No icons visible if panel is on side rather than top or bottom of screen

I'm using window picker under the Ubuntu GNOME desktop on Lucid with the panel at the right side of the screen. When I put window picker in that panel, I don't see a panel. I do see a bit of shading that seems to be the full height of the available portion of the window, and if I hover the mouse over that shading I see a tooltip for one of the open windows. It's as though the window picker is drawing the icons left-to-right despite the fact that the panel runs up-and-down.

Uh, other details: 64-bit Linux...nothing else comes to mind.