Please describe the problem:
When window list applet is on a vertical panel, if I open 8 or more windows on the same virtual desktop, gnome-panel has a 100% CPU usage and renders the desktop nearly unusable unless killed. If you don't close some windows, the problem remains.

Steps to reproduce:
1. Create a new panel.
2. Add a window list applet.
3. Open 8 or more random aplications.
4. Move the panel to the left of right side of the desktop.

You can swap steps 3 and 4, the outcome is the same.

Actual results:
gnome-panel stops responding, and it gets all the CPU usage.

Expected results:
Nothing at all. That is, gnome-panel should not hang.

Does this happen every time?
Yes.

Other information:
This behaviour did not take place with the previous version of Gnome, it started with 2.20.0. I waited a couple of updates to see if the problem got solved, but 2.20.3 is out already and the problem persists.

Also, I noticed that if the panel doesn't have the "Expand" option enabled, after opening the 8th app, window list tries to grow larger and becomes short again in an endless loop at very high speed. Then, closing a few windows solves it without having to kill gnome-panel. But still, this shouldn't happen.

Another detail is that window list will never grow beyond it's default size. Opening new windows makes their "buttons" on the list smaller and smaller, even when there's plenty of free space to grow larger. This is pretty annoying, since  makes selecting windows harder.