86 pixels. This does not appear to be crucial, but I've included it for completeness.
3. Add a Windows List panel.
4. Open several windows - exactly how many depends on the resolution of your screen, the width of your Panel and how many other applets you have.
5. Minimise/maximise windows by clicking on their Window List buttons.

This behaviour is seen on two separate machines with 64 bit Karmic and has been present since at least Heron. It is present in both fresh install and systems updated from Jaunty. It has been observed on both Nvidia GeForce 8600 GT (nvidia driver) and Intel Q43 (i915 driver) chipsets.

uname -a:
Linux cosine 2.6.31-14-generic #48+ureadahead2-Ubuntu SMP Wed Oct 28 17:28:05 UTC 2009 x86_64 GNU/Linux
Linux ebd-desktop 2.6.31-14-generic #48-Ubuntu SMP Fri Oct 16 14:05:01 UTC 2009 x86_64 GNU/Linux