Inefficient sizing in window list applet

I get really odd taskbar button sizing behaviour in the gnome-panel window list applet

I get really odd taskbar button sizing behaviour in the gnome-panel window list applet. How to reproduce:

1. Choose an empty workspace
2. Open a window
A new button in the window list appears, whose length accomodates the window title. E.g., if you launch a terminal the button will be shorter than if you launch Firefox.
3. Open another window
Even though there is a lot of empty space, the old button is shrunk to half its size and the new button is also the same size - together the buttons occupy the same size as the single one before. Of course, titles don't fit, so they are ellipsized. This is very annoying, as there is still a lot of unused space on the panel.
4. Open another window.
The "used" area increases a bit.
5. Open another window.
Now again the existing buttons are shrunk when a new one is added (there's still lots of space on the right).
...

It seems that the correct calculation is done only for every second window opened.

I tried playing with Firefox (creates a longer button) and Terminal (creates a shorter button), and no matter what the creation order, the initial two buttons occupy the size of the longer one, i.e., Terminal+Terminal gives two very short buttons; Firefox+Terminal, Terminal+Firefox and Firefox+Firefox give two slightly (but not enough) longer ones.

I don't know if this is a GNOME bug or one specific to Ubuntu.