Comment 2 for bug 733068

I can confirm this. It happens when the panel is at the right, not at the bottom. The layout changes between one-column and two-column layout whenever certain windows are activated or when I click on the panel. It's especially annoying because it's often not possible to click on a task button in order to activate a window.
At one time, when I had many open windows, it has caused very high CPU load, because it rapidly flickered between one and two column layout.